Flat plate heat exchangers are popular due to their compact size and efficiency. Flat metal plates are used to transfer heat between two fluids. Because the fluids are spread over a large area of the plates, the speed of the temperature change is greatly increased. These units are common today and small brazed types are used in combination boilers in countless applications. The domestic hot water flow rate of these boilers, due to the greater heat transfer in such a small unit, has increased dramatically, creating a great impact on the residential heating and hot water production. While the small versions tend to be brazed while the larger commercial units use gaskets between the plates.
Both brazed plate heat exchangers, and gasket plate units, use liquids contained in pipes or other conduits to exchange thermal energy between that fluid and another. The exchanger generally consists of a coiled pipe that passes through a reservoir, or chamber, containing another fluid. The walls of the pipe are conductive in nature, while the walls of the chamber have thermal insulation or are made out of non-conductive plastic for heat retention.
Brazed plate heat exchangers vary internally, from one to another, according to the plate design and the sealing methods. There are two forms of corrugations used in these products. They are intermating and chevron. The greater thermal transfer for a given increase in pressure drop is from chevrons. These are employed a great deal more in the manufacture than intermating corrugations.
For properly determining the correct size unit for a particular application the following requirements must be considered for both the source and load sides. That would be the operating pressure, the percentage of glycol, if any, and the maximum pressure drop or head pressure. Also required is the load side GPM, or BTUs and the water temperature entering and exiting on both sides. The following equation is used to determine rate of heat transfer between the hot and cold fluids passing through the plates:
U is the overall heat transfer coefficient and A symbolizes the total plate area. ΔTm represents the difference in temperatures. U is determined by the difference in the coefficients in both the hot and cold flow.