Heat recovery measures for air distribution and HVAC systems
By Thomas E. Mull, PE, Thomas Mull & Associates, St. Louis, MO
Heat recovery from air can result in substantial energy savings
There are six common energy recovery systems.
Systems can recover sensible heat, latent heat, or both, depending on design.
Many manufacturing and industrial buildings have high rates of exhaust. Frequently, processes require the removal of excessive heat or pollutants from the indoor air using a local or general exhaust system.
When air is exhausted from a conditioned space, it carries with it the energy required to condition that air. This energy may be in the form of sensible heat, latent heat, or both. Conditioning the makeup air needed as the result of exhausting is probably the most significant annual cost of a ventilation system. If makeup air is not brought in through a makeup air system, unconditioned air will be drawn into the plant through various openings.
Whenever possible, the air that might otherwise be exhausted should be returned to the conditioned space. Air that contains airborne pollutants, such as vapors or particulate matter, may be cleaned and returned to the conditioned space. Cooling coils or evaporative cooling may also remove heat from the air stream.
When it is not possible to clean the air and return it to the conditioned space, a heat recovery system should be considered. Whenever large volumes of air are exhausted, heat conservation and recovery should be considered, because it can result in substantial energy savings.
The transfer of energy from exhaust air to replacement air may be economically feasible depending upon the location of the exhaust and replacement air ducts, the dry bulb and wet bulb temperatures of the two air streams, and the nature of the contaminants being exhausted.
For air-to-air energy recovery to be economically feasible, there are a number of considerations. These include:
The cost of energy . High energy costs favor high levels of energy recovery.
The grade of waste energy. High-grade or high-temperature waste energy is generally more economical to recover than low-grade energy. Large temperature differences between the waste energy source and the makeup stream are most economical.
Coincidence and duration of waste energy supply and demand. Energy recovery is most economical when the supply is coincident with the demand and both are relatively constant.
Proximity of the supply and demand.
Operating environment. High operating temperatures or the presence of corrosives, condensables, and particulates in either air stream can result in high equipment and maintenance costs.
Effects on heating and cooling equipment. Heat recovery equipment may reduce the size requirements for primary heating and cooling equipment.
Air-to-air energy recovery equipment may be one of two basic types, sensible or total heat recovery. Sensible heat recovery systems recover only sensible heat from the exhaust air stream. Total heat recovery systems, also called enthalpy recovery, transfer both sensible heat and latent heat between the two air streams.
The type of heat recovery equipment used is based upon the type of energy to be recovered, the proximity of the air steams, the amount of cross-contamination that can be tolerated, and the cost of equipment and systems. The most common types of energy recovery systems and equipment include fixed-plate heat exchangers, rotary heat exchangers, run-around loops, heat pipes, twin tower recovery loops, and thermo-siphon heat exchangers.
Fixed-plate heat exchangers
Fixed-plate heat exchangers consist of alternate layers of plates, separated and sealed, referred to as a heat exchanger core. The exhaust and incoming air streams flow between alternating spaces between the plates. The plates, which are made of a material that readily conducts heat, transfer sensible heat between the two air streams (Fig. 1).
Fig. 1. This static device has little or no leakage between air streams.
Fixed-plate heat exchangers are typically made of aluminum because of its resistance to corrosion, ease of fabrication, heat transfer characteristics, and cost effectiveness. Heat exchangers made of steel alloys are available for temperatures exceeding 400 F.
Heat exchanger modules range in air flow capacities from 25 cfm to 10,000 cfm. One advantage of a fixed-plate heat exchanger is that it is a static device built so there is little or no leakage between the air streams. One disadvantage is it will only recover sensible heat. Fixed-plate heat exchangers can recover over 80% of the available waste exhaust sensible heat.
Rotary heat exchangers
Rotary air-to-air heat exchangers can recover latent as well as sensible heat, depending upon the heat transfer media used. A rotary air-to-air heat exchanger or heat wheel has a revolving cylinder filled with an air permeable heat exchange media with a large internal surface area.
Adjacent makeup and exhaust air streams each flow through one half of the heat exchanger. The flow of the two air streams is in opposite directions (Fig. 2).
Fig. 2. This design can recover both latent and sensible heat.
Energy is transferred from one air stream to the other as the media is exposed to the air. Sensible heat is transferred as the medium picks up and stores heat from the hot air stream and releases it to the cold air stream as the wheel rotates.
If the media is suitable for latent heat transfer, latent heat is transferred as the media condenses moisture from the air steam with the higher humidity ratio and a simultaneous release of heat. Moisture is then released through evaporation into the air stream with a lower humidity ratio. Air contaminants, dew point temperatures, exhaust air temperature, and makeup air properties determine the type of media that is most suitable.
Media for sensible heat transfer only are typically aluminum, copper, stainless steel, or Monel. Media for total energy recovery may include any number of materials treated with a desiccant such as lithium chloride or alumina. The rate of energy recovery is a function of the rotational speed of the wheel.
Run-around energy recovery loops may be used to transfer heat when two air steams are not adjacent to each other or when there can be no cross contamination between the air streams. A typical run-around energy recovery loop has extended surface, finned tube water coils in the makeup and exhaust air streams. The coils are connected by a closed loop hydronic system that includes counterflow piping, a pump, and a circulating heat-transfer liquid (Fig. 3).
Fig. 3. The air streams do not have to be in close proximity.
A principal advantage of the run-around energy recovery loop is that the two air streams need not be adjacent, or even in close proximity, to each other. The distance between the two air steams is limited only by the economics of the piping and pumping systems.
Run-around energy recovery systems have no possibility of cross contamination between the air streams. The main disadvantage of these systems is they can only transfer sensible heat between the air streams.
Heat pipe heat exchangers
Heat pipes are tubes, which have been fabricated with an integral capillary wick, evacuated, filled with a heat transfer fluid, and permanently sealed. Thermal energy applied to either end of the pipe causes the heat transfer fluid at that end to evaporate. The vapor rises, due to the difference in density between the vapor and the liquid, to the other end of the pipe, which causes it to condense into a liquid that flows by gravity back to the evaporator end, completing the cycle.
The heat pipe operates in a closed-loop evaporation/condensation cycle that is continuous as long as there is a temperature difference and the cold end is at a higher elevation than the warm end (Fig. 4).
Fig. 4. Heat pipes operate in a sealed, closed loop cycle.
Twin tower energy recovery
Twin tower energy recovery units are air-to-liquid, liquid-to-air enthalpy recovery systems. In this recovery system, a sorbent liquid is continuously circulated between the exhaust and makeup air streams, alternately contacting both air streams directly in towers. The circulated liquid, which is sorbent, transfers total heat, latent heat, and sensible heat. Another advantage of these systems is the two air steams do not have to be adjacent to each other, or even in close proximity.
One disadvantage is they are not suitable for high temperature applications. There is also the possibility of cross contamination if the air steams contain large amounts of gaseous contaminants.
Thermo-siphon heat exchangers
Thermo-siphon heat exchangers have no moving parts. They are similar to heat pipes because they employ a fluid which changes phase as part of the heat transfer process. Two-phase, thermo-siphon heat exchangers are sealed systems that consist of an evaporator, a condenser, interconnecting piping, and a heat-transfer fluid.
The heat-transfer fluid changes phase as it is heated or cooled. The change of phase, which results from a temperature difference, and the force of gravity, causes the heat-transfer fluid to circulate between the evaporator and the condenser (Fig.5).
Fig. 5. This heat exchange has unidirectional energy transfer.
Although thermo-siphon heat exchangers are similar to heat pipes, they are different in two ways. They have no wick, relying only on differences in the fluid density caused by temperature differences, and the tubes are dependent upon nucleate boiling to change phase from liquid to vapor.
A related article containing additional information on heating and cooling schemes that conserve energy (“Energy conservation measures for air distribution and HVAC system”) appears in the October issue of PLANT ENGINEERING magazine. Questions about energy conservation can be directed to author Thomas E. Mull at 636-938-6173. Article edited by Joseph L. Foszcz, Senior Editor, 630-288-8776, email@example.com .