Refrigeration Cycle Heat Recovery

Any type of refrigeration cycle system that transfers heat for beneficial purposes may be considered a heat pump. Refrigeration cycle systems that provide cooling but also recover heat rejected from the refrigerant to provide useful heating may be considered a type of heat recovery heat pump. These are differentiated from reverse cycle systems in that when they operate, they always provide cooling with conventional cycle operation. An additional feature is that they recover refrigeration cycle-generated heat, normally rejected to the outside environment, for productive use.

With refrigeration cycle heat recovery systems, there is a net gain in thermodynamic efficiency when the relatively low temperature — typically, but not always, lower than 130°F (54°C) — recovered heat can be used. Since there is 12,000 Btu/ton-h (1 kWhh/kWhr) of heat rejection, plus a portion of the cycle driving energy input to the system that also must be rejected, heat recovery can result in extremely efficient refrigeration cycle system energy utilization.

Heat recovery can be applied to almost any type of refrigeration cycle system, limited chiefly by the economic viability of using a relatively low-temperature heat source. The underlying theory is that the energy rejected from refrigeration cycles contains available thermal energy that can be recovered for productive use. Applications that heat or preheat relatively cold water (as in domestic water heating), for example, are more likely to be cost-effective. Principle obstacles are capital and operating costs associated with recovery of any relatively low-temperature energy stream, potentially negative impacts on cooling system performance resulting from installation and operation of heat recovery components, design complications associated with non-concurrence of cooling and heating loads, and potentially unstable chiller operation under low-load conditions.

Refrigeration cycle heat recovery systems typically use desuperheaters and condensers or auxiliary condensers. These are discussed below.

Desuperheaters, which are commonly used in smaller applications featuring air-cooled reciprocating and scroll compressors, function somewhat as auxiliary condensers. As shown in Figure 36-27, the desuperheater is usually connected to the system between the compressor discharge and the condenser. Heat is transferred from the hot refrigerant to the water being heated. The desuper-heated refrigerant then flows through the condenser. The temperature of the water leaving the desuperheater is dependent on the cooling (refrigeration) loads. Typically, water temperature gain ranges from 5 to 40°F (3 to 22°C).

Two important application considerations are sizing and avoiding contamination of the heating loop by the refrigerant charge. If the desuperheater is undersized, it restricts the refrigerant charge line, causing the compressor to operate inefficiently against excessive refrigerant head pressure. An oversized unit may result in insufficient discharge refrigerant gas velocities, which in turn can result in oil trapping and insufficient oil to the compressor.

Condenser heat recovery systems are typically used for larger systems featuring centrifugal or screw compressors. These systems may feature either a single condenser or a dual-condenser configuration with a dedicated heat recovery condenser. In applications in which the heating load exceeds the cooling load at all times, the chiller essentially functions as a heat pump with all of the rejected heat from the condenser serving the heating load. In applications in which the cooling load exceeds the heat recovery requirement, typical heat rejection apparatus (i.e., a cooling tower or other heat sink) is required.

In the simplest arrangement, the condenser is sized for a relatively large peak temperature differential of 30°F (17°C) or more. The hot water exiting the condenser is passed through a filter to the heat recovery (heat exchang-

Water

Water

Water Heater

Water Chiller

Water Heater

Water Chiller

Fig. 36-27 Small-Capacity Heat Recovery System Featuring Desuperheater. Source: The Trane Company er) unit and then on to the cooling tower (heat sink). To limit heat exchanger fouling and the risk of contamination by cooling tower water (or other source, such as river water), a second separate condenser is commonly used, which establishes a separate heat recovery circuit. The separate condenser is sized in accordance with the portion of the available heat energy targeted for recovery.

Dual-condenser systems, shown in Figure 36-28, operate on the principle that refrigerant will migrate to the coldest point in the system. Raising the temperature of one condenser thus forces heat rejection to the other. By modulating the flow to the heat recovery condenser or the temperature of the tower water loop, the temperature of the standard condenser and the heat rejection to the heat recovery condenser can be controlled. With centrifugal

Source: The Trane Company

chillers, care must be taken to avoid unstable operation under low load conditions, given the high head pressure imposed on the system.

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