A steam system

Abstract

A steam water-heating system 100, the steam system comprising: a heat exchanger 12 configured to heat water with steam; a steam supply line 18 supplying steam to the heat exchanger; a condensate line

Description

The present invention relates to a steam system configured to heat water and a method of controlling the steam system and a method of designing such a steam system. Steam systems are typically used to heat water for use, such as in domestic hot water systems. Such steam systems may be controlled in various ways to control the water temperature. According to a first aspect there is disclosed a steam system configured to heat water, the steam system comprising: a heat exchanger configured to receive steam and water, wherein the heat exchanger is configured to heat the water with the steam; a steam supply line configured to be connected to a steam source to supply steam to the heat exchanger; a condensate line configured to receive condensate from condensed steam from the heat exchanger and to duct the condensate away from the heat exchanger; a water supply line configured to supply water to the heat exchanger; a water discharge line configured to receive heated water from the heat exchanger and duct the heated water away from the heat exchanger; a steam valve disposed on the steam supply line and configured to control the flow of steam through the steam supply line; a condensate valve disposed on the condensate line and configured to control the flow of condensate through the condensate line; a first sensor disposed downstream of the steam valve and upstream of the condensate valve and configured to output a pressure signal indicative of pressure in the steam system between the steam valve and the condensate valve and / or a second sensor at the condensate outlet configured to output a condensate temperature signal indicative of the temperature of the condensate in the condensate line between the heat exchanger and the condensate valve; a third sensor disposed on the water discharge line and configured to output a water temperature signal indicative of the temperature of the heated water through the water discharge line; a controller configured to control opening and closing of the steam valve and the condensate valve based on the water temperature signal and at least one of the pressure signal and the condensate temperature signal. Controlling the flow with both a steam valve and a condensate valve based on the water temperature signal and at least one of the pressure signal and the condensate temperature signal enables the system to simultaneously (i) ensure subcooling of the condensate, meaning that more of the heat from the steam has been transferred to the water, which results in a more efficient heating process, (ii) deliver a stable water outlet temperature even under fast or dynamic demand load changes, and (iii) enable a high turndown ratio of the system. When controlling both a steam valve and a condensate valve based on the water temperature signal and the pressure signal, the system can further (iv) ensure that the system does not stall under any circumstances. The dual control (i.e., control with both the steam valve and the condensate valve) also means that when there are fast demand load changes from a high demand load to a low demand load, the subcooling of the condensate can be achieved more quickly, thus arriving at more efficient operation faster than could otherwise be achieved. The first sensor may be a pressure sensor. The first sensor may be disposed between the steam valve and the heat exchanger in the steam supply line to output the pressure signal indicative of pressure in the steam supply line downstream of the steam valve. The second sensor may be a temperature sensor. The third sensor may be a temperature sensor. It may be that the controller is configured to control the steam valve based on the water temperature signal, and to control the condensate valve based on the pressure signal. It may be that the controller is configured to control the condensate valve based on a comparison of the pressure signal to a set pressure. The set pressure may be a set point pressure or a set pressure range, wherein the set pressure range is a range of acceptable pressures. It may be that the controller is configured to operate in a feedback loop to control the condensate valve to at least partially close when the pressure signal indicates that the pressure in the steam supply line is below the set pressure and not rising, and / or to at least partially open when the pressure signal indicates that the pressure in the steam supply line is above the set pressure and not falling. When the set pressure is a set pressure range, the pressure signal may indicate a pressure be below the set pressure range when the pressure signal indicates a pressure below a lower end of the set pressure range. The pressure signal may indicate a pressure be above the set pressure range when the pressure signal indicates a pressure above an upper end of the set pressure range. In other words, the controller may be a PID controller which may be configured to control the condensate valve based on the pressure signal compared with a set point pressure or a set pressure range. It may be that the set pressure is higher than a pressure in the condensate line downstream of the condensate valve added to a condensate pressure drop of the condensate valve to prevent stall of the steam system. When the set pressure is a set pressure range, the lower end of the set pressure range may be higher than a pressure in the condensate line downstream of the condensate valve added to the condensate pressure drop. It may be that the set pressure is lower than a supply pressure of steam upstream of the steam valve minus a critical pressure drop of the steam valve. When the set pressure is a set pressure range, the upper end of the set pressure range may be lower than a pressure in the condensate line downstream of the condensate valve added to the condensate pressure drop. It may be that the heat exchanger is sized to ensure that the condensate is subcooled when the pressure of steam between the steam valve and the heat exchanger is at the set pressure and when the flow rate of the water is at a maximum demand. Maximum demand may relate to the mass flow rate of water through the heat exchanger and / or the inlet temperature of the water. In other words, maximum demand may correspond to a maximum mass flow rate of water through the heat exchanger and / or a minimum inlet water temperature. It may be that the controller is configured to control the steam valve based on the water temperature signal, and to control the condensate valve based on the condensate temperature signal. It may be that the controller is configured to control the condensate valve based on a comparison of the condensate temperature signal with a condensate set temperature. The condensate set temperature may be a set point temperature or a set temperature range. It may be that the controller is configured to operate in a feedback loop to control the condensate valve to at least partially close when the condensate temperature signal is above the condensate set temperature and not falling and / or to control the condensate valve to at least partially open when the condensate temperature signal is below the condensate set temperature and not rising. When the set temperature is a set temperature range, the condensate temperature signal may indicate a temperature below the set temperature range when the condensate temperature signal indicates a temperature below a lower end of the set temperature range. The condensate temperature signal may indicate a temperature above the set temperature range when the condensate temperature signal indicates a temperature above an upper end of the set temperature range. In other words, the controller may be a PID controller which may be configured to control the condensate valve based on the temperature signal compared with a set point temperature or a set temperature range. It may be that the controller controls the condensate valve so that it has a minimum opening which is not fully closed. If the condensate valve is fully closed, then the true condensate temperature cannot be measured, since the measurement is downstream of the heat exchanger. It may be that the controller is configured to control the steam valve based on a comparison of the water temperature signal to a set water temperature. The set water temperature may be a water set point temperature or a set water temperature range. It may be that the controller is configured to operate in a feedback loop to control the steam valve to at least partially open when the water temperature signal is below the water set point temperature and not rising and / or to control the steam valve to at least partially close when the water temperature signal is above the water set temperature and not falling. When the water set temperature is a set water temperature range, the water temperature signal may indicate a temperature below the set water temperature range when the water temperature signal indicates a temperature below a lower end of the set water temperature range. The water temperature signal may indicate a temperature above the set water temperature range when the water temperature signal indicates a temperature above an upper end of the set water temperature range. In other words, the controller may be a PID controller which may be configured to control the steam valve based on the water temperature signal compared with a water set point temperature or a set water temperature range. According to a second aspect there is disclosed a method of controlling a steam system according to any preceding claim, the method comprising: receiving the pressure signal or the condensate water temperature signal; receiving the water temperature signal; and controlling opening and closing of the steam valve and the condensate valve based on the water temperature signal and at least one of the pressure signal and the condensate temperature signal. The method may comprise controlling the steam valve based on the water temperature signal, and controlling the condensate valve based on the pressure signal. The method may comprise controlling the condensate valve based on a comparison of the pressure signal to a set pressure. The method may comprise operating the control of the steam valve and the condensate valve in a feedback loop by controlling the condensate valve to at least partially close when the pressure signal indicates that the pressure in the steam supply line is below the set point pressure and not rising, and / or to at least partially open when the pressure signal indicates that the pressure in the steam supply line is above the set point pressure and not falling. It may be that the set pressure is higher than a pressure in the condensate line downstream of the condensate valve added to a condensate pressure drop of the condensate valve to prevent stall of the steam system. It may be that the set pressure is lower than a supply pressure of steam upstream of the steam valve minus a critical pressure drop of the steam valve. The method may comprise controlling the steam valve based on the water temperature signal, and controlling the condensate valve based on the condensate temperature signal. The method may comprise controlling the condensate valve based on a comparison of the condensate temperature signal with a set condensate temperature. The method may comprise operating in a feedback loop to control the condensate valve to at least partially close when the condensate temperature signal is above the set condensate temperature and not falling and / or to control the condensate valve to at least partially open when the condensate temperature signal is below the set condensate temperature and not rising. The method may comprise controlling the condensate valve so that it has a minimum opening which is not fully closed. The method may comprise controlling the steam valve based on a comparison of the water temperature signal to a water set temperature. The method may comprise operating in a feedback loop to control the steam valve to at least partially open when the water temperature signal is below the water set point temperature and not rising and / or to control the steam valve to at least partially close when the water temperature signal is above the water set point temperature and not falling. According to a third aspect there is disclosed a method of designing a steam system according to the first aspect, the method comprising: receiving a steam supply pressure of an environment in which the steam system is to be installed; determining a steam critical pressure drop across a steam valve; determining a maximum demand load for a heat exchanger; and selecting a heat exchanger size based on the steam supply pressure, steam critical pressure and maximum demand load. The method may further comprise receiving a condensate return pressure of the environment in which the steam system is to be installed; determining a condensate pressure drop across a condensate valve; and selecting the heat exchanger size based further on the condensate return pressure and the condensate critical pressure. The method may comprise selecting a set pressure at an inlet for the heat exchanger based on the steam supply pressure and the steam critical pressure; wherein selecting the heat exchanger size comprises determining the heat exchanger size at which condensate is subcooled when an inlet pressure to the heat exchanger is the set pressure and when there is a maximum demand load. The skilled person will appreciate that except where mutually exclusive, a feature or parameter described in relation to any one of the above aspects may be applied to any other aspect. Furthermore, except where mutually exclusive, any feature or parameter described herein may be applied to any aspect and / or combined with any other feature or parameter described herein. Embodiments will now be described, by way of example only, with reference to the accompanying Figures, in which: Figure 1 schematically shows a first example steam system for heating water; Figure 2 schematically shows a second example steam system for heating water; Figure 3 is a flow chart showing steps of a basic method of heating water with an example steam system; Figures 4a and 4b are flow charts showing steps of a more detailed method of heating water with an example steam system; and Figure 5 is a flow chart showing steps of an example method for designing an example steam system such as in Figures 1 and 2. Figure 1 shows a first example steam system 100 configured to heat water, for example in a domestic hot water system. The first example steam system 100 comprises a heat exchanger 12 which is configured to receive steam on a supply side 14 and to receive water on and demand side 16 and to heat the water with the steam. In this example, the steam and the water are kept separate. In this example, the steam system 100 comprises a steam supply line 18 which is configured to be connected to a steam source 20. The steam supply line 18 in this example is connected to a supply side inlet 22 (also called a steam inlet 22) of the heat exchanger 12 to supply steam to the heat exchanger 12 on the supply side 14. In this example, the steam source 20 may supply steam at a constant pressure, such as 400 kPa. The steam supplied via the steam supply line 18 to the supply side 14 condenses as it passes through the supply side 14 and transfers heat to the demand side 16 of the heat exchanger 12. In this example, the steam system 100 comprises a condensate line 24 which is configured to be connected to a condensate drain 26. The condensate line 24 is connected to a supply side outlet 28 (also called a condensate outlet 28) so that it is configured to receive condensate (from the condensed steam) from the supply side 14 of the heat exchanger 12 and to duct the condensate away from the heat exchanger 12, delivering it to the condensate drain 26. In some examples, condensate received in the condensate drain 26 may be heated in another system and may be connected to the steam source 20 to provide a closed loop system providing steam for the steam source 20. In this example, the steam system 100 comprises a water supply line 30 which is configured to be connected to a demand side inlet 34 (also called a water inlet 34) of the heat exchanger 12 on the demand side 16. The water supply line 30 is configured to supply water to the demand side 16 of the heat exchanger 12. The water supplied to the water inlet 34 may be relatively cold water. In this example, the water supply line 30 is configured to connect to a water source 32, for example a domestic water system. In the heat exchanger 12, the water received on the demand side 16 is heated with the steam received on the supply side 14, so that the steam condenses and leaves the heat exchanger 12 as condensate, and the water is heated to leave the heat exchanger 12 as heated water. In this example, the steam system 100 comprises a water discharge line 36 which is connected to a demand side outlet 38 (also called a water outlet 38) on the demand side 16 of the heat exchanger 12 to receive heated water from the demand side 16 of the heat exchanger 12 and to duct the heated water away from the heat exchanger 12 towards a water drain 37. The water at the water inlet 34 is relatively cooler than the heated water at the water outlet 38 when the heat exchanger is operating to receive steam at the steam inlet 22. In some examples, the water drain 37 may be connected in a closed loop to the water source 32, such as by a domestic water system. The heated water from the water drain 37 may lose its heat through the domestic water system, and arrive at the water source 32 relatively cooler. In this example, a steam valve 40 is disposed on the steam supply line 18 and is configured to control the flow of steam through the steam supply line 18. The steam valve 40 may be a globe valve, for example. In other examples, the steam valve may be any suitable valve such as a slide valve, a piston valve or a ball valve. In this example, the steam valve 40 is connected to a controller 50 which is configured to control the opening and closing of the steam valve 40. In other words, the steam valve 40 may be actively controlled by the controller 50 to control the flow of steam through the steam supply line 18. In this example, the steam valve 40 is a variable control valve, and by incrementally opening the steam valve 40, the mass flow rate of steam increases across the steam valve 40 to supply more steam to the supply side 14 of the heat exchanger 12, and by incrementally closing the steam valve 40, the mass flow rate of steam decreases across the steam valve 40 to supply less steam to the supply side 14 of the heat exchanger 12. The steam valve 40 in this example may be sized based on the pressure of steam supplied by the steam source 20 so that, when it is maximally opened, it will have at least a critical pressure drop (i.e., the pressure drop at which any further pressure drop will not result in an increase in steam velocity) across the steam valve 40. This would ensure that the incrementally closing the steam valve 40 from its maximum opening would reduce the mass flow rate across the steam valve 40, thereby enabling use of the steam valve 40 across its entire opening spectrum. In this example, a condensate valve 42 is disposed on the condensate line 24 and is configured to control the flow of condensate through the condensate line 24. In this example, the condensate valve 42 is connected to the controller 50 which is configured to control the opening and closing of the condensate valve 42 in a similar manner to the steam valve 40. In other examples, the condensate valve 42 and the steam valve 40 may be different types of valves which are controlled differently. In this example, a pressure sensor 44 is disposed in the steam supply line 18 between the steam valve 40 and the steam inlet 22 of the heat exchanger 12. The pressure sensor 44 is configured to output a pressure signal indicative of pressure in the steam supply line 18 downstream of the steam valve 40 (i.e., between the steam valve 40 and the heat exchanger 12). In some examples, the pressure sensor may be disposed anywhere downstream of the steam valve 40 and upstream of the condensate valve 42 to output a pressure signal indicating a pressure in the supply side 14 of the steam system between the steam valve 40 and the condensate valve 42. In other examples, the pressure sensor may be any type of sensor or collection of sensors which can be used to indicate a pressure in the steam system between the steam valve 40 and the condensate valve 42. In this example, a water temperature sensor 46 is disposed on the water discharge line 36 and is configured to output a water temperature signal indicative of the temperature of the heated water in the water discharge line 36. In other examples, the temperature sensor may be any type of sensor or collection of sensors which can be used to indicate a temperature of the water in the water discharge line 36. In this example, the controller 50 is configured to control opening and closing of the steam valve 40 and the condensate valve 42 based on the water temperature signal and the pressure signal. This simultaneous control of the steam valve 40 and the condensate valve 42 can control the level (or height) of condensate in the heat exchanger 12. For example, if the steam valve 40 is open while the condensate valve 42 is closed, or the steam valve 40 is opened more than the condensate valve 42, such that mass flow rate of steam across the steam valve 40 (and therefore into the steam inlet 22) is greater than the mass flow rate of condensate across the condensate valve 42 (and therefore out of the condensate outlet 28), the level of condensate in the heat exchanger will rise. Conversely, if the steam valve 40 is closed while the condensate valve 42 is opened, or the steam valve 40 is opened less than the condensate valve 42, such that mass flow rate of steam across the steam valve 40 (and therefore into the steam inlet 22) is smaller than the mass flow rate of condensate across the condensate valve 42 (and therefore out of the condensate outlet 28), the level of condensate in the heat exchanger will fall. Controlling the level of condensate in the heat exchanger 12 can help to control the amount of heat which is conducted across the heat exchanger 12 by changing the area of heating surface exposed to steam. By controlling the level over condensate, the steam system 100 can ensure that the condensate sub-cools in the heat exchanger 12 before it is discharged. This can considerably reduce the amount of flash steam downstream, which may improve the performance of the system and also reduce heat losses. It also helps to prevent stall of the steam system 100 as increasing the condensate level reduces the heat transfer which increases the pressure of the steam in the heat exchanger 12, thereby giving enough pressure to discharge the condensate through the condensate valve 42. A heat exchanger may be typically sized to work with a predetermined steam inlet pressure, at which the heat exchanger is configured to subcool the condensate when the water is at a maximum demand. For some applications, maximum demand corresponds to a maximum flow rate of water through the heat exchanger 12. In other applications, maximum demand may correspond to a minimum inlet water temperature. In further applications, maximum demand may be related to both the mass flow rate of water through the heat exchanger 12 and the minimum inlet temperature of the water. In this example, the heat exchanger 12 may be sized to work with a predetermined steam inlet 22 pressure which is the same as or lower than a supply pressure of steam upstream of the steam valve 40 minus a critical pressure drop of the steam valve 40, where it is configured to subcool the condensate when receiving steam at, or below, the predetermined steam inlet 22 pressure. This maximises efficiency of the steam system 100, since it ensures that the condensate will always be subcooled if the inlet pressure is kept at, or below, the predetermined steam inlet pressure. Therefore, this would enable the steam system 100 to be able to subcool the condensate under all load conditions. In this example, there is a set pressure for the steam between the steam valve 40 and the condensate valve 42. In this example, the set pressure is a set point pressure, for example X kPa. The set point pressure may be set to the same predetermined steam inlet 22 pressure, which the heat exchanger 12 is sized for. In order to ensure that the steam system 100 does not stall at any point (i.e., to ensure that the pressure in the condensate line 24 upstream of the condensate valve 42 is always high enough to drive steam flow through the condensate valve 42 to the condensate drain 26), the set pressure, in this example, is set to be higher than a pressure in the condensate line 24 downstream of the condensate valve 42 added to a condensate pressure drop of the condensate valve. Having the set pressure set to be higher than a pressure in the condensate line 24 downstream of the condensate valve 42 added to a condensate pressure drop of the condensate valve, and lower than or equal to a supply pressure of steam upstream of the steam valve 40 minus a critical pressure drop of the steam valve 40 ensures both that the steam system 100 will not stall at any point, and that the condensate will always be subcooled, thus increasing reliability and efficiency of the system. Therefore, as will be explained in more detail with reference to the flow charts in Figure 3-4, the dual, simultaneous control of the supply side 14 flow into and out of the heat exchanger 12 with both the steam valve 40 and a condensate valve 42 based on the water temperature signal and the pressure signal enables the system to simultaneously (i) ensure subcooling of the condensate, meaning that more of the heat from the steam has been transferred to the water, which results in a more efficient heating process, (ii) deliver a stable water outlet temperature even under fast or dynamic demand load changes, (iii) enable a high turndown ratio of the system, and (iv) ensure that the system does not stall under any conditions. The dual control (i.e., control with both the steam valve 40 and the condensate valve 42) also means that when there are fast demand load changes from a high demand load to a low demand load, the subcooling of the condensate can be achieved more quickly, thus arriving at more efficient operation faster than could otherwise be achieved. The controller 50 in this example is configured to control the condensate valve 42 based on a comparison of the pressure signal to the set point pressure. The controller 50 in this example operates in a feedback loop to control the condensate valve 42 to control the pressure at the pressure sensor to be at the set point pressure. In other words, when the pressure signal indicates that the pressure should be increased, the controller 50 may control the condensate valve 42 to at least partially close, and when the pressure signal indicates that the pressure should be decreased, the controller 50 may control the condensate valve 42 to at least partially open. In some examples, the controller 50 may be a PID (proportional integral derivative) controller, which is configured to operate in a feedback loop to control the condensate valve to at least partially close when the pressure signal indicates that the pressure in the steam supply line 18 is below the set pressure and not rising, and to at least partially open when the pressure signal indicates that the pressure in the steam supply line is above the set pressure and not falling. This accounts for the lag time in making a change to the condensate valve 42 and observing the change. The PID controller may control the amount of partial opening or closing of the condensate valve 42 based on how far the steam pressure is from the set pressure and the rate of change of the steam pressure. In other examples, the set pressure may be a set pressure range which may encompass a range of acceptable pressures, such as Y-Z kPa, and the condensate valve 42 may be controlled to ensure that the pressure between the steam valve 40 and the condensate valve 42 is within the set pressure range. When the set pressure is a set pressure range, the pressure signal may indicate a pressure being below the set pressure range when the pressure signal indicates a pressure below a lower end of the set pressure range. The pressure signal may indicate a pressure be above the set pressure range when the pressure signal indicates a pressure above an upper end of the set pressure range. When the set pressure is a set pressure range, the lower end of the set pressure range may be set to be higher than a pressure in the condensate line 24 downstream of the condensate valve 42 added to the condensate pressure drop. In such examples, the upper end of the set pressure range may be lower than or equal to the supply pressure of steam upstream of the steam valve 40 minus the critical pressure drop of the steam valve 40. The controller 50 in this example is further configured to control the steam valve 40 based on a comparison of the water temperature signal to a water set temperature. In this example, the water set temperature is a water set point temperature, for example 60 degrees centigrade. The controller 50 in this example operates in a feedback loop to control the steam valve 40 to control the temperature at the water temperature sensor 46 (in the water discharge line 36) to be at the water set point temperature. In other words, when the water temperature signal indicates that the water temperature should be increased (i.e., the water temperature is below the water set point temperature), the controller 50 may control the steam valve 40 to at least partially open, and when the water temperature signal indicates that the water temperature should be decreased (i.e., the water temperature is above the water set point temperature), the controller 50 may control the steam valve 40 to at least partially close. In this example, the controller 50 is PID (proportional integral derivative) controller, which is configured to operate in a feedback loop to control the steam valve 40 to at least partially close when the water temperature signal indicates that the water temperature in the water discharge line 36 is above the water set point temperature and not falling, and to at least partially open when the water temperature signal indicates that the pressure water temperature in the water discharge line 36 is below the water set point temperature and not rising. This accounts for the lag time in making a change to the steam valve 40 and observing the change. The PID controller may control the amount of partial opening or closing of the steam valve 40 based on how far the water temperature is from the water set point temperature and the rate of change of the water temperature. In some examples, the water set temperature may be a set water temperature range which may encompass a range of acceptable temperatures, such as 55-65 degrees centigrade. In such examples, the water temperature signal may indicate a temperature below the set water temperature range when the water temperature signal indicates a temperature below a lower end of the set water temperature range. The water temperature signal may indicate a temperature above the set water temperature range when the water temperature signal indicates a temperature above an upper end of the set water temperature range. Figure 2 shows a second example steam system 200. The second example steam system 200 is similar to the first example steam system 100, with like reference numerals denoting like features. The second example steam system 200 differs from the first example steam system 100 in that it does not comprise a pressure sensor, but rather it comprises a condensate temperature sensor 48 which is disposed at the condensate outlet 28 on the condensate line 24 (i.e., between the condensate outlet 28 and the condensate valve 42) and which is configured to output a condensate temperature signal indicative of the temperature of the condensate in the condensate line 24 between the heat exchanger 12 and the condensate valve 42. In other examples, the condensate temperature sensor may be any type of sensor or collection of sensors which can be used to indicate a temperature in the condensate line 24. In yet further examples, a steam system may comprise both the pressure sensor 44 as in the first example steam system 100, and the condensate temperature sensor 48 as in the second example steam system 200. In this example, the controller 50 is configured to control opening and closing of the steam valve 40 and the condensate valve 42 based on the water temperature signal and the condensate temperature signal. Controlling the steam valve 40 and the condensate valve 42 based on the condensate temperature signal enables the steam system 200 to ensure that the condensate is subcooled, by directly controlling the temperature of the condensate. Controlling the steam valve 40 and the condensate valve 42 based on the water temperature signal also enables delivery of a stable water outlet temperature even under fast or dynamic demand load changes, and enables a high turndown ratio of the system 200 in a similar manner to the first example steam system 100. The second example steam system 200 is more vulnerable to stall than the first example steam system 100 since it is not directly controlling the pressure in the supply side 14. The controller 50 in this example may be configured to operate in a feedback loop to control the steam valve 40 to control the temperature at the temperature sensor (in the water discharge line 36) to be at the water set point temperature, in the same manner as in the first example steam system 100. The controller 50 in the second example steam system 200 differs from the controller in the first example steam system 100 in that it is configured to control the condensate valve 42 based on a comparison of the condensate temperature signal to a condensate set temperature. In this example, the condensate set temperature is a condensate set point temperature, such as 90 degrees centigrade. Ideally, the condensate set point temperature is below 100 degrees centigrade in order to ensure subcooling of the condensate. The controller 50 in this example operates in a feedback loop to control the condensate valve 42 to control the condensate temperature at the condensate temperature sensor 48 to be at the condensate set point temperature. In other words, when the condensate temperature signal indicates that the condensate temperature should be decreased (i.e., the condensate temperature is determined to be above the condensate set point temperature based on the condensate temperature signal), the controller 50 may control the condensate valve 42 to at least partially close, and when the condensate temperature signal indicates that the condensate temperature should be increased (i.e., the condensate temperature is determined to be below the condensate set point temperature based on the condensate temperature signal), the controller 50 may control the condensate valve 42 to at least partially open. In some examples, the controller 50 may be a PID controller, which is configured to operate in a feedback loop to control the condensate valve 42 to at least partially open when the condensate temperature signal indicates that the condensate temperature is below the set condensate temperature and not rising, and to at least partially close when the condensate temperature signal indicates that the condensate temperature is above the condensate set point temperature and not falling. This accounts for the lag time in making a change to the condensate valve 42 and observing the change. The PID controller may control the amount of partial opening or closing of the condensate valve 42 based on how far the condensate temperature is from the set condensate temperature and the rate of change of the condensate temperature. In other examples, the set condensate temperature may be a set condensate temperature range which may encompass a range of acceptable temperatures, such as 85-90 degrees centigrade, and the condensate valve 42 may be controlled to ensure that the condensate temperature between the heat exchanger 12 and the condensate valve 42 is within the set condensate temperature range. When the set condensate temperature is a set condensate temperature range, the condensate temperature signal may indicate a condensate temperature being below the set condensate temperature range when the condensate temperature signal indicates a condensate temperature below a lower end of the set condensate temperature range. The condensate temperature signal may indicate a condensate temperature be above the set condensate temperature range when the condensate temperature signal indicates a condensate temperature above an upper end of the set condensate temperature range. In this example, where the condensate temperature is monitored with the condensate temperature sensor 48, the controller 50 may be configured to control the condensate valve 42 so that it has a minimum opening which is not fully closed. If the condensate valve 42 is fully closed, then the true condensate temperature cannot be measured, since the measurement is downstream of the heat exchanger 12, and closing the condensate valve 42 would prevent condensate from flowing out of the heat exchanger 12. Figure 3 is a flow chart showing the basic steps of a method 300 of controlling a steam system such as the first example steam system 100 or the second example steam system 200. In block 302, the method 300 comprises the controller 50 receiving a pressure signal in the first example steam system 100 or a condensate temperature signal in the second example steam system 200. In some examples combining first and second example steam systems 100, 200, the controller 50 may receive both a pressure signal and a condensate temperature signal. In block 304, the method 300 comprises the controller 50 receiving a water temperature signal. Blocks 302 and 304 may occur concurrently, or one after another in any order. In block 306, the method 300 comprises the controller 50 controlling opening and closing of the steam valve 40 and the condensate valve 42 based on the received water temperature signal and at least one of the received pressure signal and condensate temperature signal. Figures 4a and 4b are flow charts showing a more detailed example method 400 of controlling the steam system 100, 200. Figures 4a and 4b are configured to run simultaneously and concurrently, although they can run sequentially. Figure 4a starts with block 302 from Figure 3, and Figure 4b starts with block 304 from Figure 3. From block 302, the method 400 in this example is configured to control the condensate valve 42 based on the pressure signal and / or the condensate temperature signal. From block 304, the method 400 in this example is configured to control the steam valve 40 based on the water temperature signal. From block 302, the method 400 proceeds to control the condensate valve 42 based on a comparison of the pressure signal to the set pressure, or a comparison of the condensate temperature signal to the set condensate temperature. In this example, from block 302, the method 400 proceeds to block 402, where it is determined whether the pressure needs to be raised or the condensate temperature needs to be lowered. In other words, where there is a set pressure, block 402 determines if the pressure signal indicates that the pressure is below the set pressure, and where there is a set condensate temperature, block 402 determines if the condensate temperature signal indicates that the condensate temperature is above the set condensate temperature. If it is determined that the pressure is below the set pressure, or the condensate temperature is above the set condensate temperature, then the method proceeds to block 404 in which the condensate valve 42 is controlled to close (for example, incrementally), and then back to block 302 in a feedback loop. If the controller 50 is a PID controller, determining if the pressure needs to be raised may involve determining whether pressure signal indicates that the pressure in the steam supply line is below the set point pressure and not rising (i.e., the derivative of the pressure signal over time indicates a negative trend), and determining if the condensate temperature needs to be lowered may involve determining whether the condensate temperature signal indicates that the condensate temperature is above the set condensate temperature and not falling (i.e., the derivative of the pressure signal overtime indicates a positive trend). If it is determined that the pressure does not need to be raised or the condensate temperature does not need to be lowered, the method 400 proceeds to block 406. In block 406, the method comprises determining if the pressure needs to be lowered or the condensate temperature needs to be raised. If it is determined that the pressure needs to be lowered or the condensate temperature needs to be raised, the method proceeds to block 408, in which the condensate valve 42 is controlled to open (for example, incrementally), and then back to block 302 in a feedback loop. If the controller 50 is a PID controller, determining if the pressure needs to be lowered may involve determining whether pressure signal indicates that the pressure in the steam supply line is above the set point pressure and not falling (i.e., the derivative of the pressure signal over time indicates a positive trend), and determining if the condensate temperature needs to be raised may involve determining whether the condensate temperature signal indicates that the condensate temperature is below the set condensate temperature and not rising (i.e., the derivative of the pressure signal over time indicates a negative trend). If it is determined in block 406 that the pressure does not need to be lowered or the condensate temperature does not need to be raised, then the method 400 proceeds to block 410 in which the condensate valve 42 is not controlled to change its opening, and then back to block 302 in a feedback loop. If the controller is a PID controller, the condensate valve 42 may not be controlled to open or close, even if the pressure is below or above the set pressure or the condensate temperature is below or above the set condensate temperature, as a previous control may already be raising or lowering the pressure or condensate temperature. Therefore, the PID controller which may monitor the derivative of the pressure signal and / or the condensate signal, can be used to account for the lag time between implementing a control of the condensate valve 42 and observing a change in the conditions in the steam system. The PID controller may control the amount of opening or closing of the condensate valve 42 based on how far the condensate temperature is from the set condensate temperature and the rate of change of the condensate temperature. Where the controller 50 receives a condensate temperature signal (for example in the second example steam system 200), there may be an additional block between block 402 and block 404 in which it is determined whether incrementally closing the condensate valve 42 further will fully close the condensate valve 42, or whether the condensate valve is at a minimum opening. If it is determined that the condensate valve 42 would be fully closed or is already at the minimum opening, the method may proceed directly to block 410 in order to avoid controlling the condensate valve to close fully or beyond the minimum opening. This ensures that the condensate temperature can continue to be accurately measured, as it will not block flow out of the heat exchanger 12. From block 304, the method 400 proceeds to control the steam valve 40 based on a comparison of the water temperature signal to the set water temperature. In this example, from block 304, the method 400 proceeds to block 412, where it is determined whether the water temperature needs to be raised. In other words, block 412 determines if the water temperature signal indicates that the water temperature is below the set water temperature. If it is determined that the water temperature is below the set water temperature, then the method proceeds to block 414 in which the steam valve 40 is controlled to open (for example, incrementally), and then back to block 304 in a feedback loop. If the controller 50 is a PID controller, determining if the water temperature needs to be raised may involve determining whether water temperature signal indicates that the water temperature is below the set point water temperature and not rising (i.e., the derivative of the water temperature signal over time indicates a negative trend). If it is determined that the water temperature does not need to be raised, the method 400 proceeds to block 416. In block 416, the method comprises determining if the water temperature needs to be lowered. If it is determined that the water temperature needs to be lowered, the method proceeds to block 418, in which the steam valve 40 is controlled to close (for example, incrementally), and then back to block 304 in a feedback loop. If the controller 50 is a PID controller, determining if the water temperature needs to be lowered may involve determining whether water temperature signal indicates that the water temperature in the water discharge line 36 is above the set point water temperature and not falling (i.e., the derivative of the water temperature signal over time indicates a positive trend). If it is determined in block 416 that the water temperature does not need to be lowered, then the method 400 proceeds to block 420 in which the steam valve 40 is not controlled to change its opening size, and then back to block 304 in a feedback loop. If the controller is a PI D controller, the steam valve 40 may not be controlled to open or close, even if the water temperature is below or above the set water temperature, as a previous control may already be raising or lowering the water temperature. Therefore, the PID controller which may monitor the derivative of the water temperature signal, can be used to account for the lag time between implementing a control of the steam valve 40 and observing a change in the conditions in the steam system. The PID controller may control the amount of opening or closing of the steam valve 40 based on how far the water temperature is from the set point water temperature and the rate of change of the water temperature. Figure 5 is a flow chart showing steps of a method 500 of designing a steam system, such as the first example steam system 100 or the second example steam system 200. In block 502, the method comprises receiving a steam supply pressure of an environment in which the steam system is to be installed. In block 504, the method comprises determining a critical pressure drop across a steam valve 40. In block 506, the method may comprise receiving a condensate return pressure of the environment in which the steam system is to be installed. In block 508, the method may comprise determining a condensate pressure drop across a condensate valve 42. In some examples, block 506-508 may be omitted. In block 510, the method comprises determining a maximum demand load for the heat exchanger. This may be based on the maximum mass flow rate of water through the system and / or the maximum temperature difference between the water inlet 34 at the heat exchanger 12 and the set water temperature. In block 512, the method comprises selecting a heat exchanger size based on at least the steam supply pressure, the steam critical pressure drop and the maximum demand load, and optionally further on the condensate return pressure and the condensate pressure drop. For example, the heat exchanger may be sized to ensure subcooling of the condensate at a maximum demand load, and with an inlet steam pressure is at a set pressure which is equal to or less than the steam supply pressure minus the critical pressure drop. The set pressure may also be based on the condensate return pressure and the condensate pressure drop. For example, the set pressure may be more than the condensate return pressure added to the condensate pressure drop. It will be understood that the invention is not limited to the embodiments above- 5 described and various modifications and improvements can be made without departing from the concepts described herein. Except where mutually exclusive, any of the features may be employed separately or in combination with any other features and the disclosure extends to and includes all combinations and sub-combinations of one or more features described herein. 10 13 06 25

Claims

1. A steam system configured to heat water, the steam system comprising:a heat exchanger configured to receive steam and water, wherein the heat5 exchanger is configured to heat the water with the steam;a steam supply line configured to be connected to a steam source to supply steam to the heat exchanger;a condensate line configured to receive condensate from condensed steam from the heat exchanger and to duct the condensate away from the heat exchanger;10 a water supply line configured to supply water to the heat exchanger;a water discharge line configured to receive heated water from the heat exchanger and duct the heated water away from the heat exchanger;a steam valve disposed on the steam supply line and configured to control the flow of steam through the steam supply line;15 a condensate valve disposed on the condensate line and configured to control the flow of condensate through the condensate line;a first sensor disposed downstream of the steam valve and upstream of the condensate valve and configured to output a pressure signal indicative of pressure in the steam system between the steam valve and the condensate valve and / or a second20 sensor at a condensate outlet configured to output a condensate temperature signal indicative of the temperature of the condensate in the condensate line between the heat exchanger and the condensate valve;a third sensor disposed on the water discharge line and configured to output a water temperature signal indicative of the temperature of the heated water through the25 water discharge line;a controller configured to control opening and closing of the steam valve and the condensate valve based on the water temperature signal and at least one of the pressure signal and the condensate temperature signal.30 2. A steam system according to claim 1, wherein the controller is configured tocontrol the steam valve based on the water temperature signal, and to control the condensate valve based on the pressure signal.

3. A steam system according to claim 2, wherein the controller is configured to35 control the condensate valve based on a comparison of the pressure signal to a set pressure.13 06 254. A steam system according to claim 3, wherein the controller is configured to operate in a feedback loop to control the condensate valve to at least partially close when the pressure signal indicates that the pressure in the steam supply line is below 5 the set pressure and not rising, and / or to at least partially open when the pressure signal indicates that the pressure in the steam supply line is above the set pressure and not falling.

5. A steam system according to claim 3 or 4, wherein the set pressure is higher than 10 a pressure in the condensate line downstream of the condensate valve added to a condensate pressure drop of the condensate valve to prevent stall of the steam system.

6. A steam system according to any of claims 3-5, wherein the set pressure is lower 15 than a supply pressure of steam upstream of the steam valve minus a critical pressure drop of the steam valve.

7. A steam system according to any of claims 3-6, wherein the heat exchanger is sized to ensure that the condensate is subcooled when the pressure of steam between 20 the steam valve and the heat exchanger is at the set pressure and when the flow rateof the water is at a maximum demand.

8. A steam system according to claim 1, wherein the controller is configured to control the steam valve based on the water temperature signal, and to control the 25 condensate valve based on the condensate temperature signal.

9. A steam system according to claim 8, wherein the controller is configured to control the condensate valve based on a comparison of the condensate temperature signal with a condensate set temperature.3010. A steam system according to claim 9, wherein the controller is configured to operate in a feedback loop to control the condensate valve to at least partially close when the condensate temperature signal is above the condensate set temperature and not falling and / or to control the condensate valve to at least partially open when the 35 condensate temperature signal is below the condensate set temperature and not rising.13 06 2511. A steam system according to according to claim 10, wherein the controller controls the condensate valve so that it has a minimum opening which is not fully closed.5 12. A steam system according to according to any of claims 2-11, wherein thecontroller is configured to control the steam valve based on a comparison of the water temperature signal to a set water temperature.

13. A steam system according to according to claim 12, wherein the controller is 10 configured to operate in a feedback loop to control the steam valve to at least partially open when the water temperature signal is below the water set point temperature and not rising and / or to control the steam valve to at least partially close when the water temperature signal is above the water set temperature and not falling.15 14. A method of controlling a steam system according to any preceding claim, themethod comprising:receiving the pressure signal or the condensate water temperature signal;receiving the water temperature signal; andcontrolling opening and closing of the steam valve and the condensate valve 20 based on the water temperature signal and at least one of the pressure signal and the condensate temperature signal.

15. A method according to claim 14, comprising controlling the steam valve based on the water temperature signal, and controlling the condensate valve based on the25 pressure signal.

16. A method according to claim 15, comprising controlling the condensate valve based on a comparison of the pressure signal to a set pressure.30 17. A method according to claim 16, comprising operating the control of the steamvalve and the condensate valve in a feedback loop by controlling the condensate valve to at least partially close when the pressure signal indicates that the pressure in the steam supply line is below the set point pressure and not rising, and / or to at least partially open when the pressure signal indicates that the pressure in the steam supply 35 line is above the set point pressure and not falling.13 06 2518. A method according to claim 16 or 17, wherein the set pressure is higher than a pressure in the condensate line downstream of the condensate valve added to a condensate pressure drop of the condensate valve to prevent stall of the steam system.

519. A method according to any of claims 16-18, wherein the set pressure is lower than a supply pressure of steam upstream of the steam valve minus a critical pressure drop of the steam valve.10 20. A method according to claim 14, comprising controlling the steam valve based onthe water temperature signal, and controlling the condensate valve based on the condensate temperature signal.

21. A method according to claim 20, comprising controlling the condensate valve 15 based on a comparison of the condensate temperature signal with a set condensate temperature.

22. A method according to claim 21, comprising operating in a feedback loop to control the condensate valve to at least partially close when the condensate20 temperature signal is above the set condensate temperature and not fallingand / or to control the condensate valve to at least partially open when the condensate temperature signal is below the set condensate temperature and not rising.

23. A method according to according to claim 22, comprising controlling the 25 condensate valve so that it has a minimum opening which is not fully closed.

24. A method according to according to any of claims 15-23, comprising controlling the steam valve based on a comparison of the water temperature signal to a water set temperature.3025. A method according to according to claim 24, comprising operate in a feedback loop to control the steam valve to at least partially open when the water temperature signal is below the water set point temperature and not rising and / or to control the steam valve to at least partially close when the water temperature signal is above the 35 water set point temperature and not falling.13 06 2526. A method of designing a steam system according to claim 1, the method comprising:receiving a steam supply pressure of an environment in which the steam system is to be installed;5 determining a steam critical pressure drop across a steam valve; determining a maximum demand load for a heat exchanger; and selecting a heat exchanger size based on the steam supply pressure, steam critical pressure and maximum demand load.10 27. A method according to claim 26, further comprising:receiving a condensate return pressure of the environment in which the steam system is to be installed;determining a condensate pressure drop across a condensate valve; and selecting the heat exchanger size based further on the condensate return15 pressure and the condensate critical pressure.

28. A method according to claim 26 or 27, comprising selecting a set pressure at an inlet for the heat exchanger based on the steam supply pressure and the steam critical pressure;20 wherein selecting the heat exchanger size comprises determining the heat exchanger size at which condensate is subcooled when an inlet pressure to the heat exchanger is the set pressure and when there is a maximum demand load.

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