Control of dehumidifiers

Abstract

A dehumidifier having a vapour compression circuit 1 including an evaporator 5 and a condenser 3, and means (fan) 6 to produce a flow of incoming air over the evaporator and the condenser before being

Description

TECHNICAL FIELD OF THE INVENTION This invention relates to dehumidifiers. BACKGROUND The main function of a dehumidifier is to remove excess moisture from a building. A common form of dehumidifier has a vapour compression circuit which includes an evaporator and a condenser. Ambient air is drawn into the dehumidifier to pass over the evaporator by which the air is cooled causing moisture to condense out. The drier air then passes over the condenser by which the air is warmed again before being discharged from the dehumidifier. Most vapour compression dehumidifiers are controlled by a humidistat which senses the relative humidity (RH) of the air which is drawn in. The dehumidifier is configured to run when the RH exceeds a certain level which is adjustable by the user. Relative humidity is expressed as a percentage. It is the ratio of the absolute humidity at a given time to the highest possible humidity (100%), which depends on current air temperature. Relative humidity does not indicate how much water vapour is in the air, but what percentage of the maximum vapour pressure has been reached. The structure and fittings of a house or other building tend to absorb water vapour so that when a dehumidifier is operated in the building for the first time it needs to run continuously for several weeks until the excess moisture is removed. Once the building and its contents have dried out, the dehumidifier only needs to remove peaks of moisture which may be caused by cooking, running a shower, drying clothes etc. Apart from peaks of moisture, water vapour is also continuously added to the air within a building from various sources such as through occupants breathing out water vapour. The RH in a building varies with the seasons. During summer months the RH is often relatively high, but in winter the RH tends to be lower due to lower external humidity and heating within the building. For example, in winter the optimum RH could be below 40%, but when it is warmer outside the optimum could be above 50%. The RH will also be higher in unheated rooms or adjacent to a window where the air temperature will be lower. Thus, measuring the RH with a centrally-placed humidistat may give an RH reading of only 50% whereas colder areas of the same room could be as high as 80% or even 100% on windows and other cold surfaces giving rise to condensation. Thus, using a humidistat which aims for a particular R.H is not an effective and energy-efficient way of preventing moisture problems. Even if the R.H setting of the dehumidifier has been adjusted to achieve an acceptable water extraction rate under the prevailing conditions, should the conditions change, for example due to a lower outdoor temperature, the indoor heating may come on causing the R.H to fall. The humidistat will then switch off the dehumidifier. On the other hand, external walls of the building will tend to become colder causing the R.H to increase adjacent to these walls. Under these conditions dampness will often cause problems. Moreover, if the R.H setting is increased sufficiently for the unit to run under colder external conditions and increase the rate of water extraction, should the weather get warmer a humidistat-controlled dehumidifier may never switch off. It should also be noted that the hysteresis of a typical humidistat will frequently result in the dehumidifier failing to switch because the R.H change may be quite small even though it is removing sufficient water. Thus, a humidistat-controlled dehumidifier will frequently run for much longer than is necessary, and may stop running altogether, depending on the the weather conditions and internal conditions such as room temperature. A timer is not an efficient means of control because the extraction rate changes with conditions within a household. GB 2 400 457-A goes some way to addressing this problem. This describes a dehumidifier which monitors changes in RH over a period of time to establish a reference level. Operation of the dehumidifier is determined by whether the current RH is above or below the established reference level, which means that the dehumidifier operates when the base RH level changes. The present invention seeks to provide a better way of controlling a dehumidifier which provides more effective control of humidity levels and uses less energy. SUMMARY OF THE INVENTION The present invention lies in a control system for a dehumidifier of the kind having a vapour compression circuit. The control system is configured to allow manual setting of a target amount of water which is to be extracted in a predetermined period, typically 24 hours, for example using a rotary switch, a potentiometer, a keypad, a mobile phone application, or other human interface device. This setting is referred to herein as the 'target extraction rate'. The control system is also arranged to determine the current rate of water extraction during operation of the vapour compression circuit under the prevailing operating conditions, which is referred to herein as the 'run-time extraction rate'. This extraction rate will vary with ambient conditions but will normally be substantially higher than the desired target extraction rate. The control system uses the difference between the two extraction rates to adjust the amount of time which the vapour compression circuit runs within the predetermined period to extract the target amount of water. The ambient conditions can change within the predetermined period, and hence the run-time extraction rate will also change, but this will be relatively small over a shorter period. Preferably therefore, the predetermined period is divided into a number of shorter on-off periods, e.g. one hour, within which the vapour compression circuit is on for part of the period and off for the remainder. The run-time extraction rate is correspondingly updated at intervals within the predetermined period. Various ways of in which the run-time extraction rate can be ascertained are described in the specific embodiments below. BRIEF DESCRIPTION OF THE DRAWINGS The following description and the accompanying drawings referred to therein are included by way of non-limiting example in order to illustrate how the invention may be put into practice. In the drawings: Figure 1 is a schematic diagram of a first embodiment of a dehumidifier in accordance with the invention; Figure 2 is a schematic diagram of a second embodiment of the dehumidifier; Figure 3 is a schematic diagram of a third embodiment of the dehumidifier. DETAILED DESCRIPTION OF THE DRAWINGS Referring firstly to Fig. 1, the dehumidifier described in each of the embodiments incorporates a vapour compression circuit 1 in which a compressor 2 circulates refrigerant vapour under pressure through a condenser 3 wherein the refrigerant condenses and gives out heat. Condensed refrigerant then passes through an expansion device 4, such as a valve or small bore tube, providing a restriction which reduces the temperature and pressure of the refrigerant entering evaporator coils 5. Vaporisation of the refrigerant in the evaporator 5 absorbs heat from ambient air so that the resulting temperature drop causes water vapour in the air to condense out on the surfaces of the evaporator coils. The ambient air is drawn into the dehumidifier by a fan 6, and after passing over the evaporator coils the air passes over the condenser to receive heat from the refrigerant so that the dehumidifier discharges warm air back into the external environment. Refrigerant vapour from the evaporator coils is recirculated by the compressor 2 in a continuous cycle of condensation and evaporation. Water which condenses on the evaporator 5 normally drains into a collection vessel which is periodically emptied. The operation of the dehumidifier is overseen by an electronic control unit 11 which controls the compressor 2 and the fan 6 such that the vapour compression circuit may be in a running state or turned off (on or off). The control unit receives input from a first moisture sensor 12 arranged to monitor the moisture content of ambient air at the point where it enters the dehumidifier before contact with the evaporator 5. A human interface device 13, such as a rotary switch, allows the user to set a target extraction rate, namely the quantity or amount of water to be removed in a predetermined period, e.g. the number of litres per day. The amount of excess moisture to be removed is determined by a combination of the behaviour of the occupants and the home materials and design and therefore tends to be the same each day. Setting the amount of water to be extracted is somewhat arbitrary, but it can be changed by the user in the light of experience. The moisture sensor 12 preferably measures absolute humidity using the principle of a hygrometer. Absolute moisture levels may be expressed in grams of water per kilogram of dry air or grams of water per cubic metre of dry air (g / m3). There are several types of hygrometer which could be used. One type measures humidity based on resistance or capacitance using electronic sensors in the air stream. A device which measures capacitance does so by utilizing two metal plates with space between them and measures the amount of moisture in the air by measuring the static electric charge between the two plates. Another type of hygrometer uses an electrical current passing through a ceramic material (or another moisture-absorbent substrate) to test the moisture in the air. When the ceramic component comes into contact with moist air the electrical current changes due to the resistance provided by the water, and the measured resistance correlates with the amount of moisture in the air. The control unit 11 uses the absolute moisture content of the incoming ambient air to determine the run-time extraction rate of the vapour compression circuit using a lookup table stored in non-volatile memory. The table contains a stored list of values of absolute moisture content, e.g. grams of water per kilogram of dry air, covering the expected range measured by the moisture sensor 12. Each value is associated with a figure representing the expected run-time extraction rate of the vapour compression circuit for that particular water content. If a reading from the sensor 12 does not correspond exactly with values the stored in the table, the nearest stored value can be used. Having obtained a figure for the run-time extraction rate at the prevailing moisture content of the air, the control unit 11 is able to calculate the extraction time required to remove the desired amount of water set by the selector 13. By way of example, if the water content signalled by the moisture sensor 12 indicates that a run-time extraction rate of 100 ml per hour will be achieved (2,400 ml in 24 hours) and a target extraction rate of 1 litre per day (1,000 ml per 24 hours) is set by the selector 13, the vapour compression circuit needs to run for 42% of the time. It is desirable to remove a constant amount of water each hour to achieve the daily figure, e.g. 1 litre, and thus maintain a relatively constant humidity level throughout the 24 hour period. This is achieved by dividing the day into shorter periods, e.g. one hour, so that, in the example just given, the vapour compression circuit would be on for 25 minutes and off for 35 minutes in every hour. Moreover, a more accurate target extraction rate is achieved, taking account of changes in moisture content and temperature of the incoming air, by updating the run-time extraction rate several times within the predetermined 24 hour period. The target extraction rate setting device 13 aims to take out substantially the same amount of water each day. In a preferred embodiment the control unit 11 may be configured to update the output values stored in the table over an extended running period, e.g. several days, to take account of the prevailing ambient conditions. For example, if the operating conditions require a greater extraction rate to be achieved to reduce the ambient moisture level the stored values obtained from the table can be decremented. This will increase the running time and increase in the amount of water extracted. It is also possible to vary the output values across part of the range, e.g. to achieve greater extraction rates at higher humidity levels or lower ambient air temperatures. Compared with a dehumidifier which relies on pre-programmed calculations to take account of operating conditions the use of a lookup table is highly adaptable. The dehumidifier can readily respond to changes in the operating conditions by a process which is essentially a form of machine learning. In some embodiments the fan could be operated at two speeds -a relatively low idle speed which maintains a continuous base level of water extraction, and a higher boost speed which uses the lookup table to determine the running time when it is required to actively lower the humidity level within the building. Other ways of controlling the vapour compression circuit to extract the target amount of water are possible. In the dehumidifier of Fig. 2 the vapour compression circuit is as described above in relation to Fig. 1. The control unit 11 receives input from a first moisture sensor 12 arranged to monitor the moisture content of ambient air at the point where it enters the dehumidifier. A second moisture sensor 14 is arranged to monitor the moisture content of ambient air at the point where it is discharged from the dehumidifier after passing over the condenser 3. Both of the moisture sensors 12 and 14 may be as described above. A manual selector 13, such as a rotary switch, again allows the user to set a target extraction rate for the quantity or amount of water to be removed in a 24 hour period. If the air flow through the dehumidifier is known, e.g. in cubic metres per hour (m3 / hr), the control unit 11 can use the difference in absolute moisture levels between the two sensors 12 and 14 to calculate the run-time extraction rate which is being achieved. The control unit can therefore operate the vapour compression circuit to run for the required amount of time to achieve the target water extraction rate, as described. Another way of controlling the run time of the dehumidifier is to directly measure the water which is removed by the evaporator. In the dehumidifier of Fig. 3 the vapour compression circuit is as described above but in this embodiment the container 15 which is used to collect water from the evaporator 5 is provided with a force transducer 16, such as a load cell, which senses the weight of the container and its contents. The control unit 11 monitors the load cell to detect the amount of water which is collected over a preset operating period of the vapour compression circuit, which may comprise the total running time excluding any off periods. This again gives the run-time extraction rate which the control unit can use to operate the vapour compression circuit for the required amount of time to achieve the target water extraction rate. The proposed dehumidifier control system is thus able to provide better user control of the internal moisture levels within a building whilst at the same time significantly reducing energy consumption, avoiding operation of the vapour compression circuit when water extraction is not necessary. Whilst the above description places emphasis on the areas which are believed to be new and addresses specific problems which have been identified, it is intended that the features disclosed herein may be used in any combination which is capable of providing a new and useful advance in the art.

Claims

1. A dehumidifier of the kind having a vapour compression circuit which includes an evaporator (5) and a condenser (3), and means (6) to produce a flow of incoming air over the evaporator and the condenser before being discharged to the external environment,wherein the dehumidifier has a control system (11) which includes a human interface device (13) for manually setting a target extraction rate which is to be achieved over a predetermined period, and means (12; 12, 14; 16) for determining the run-time extraction rate under the prevailing conditions,wherein the control system is configured to use the difference between the two extraction rates to adjust the amount of time which the vapour compression circuit runs within the predetermined period to extract the target amount of water.

2. The dehumidifier of claim 1 wherein the predetermined period is divided into a plural number of shorter on-off periods within which the vapour compression circuit is on for part of the period and off for the remainder.

3. The dehumidifier of claim 1 or 2 wherein the run-time extraction rate is updated at intervals within the predetermined period.

4. The dehumidifier of claim 1, 2 or 3 wherein the means for determining the run-time extraction rate comprises a first moisture sensor (12) configured to sense the water content of the incoming air.

5. The dehumidifier of claim 4 wherein the control system (11) is configured to determine the run-time extraction rate using a lookup table which correlates water content readings from the first moisture sensor (12) with stored extraction rates.

6. The dehumidifier of claim 4 wherein the means for determining the run-time extraction rate comprises a second moisture sensor (14) which is arranged to sense the water content of the discharged air.

7. The dehumidifier of claim 6 wherein the control system (11) is configured to determine the run-time extraction rate from the difference in water content between the incoming air and the discharged air.

8. The dehumidifier of any of claims 4 to 7 wherein the or each moisture sensor (12, 14) measures absolute humidity using the principle of a hygrometer.

9. The dehumidifier of any of claims 1 to 3 wherein the means for determining the run-time extraction rate comprises a water collection vessel (15) which is associated with a transducer (16) arranged to sense the weight of water collectedwithin the vessel.

10. The dehumidifier of claim 9 wherein the run-time extraction rate is calculated by measuring the weight of water collected within the collection vessel (15) over a preset collection period.

11. The dehumidifier of claim 10 wherein the preset collection period comprises the total running time of the vapour compression circuit excluding any off periods.

12. A method of operating a dehumidifier of the kind having a vapour compression circuit which includes an evaporator (5) and a condenser (3), and means (6) to produce a flow of incoming air over the evaporator and the condenser before being discharged to the external environment,wherein the dehumidifier has a control system (11) which includes a human interface device (13) for manually setting a target extraction rate which is to be achieved over a predetermined period, and means (12; 12, 14; 16) for determining the run-time extraction rate under the prevailing conditions,wherein the control system is configured to use the difference between the two extraction rates to adjust the amount of time which the vapour compression circuit runs within the predetermined period to extract the target amount of water.

13. The dehumidifier of claim 12 wherein the predeterminedperiod is divided into a plural number of shorter on-off periods within which the vapour compression circuit is on for part of the period and off for the remainder.

14. The dehumidifier of claim 12 or 13 wherein the run-time extraction rate is updated at intervals within the predetermined period.

15. The dehumidifier of claim 12, 13 or 14 wherein the means for determining the run-time extraction rate comprises a first moisture sensor (12) configured to sense the water content of the incoming air.

16. The dehumidifier of claim 15 wherein the control system (11) is configured to determine the run-time extraction rate using a lookup table which correlates water content readings from the first moisture sensor (12) with stored extraction rates.

17. The dehumidifier of claim 15 wherein the means for determining the run-time extraction rate comprises a second moisture sensor (14) which is arranged to sense the water content of the discharged air.

18. The dehumidifier of claim 17 wherein the control system (11) is configured to determine the run-time extraction rate from the difference in water content between the incoming air and the discharged air.

19. The dehumidifier of any of claims 15 to 18 wherein the or each moisture sensor (12, 14) measures absolute humidity using the principle of a hygrometer.

20. The dehumidifier of any of claims 12 to 14 wherein the means for determining the run-time extraction rate comprises a water collection vessel (15) which is associated with a transducer (16) arranged to sense the weight of water collected within the vessel.

21. The dehumidifier of claim 20 wherein the run-time extraction rate is calculated by measuring the weight of water collected within the collection vessel (15) over a preset collection period.

22. The dehumidifier of claim wherein the preset collection period comprises the total running time of the vapour compression circuit excluding any off periods.

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