How do they work?
The basic principle of a heat exchanger is to transfer heat when a temperature difference exists between two fluid flows. A simple formula used to evaluate heat exchange can be expressed in the following ways:
BTU = GPM X ΔT x 498
GPM = BTU / ΔT x 498
ΔT = BTU / GPM x 498
This formula helps to find different ways of satisfying the goal of ensuring enough heat is transferred to match the load. In this case, heat is measured in British thermal units (Btus). A Btu is the amount of energy required to raise the temperature of 0.45 kg (1 lb) of water by one degree, while the flow of liquid through the heat exchanger is measured in lpm/gpm. The constant ‘498’ represents the specific properties of water. If the liquid is changed to another medium, this constant also changes. Finally, ΔT is the temperature difference experienced by the water across the heat exchanger.
For example, if the load requires 100,000 Btus, the solution can be met in the following ways:
Water volume
One way to exchange heat is to increase the volume of liquid passing through the heat exchanger. For a specific Btu heat exchange, as the flow increases the temperature difference (ΔT) is reduced (e.g., to achieve a five-degree temperature difference an approximate flow rate of 151 lpm (40 gpm) would be required, while a 50-degree temperature difference would only require 15 lpm (4 gpm).
Temperature difference
Another way to exchange heat is to increase the temperature difference across the heat exchanger. For instance, by keeping the flow rate a constant the larger the temperature difference the more heat will be transferred (e.g., setting the flow rate at 38 lpm (10 gpm) the temperature difference required would be approximately 20 degrees).
Hotter temperature is not better
What about the effect of lowering EWT? Using the examples above, if the supply EWT came in at 82 C (180 F) and left at 71 C (160 F) it would achieve the same result as if the supply EWT came in at 48.8 C (120 F) and left at 37.7 C (100 F). Therefore, all things being equal, it seems as though EWT is not a factor; it appears to be all about the temperature difference.
However, EWT is a factor. In fact, hotter water is less efficient to produce as the heater’s chimney throws out hotter exhaust gases. When using energy efficient heating sources, it is all about the supply water temperature. The lower the temperature required to heat the load, the higher the rate of return on the source heating system. Therefore, designers should always look at their maximum or primary load temperature and see what can be done to bring that temperature lower. Lower heating system temperatures will reduce losses at the chimney, equipment and piping, and will provide a larger amount of heat that can be efficiently exchanged.
