How To calculate the efficiency of a plate heat exchanger ?
Plate heat exchangers are efficient heat recovery units in commercial, industrial and residential settings. Plate heat exchangers can reduce a building’s energy costs and environmental impact by extracting sensible energy from exhaust air and using it to cool or heat incoming air.
Simple engineering principles are used to build them. A heat exchange core consists of layers of aluminium or polymer plates with gaps between them that allow air to flow freely. The exhaust air is funnelled between some of the layers. In the meantime, incoming air is funnelled in the other direction between the other layers.
Depending on the climate, the exhaust air heats or cools the plates. This sensible energy is then transferred to the incoming air. Modern plate heat exchangers can recover the vast majority of sensible energy, so their impact is significant.
What Is Heat Exchanger Efficiency?
Comparison of real-world performance with ideal performance is efficiency; it is the ratio between the heat transferred in an actual heat exchanger and the heat transferred in an ideal heat exchanger. Optimal performance is determined via modelling and includes the limitations imposed by factors such as the second law of thermodynamics, which states that more energy is wasted each time it is transferred or transformed.
Defining the levels involved in optimal plate heat exchanger efficiency, which transfers the maximum amount of heat and generates the least entropy, provides a benchmark against which existing plate heat exchangers can be measured.
Ideal Heat Exchanger System
In most cases, the mean temperature difference is given. UA is the product of surface area and heat transfer coefficient and determines the heat transfer rate. A new heat exchanger design or a larger heat exchanger can always be improved on the UA. Therefore, we do not have a practical definition of an ideal heat exchanger.
Overall heat transfer equation
For any heat exchanger system, the overall heat transfer rate (Q) is defined as –
Q = U×A×ΔT
where, U is the overall heat transfer coefficient
A is the overall heat transfer surface area
and ΔT is the mean temperature difference between hot and cold side
There are two main models for calculating the performance efficiency of a plate heat exchanger. Calculating the rate of heat transfer using the log-mean temperature difference method (LMTD) is as follows:
Q = UA(FΔTlm)
This equation defines U as the overall heat transfer coefficient, A as the total area of heat transfer, *Tlm as the log-mean temperature difference, and F as the log-mean temperature difference correction factor. LMTD is most commonly used when the inlet and exit temperatures are known, but the size of the heat exchanger is not. In contrast to the LMTD method, the thermal effectiveness method compares the real-world heat transfer inside the heat exchanger with the maximum possible heat transfer. The ratio is then calculated.
E = Q / Qmax
When building managers or engineers are attempting to determine the heat transfer rate and fluid exit temperatures, and they already know the size of the heat exchanger and the inlet temperatures, this method is most commonly used.
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