Concentric Hot Water Pipe Circulation to Prevent Legionella Growth
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Solution Overview
Problem
Large hot water systems in commercial settings face challenges in maintaining sanitary conditions, particularly in preventing the growth of harmful bacterial colonies like Legionella, due to temperature fluctuations and dead legs in the piping, which current systems fail to fully address.
Innovation Solution
A circulating hot water system design featuring concentric outflow and return conduits with a common wall, a pump in the return conduit, and a control system that adjusts heating and flow based on demand, eliminating static water volumes and ensuring continuous hot water supply at adequate temperatures to inhibit bacterial growth.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Temperature
If water is circulated continuously through the system to maintain hot water availability, then hot water is always present at outlets, but water cools in the intervening pipes during periods of non-use and must be run off in large volumes before hot water emerges
Solution Approach 1:
The system pre-heats water in the return pipes using heat exchangers before it reaches the outlets. This preliminary heating action prevents the water from cooling down completely during circulation, ensuring that hot water is always available at outlets without wasting large volumes of cooled water.
Solution Approach 2:
The circulation system maintains continuous movement of water through the pipes, preventing stagnation and ensuring that heated water continuously reaches all outlets. The heat exchangers continuously reheat the returning water, maintaining the temperature gradient and ensuring uninterrupted hot water supply.
2Reliability
If the water temperature is raised above 45°C to kill Legionella bacteria, then bacterial colonies are eliminated, but hot water can be dangerous to users
Solution Approach 1:
The system segments the water circulation into separate circuits: a hot water circulation circuit for killing bacteria (maintained at high temperatures) and a cold water supply circuit for user safety. Heat exchangers transfer heat from the hot circuit to the cold circuit, allowing the hot water to reach sanitizing temperatures while the user-facing outlets receive temperature-controlled mixed water.
Solution Approach 2:
Heat exchangers act as intermediaries between the hot water circulation system and the cold water supply system. They transfer thermal energy from the high-temperature water to the lower-temperature supply water without direct mixing, enabling bacterial elimination in the hot circuit while maintaining safe temperatures at user outlets through controlled mixing with cold water.
3Quantity of substance
If a substantial reservoir of heated water is provided to ensure adequate hot water at multiple outlets, then hot water availability is improved, but the system complexity and energy consumption increase
Solution Approach 1:
The system extracts the storage function from the circulation design by using the circulation itself to maintain hot water availability. Instead of requiring a large reservoir, the continuous circulation with heat exchanger reheating creates an on-demand hot water supply, extracting only the necessary water volume for each outlet request from the circulating system.
Solution Approach 2:
The circulation system serves itself by continuously moving water through heat exchangers that automatically reheat the returning water. This self-sustaining circulation eliminates the need for external storage reservoirs, as the system maintains its own hot water supply through continuous circulation and periodic reheating at strategic points in the circuit.
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
This design effectively prevents bacterial colony formation, reduces energy consumption, and ensures a reliable hot water supply at all user points, enhancing safety and hygiene in commercial water systems.
Implementation Method 1
The outflow conduit (11) and the return conduit (12) are surrounded by a common wall which forms a heat exchanger
Implementation Method 2
The outflow conduit (11) and the return conduit (12) are surrounded by a common wall which forms a heat exchanger
Implementation Method 3
a pump in the return conduit to drive circulation of the water
Data Source
Figure 1
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AI summary
A circulating hot water system has a hot water flow circuit defined by pipework leading out from and back to an in-line heater (20), and including a pump (8) to drive circulation of the hot water. Each of multiple user points (4) has an outflow branch conduit (141) and a return flow branch conduit (142) with a common wall for heat exchange, as does the main flow circuit: the outflow conduit surrounds the return conduit. Water is fed into the system from a pressurised cold water supply main (100) through a check valve (110). Sensors are used to monitor water temperatures and flow conditions around the system. A programmed control processor (7) can control heating and pumping rates in various regimes, e.g. to maintain system temperature above a predetermined threshold. An isolation valve adapted for concentric double pipes is also described.