Heat Recovery Systems

Applications and Capabilities

Heat reclaim coils (also referred to as hot gas reheat coils) take advantage of the hot gas in the compressor discharge line as a source for heat. They can be piped either in parallel or in series with the normal condenser. When piped in parallel, the reclaim coil is typically sized to handle the entire condensing load, since the primary condenser coil will not be in operation in reclaim mode. When piped in series (e.g., supply air-reheating), the primary condenser and the reclaim coil will both be in operation in reclaim mode. Therefore, reclaim coils piped in series typically have capacities of 50% or less of the total heat rejection of the system. It is preferable to have saturated gas leaving the heat reclaim coil (de-superheated gas) in lieu of condensed liquid.

“Run-Around” Heat Recovery System

A “Run-Around” system is heat recovery method in which two or more multi-row coils are connected to each other by a circulating pump and pipe network. The fluid circulated through the system can be water or, if freezing is a potential problem, a water/glycol mixture. When only two coils are involved, the “cooling” coil is installed in the exhaust air stream and the “heating” coil is installed in the supply air stream. The fluid flows through the “cooling” coil, picking up heat (both sensible and latent, depending on the entering air conditions) from the exhaust air and is then pumped to the “heating” coil where it gives up that heat to the supply air. The fluid then returns to the “heating” coil in this closed loop piping system.

This type of heat recovery system is limited to situations where the exhaust and supply air streams are separate. Efficiency is typically limited to a maximum of 40% – 50% and will vary seasonally with the difference between the exhaust and supply air temperatures. Four, 6 or 8 row deep coils are typically most efficient (remember that fans must overcome the additional air pressure drop of thicker coils or coils with increased fin spacing).

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