More Efficient Heat Recovery

By James O. Edwards, P.E., Senior Engineer, Project Manager, URS Corporation, Denver September 1, 2006

With today’s ever-increasing energy costs, it is as important as ever to design HVAC systems that minimize energy use. Given the high ventilation requirements of many applications, engineers must design cost-effective heat-recovery systems.

Air-to-air heat exchanger systems require the air distribution ductwork for the two airstreams to be in close proximity. However, many times it is impractical to configure ductwork to use this approach.

Run-around hydronic loop systems allow for heat recovery from a system where the airstreams are not in close proximity. Unfortunately, the amount of heat recovered depends on the relative temperature differentials between the two airstreams, as well as on the associated water temperature differential that can be generated within the run-around loop. These limitations many times result in an unacceptable payback for these systems.

A different approach

A cost-effective alternative to these systems uses heat pipe technology. The heat pipe system is similar to the run-around hydronic loop, except that it uses refrigerant for the heat transfer media. A refrigerant coil is placed within each airstream, and the two coils are connected by refrigerant piping. The system utilizes the latent heat of evaporation and condensation to transport the energy. The refrigerant evaporates within the coil located in the hot airstream. The expansion of the refrigerant within the coil causes the refrigerant to flow out of the coil and back to the coil located in the cold airstream. The refrigerant condenses within the coil in the cold airstream, thereby giving up the heat of condensation to the cold airstream. This approach allows for two distinct advantages:

First, significantly more heat per lb. of transfer media is produced, resulting in smaller system components, less investment cost and improved economic performance. A comparison of the two systems indicates that:

1) R-22 heat pipe system transfers 84.5 BTU/lb., a water system transfers 1 BTU/lb. and a 50% glycol system transfers 0.93 BTU/lb. of media.

2) The same amount of heat is transferred with:

• 26.1 gal. per min. of R-22 using the heat pipe system.

• 275 gal. per min. of water using a traditional run-around loop system.

• 300 gal. per min. of 50% glycol using a traditional run-around loop system.

The second advantage is that the system does not require a pump to transfer the media if the exhaust and outside airstreams are relatively close.

Heat pipe technology can also be used to provide dehumidification within HVAC air-handling systems in a manner that is significantly more efficient than conventional methods.

Conventional systems dehumidify supply air by overcooling it to the point where the moisture condenses out of the airstream. The air is then reheated back up to the required supply air temperature. The heat pipe system, however, uses the heat pipe heat-recovery system to absorb heat from the supply airstream, thereby pre-cooling the air and condensing the moisture from it. The heat that is absorbed from the supply air is then used to reheat the air after the moisture is removed.

Heat pipe advantages

• Produces more heat per lb. of transfer media

• Doesn’t require a pump to transfer the media if airstreams are close to each other

• Can provide more efficient dehumidification within an HVAC system