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For installations requiring a high percentage of outside air for ventilation, the energy consumption to cool and to heat outside air, especially in cold winter and hot summer climates, is usually very high. These requirements often foster high humidity problems, especially at part-load conditions in hot and humid areas. To reduce ventilation-based energy consumption, heat recovery systems such as heat wheels, air-to-air heat exchangers, heat pipes, or runaround loop systems can be used to recover heat from exhaust airstreams.
To reduce humidity problems, a reheat coil after a cooling coil is normally provided to dehumidify the space by adding sensible heat to the room cooling load. However, energy usage associated with the reheat and corresponding additional cooling load is very high. A runaround loop heat recovery system consisting of two coils, a glycol circulating pump and interconnection piping, and a reheat coil is shown in Figure 1. This loop, together with the cooling coil, becomes an enhanced dehumidifier in the summer by adding a fluid-type reheat coil (third coil) after the cooling coil and circulating glycol solution between the first and third coils (Figure 2). The runaround loop heat recovery/dehumidification system decreases the mechanical cooling effect required and provides reheat at the same time, thus saving both cooling and reheat energy and decreasing the size of the mechanical cooling equipment.
A 1,000 ft^sup 2^ (93 m^sup 2^) smoking room located in the interior space of a building will be used as a model to explain the process and to compare the cooling and reheat loads of each system. The design parameters are in Table 1.
The smoking room will be ventilated with 100% outside air. Supply air temperature to the room is calculated as follows:
(20,030 Btuh) / 1.08 / (3,000 cfm) = 6.2 deg F (delta)T
(5870 W) / 1.2 / ( 1420 L/s) = 3.4C (delta)T
Room supply air temperature = 75 deg F - 6.2 deg F = 68.8 deg F
= 23.9 deg C-3.4 deg C=20.5 deg C
Dehumidifying with Reheat System
(No Heat Recovery Coil)
The reheat process is shown in a psychrometric chart, Figure 3. The outside air condition is located at A, and room condition is located at B. Room supply air condition, C, is located between the...