Heat exchangers transfer thermal energy from one medium to another. For example, refrigerators use heat exchangers to transfer heat from your groceries to the surrounding air. There are probably heat exchangers in your office building which transfer heat from hot water to the ventilation system in the winter and from the indoor air to the cooling system in the summer. Heat exchangers come in two varieties, namely parallel and counter flow.
Heat exchangers can transfer energy between two liquids or between liquid and air. Shell and tube heat exchangers, the most common liquid-to-liquid heat exchangers, are used commonly in large buildings as a part of the HVAC system. A common application of this type of heat exchanger is to use steam to heat hot water. In this scenario, the hot water flows into the shell and the steam travels through the shell in a series of small tubes. Pumps move the hot water from the mechanical room throughout the building. A separate set of pumps move the steam through a loop between the boiler and the heat exchanger.
Parallel Heat Exchanger
In a parallel heat exchanger the hot water and the steam flow in the same direction. The hottest steam and the coldest water enter the exchanger at one end and the cooled steam and the heated water exit at the other end.
Counter flow Heat Exchanger
In a counter flow heat exchanger, the two media are flowing in opposite directions. The hottest steam enters on the left side and the hot water exits on the left side, while the cooled steam exits on the right and the cooler water enters on the right.
Pros and Cons
If the designs of two systems are identical, the counter flow type exchanger can transfer more energy than the parallel design because the exit temperature of the cooler water can get close to the highest temperature of the steam. Parallel flow heat exchangers undergo a lot more thermal stress because of the extreme temperature differences at the entrance to the device. Parallel heat exchangers are best suited when the goal is to bring the two fluids to the same temperature.