Finned pack heat exchangers — coils — find wide application in air handling units (AHU) and ventilation systems. Engineers often approach the rating of finned pack heat exchangers from one side only, whether that is the fluid side or the air side. As a result, a number of myths and suppositions endure in the industry, leading to errors and misunderstandings. This article deals with water heating and cooling coils, and focuses on the properties of the water or brine (glycol).
A typical example is an air handling unit with a water heating coil. The requirement is a coil of 100 kW capacity, with the inlet and outlet water temperatures at 90 and 70° C respectively. A simple calculation based on the heat capacity of water gives the required water flowrate as 4.4 m3/h. The vendor sends the buyer a technical proposal for the coil, and it is no surprise that the figures given exactly match the specified heat duty and temperatures. The unit is bought and installed.
A few months later, for whatever reason, it is necessary to increase the heat duty by 20 percent, to 120 kW. What do we do? Simple reasoning says that to supply the necessary heat we must increase the water flow by 20 percent, i.e. to 5.3 m3/h. We check the performance of the water heater and circulation pump, and up-rate them if required. Job done, right? Wrong – this could be a critical mistake.
Effects of Increased Water Flow in Heat Exchanger
Even with the increased water flowrate, the heating capacity may be more than 15 percent short of the required 120 kW. Most probably we will have to compensate by increasing the water inlet temperature or decreasing the outlet temperature. A smart reader has probably already figured out why: when changing the specification, we took into consideration neither the temperature difference nor the surface area and design of the coil, all of which remained unchanged.
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