What are heat exchangers and what are they used for?
Heat exchanges are used for heat transfer wherever heat cannot be transferred directly from a source to an application. Typically for two different heat transfer media (antifreeze fluid – water, heating water – sanitary water, district heating – heating water etc.) or different pressures in the circuits (district heating up to 20 bar – heating system 3 bar max.).
Since 100% thermal efficiency cannot be reached in any thermal heat exchanger, there is always a certain temperature difference betweeh the incoming heating medium and outgoing heated medium. A heat exchanger design is very important in any system, and usually the overall system performance and efficiency depends on it.
E.g. a solar system or a heat pump is a typical example of such a relation. If an insufficiently sized heat exchanger is installed into a heating system powered by such an energy-efficient source, it does not manage transferring the energy and hot medium returns to the source, making it work at higher temperature than designed. This has a direct influence on its efficiency – in solar systems the heat gains are lower due to reduced collector efficiency, and a heat pump’s C.O.P. sinks.
Parameters and design of a system with a heat exchanger
The most important feature of a heat exchanger is without doubt its performance. However, it keeps changing with many factors, esp. with the temperature difference between the heating and heated side of a heat exchanger (called a temperature drop of a heat exchanger or a log mean temperature difference, LMTD), with the flow in both heating and heated side, and with the medium type.
For this reason, heat exchanger performance cannot be established without specifying its working conditions. It is valid in general that the higher the temperature difference between the heating and heated side, and the higher flow rates on both sides of a heat exchanger, the higher is the heat exchanger output.
Heat exchangers shall always be designed for specific applications and working conditions. For applications of heat exchangers with a solar system, heat pump or condensing boiler it is necessary to design the heat exchanger for a low temperature drop, i.e. a small difference between the heating and heated side.
A higher temperature drop means higher working temperature of a solar system or a heat pump, its worse efficiency or lower C.O.P. respectively. The higher temperature drop is designed for a heat exchanger, the smaller (and less costly) the heat exchanger can be. Then for systems or sources that do not require a very low source temperature, smaller and less expensive heat exchangers will be sufficient. Typical examples are e.g. systems with a standard gas boiler, electric boiler or solid-fuel boiler as their efficiency does not drop with their higher working temperature.
Heat exchanger types
There are two principal types of heat exchangers that differ in design, principal parameters and use – tube heat exchangers and plate heat exchangers.
Tube heat exchangers
Tube heat exchangers are simple devices consisting of one or more coiled tubes and their outer surface is the heat transfer surface. Typically, these are mostly heat exchangers integrated into storage water heaters or Thermal Stores. They are simple, feature low pressure drop (being just a tube) and cost very little.
Unfortunately, they also have a very low heat transfer coefficient which means they need a large heat transfer surface area in order to transfer even a relatively low output. For this reason, these heat exchangers are used in systems of tens of kilowatts max. and at the same time, there is no need for a small temperature difference between the heating and heated medium. Their use is then limited to relatively small solar systems (one collector needs about 0.5 m2 of a tube collector for optimum operation), to heat pumps of output up to circa 10 kW and to high-temperature sources.
Plate heat exchangers
Heat transfer coefficient of plate heat exchangers is as much as 10 times higher than that of tube heat exchangers and their output may be in hundreds of kilowatts. Plate heat exchangers cannot be used as integrated which brings their disadvantage – a circulation pump shall be used on both heating and heated side while tube heat exchangers will usually do with just one pump. A plate heat exchanger usually requires higher investment costs as well as operation costs because the power input of the circulation pumps is almost twice as much.
A great advantage of these heat exchangers lies in their variability of sizes and types, in the chance to design a heat exchanger even for an extremely small temperature drop (the best gain of a solar system) and for transfer of a high heating output (instantaneous DHW heating, district heating, large solar system etc.).