Enhancing Dehumidification with Heat Pipes

From Volume 7, No. 6 -- June 1998

This article was originally published in Environmental Building News, the leading newsletter on environmentally responsible design and construction; subscribe and read more!

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Air conditioners cool air in two ways: they reduce air temperature directly (removing sensible heat) and they remove moisture from air, reducing its latent heat. In some cases the relative balance between these two functions is acceptable, but there are also many applications for which additional dehumidification is needed. The traditional approach to these situations has been to overcool the air (thereby extracting more moisture), and then to reheat it to the desired temperature with gas or electricity. This approach wastes energy twice: once for the additional cooling and again for the reheating.

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Heat Pipe Technologies custom-makes these wrap-around coils to increase dehumidification in commercial and industrial-sized HVAC systems. Other products are available off-the-shelf for residential applications. Photo: Heat Pipe Technologies, Inc.

There are several ways to increase the dehumidification capability of an air conditioner (AC) that are not as wasteful. One of the best ways for many situations was adapted from NASA technology by inventor Khanh Dinh, president of Heat Pipe Technologies, Inc. (HPT). Dinh's heat pipes precool the air before it reaches the evaporator coils (or chilled water coils) of the AC unit, which increases the amount of moisture that those coils extract. The heat pipes also reheat the outgoing air, which leaves the system just slightly warmer than it would have been had the heat pipes not been there at all. This reheated air is also significantly drier than it otherwise would have been. The amazing thing is that both the precooling and reheating are totally passive--requiring no added energy or moving parts.

The heat pipes work because they contain a refrigerant that evaporates as the warm incoming air passes by the pipe, which removes heat from that air. The refrigerant, now a gas, then rises up along the pipe to the other side of the cooling coil. On this side it encounters the chilled air, condenses into a liquid, and runs back down to where it started. As the refrigerant condenses, it returns the heat that it had extracted from the air before the cooling coil (see schematic).

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Heat pipes increase moisture removal by precooling the air before it reaches the cooling coils, then returning the heat to the air on the other side of the coils. Photo: Heat Pipe Technologies, Inc.

While heat pipes are widely used in mechanical systems to reclaim energy from exhaust air in preconditioning incoming fresh air, HPT is one of very few companies using them to provide enhanced dehumidification. HPT holds several patents on various aspects of this process. For example, some of their heat pipes are configured in a loop rather than a single pipe, so that the gas rising up the pipe is not moving against the liquid running down.

In engineering lingo, the sensible heat ratio (SHR) is the amount of sensible cooling an AC unit provides as a fraction of the total cooling. Thus, an AC unit with a typical SHR of 0.75 removes 75% sensible heat and 25% latent heat (moisture). That ratio may be appropriate in some cases, but there are many situations, especially in the hot-and-humid Southeast, when more latent heat removal is desirable. In general, the need for enhanced moisture removal depends on factors such as the outdoor humidity, the amount of outdoor air coming into the building, the amount of humidity generated indoors, and the desired indoor humidity level.

For example, the recent proposed revisions to ASHRAE Standard 62 on Indoor Air Quality recommend levels of fresh air that can be very hard to provide in Florida without introducing excessive humidity. In less humid regions, buildings with indoor swimming pools might have high internal moisture gain to control, while many industrial and manufacturing applications require lower relative humidity (RH) levels than a standard AC can provide. In office buildings, the highly touted displacement air distribution systems used with access floors (see EBN Vol. 7, No. 1) can save energy because they deliver air at a higher temperature than conventional distribution, but in humid climates such a system may require enhanced dehumidification.

In homes, these enhanced dehumidification systems can improve indoor air quality by keeping the RH below 60%, with little or no energy penalty. In fact, if the occupant is otherwise overcooling the home to control humidity, the heat pipes will even save some energy. Sixty percent RH is generally regarded as a threshold level for the growth of mold and mildew in a house. The company also points out that by dehumidifying the air before it goes through the ducts, HPT's products reduce the risk of mold and mildew growth in ductwork.

Dehumidification heat pipes from HPT come in three basic models: the Z-Coil, which replaces the standard evaporator coil in a typical AC's air handler; the flat panel Dehumidification Heat Pipe (DHP), which can be retrofitted into the AC's supply and return ducts when the ducts are side-by-side; and the Wrap-Around Coil, which is custom made to fit into a commercial-scale AC unit, putting heat pipes on either side of the main coil. There are a number of variations on the Wrap-Around model to accommodate specific needs and situations.

The residential-scale Z-Coil and DHP systems can be integrated into a typical three- or four-ton (10 to 14 kW) AC for an upcharge of about $500, according to Chuck Yount, the residential systems manager at HPT. Retrofitting either system onto an existing AC costs about twice as much. Pricing for the commercial Wrap-Around Heat Pipes depends on the specifics of the installation.

When enhanced dehumidification is needed, heat pipes are clearly an option worth considering. In an example worked out by researchers at the Florida Solar Energy Center, the Heat Pipe system cost 14% less to install than a system with electric reheat, while reducing energy use by 56% and saving a whopping 78% on the peak load. The first-cost savings come primarily from the fact that with reheat the AC must be significantly oversized to supercool the air. The energy savings occur because both the overcooling and the reheating loads are avoided.

For all its elegance, the enhanced dehumidification provided by heat pipes is not free, but comes at the expense of a reduction in sensible cooling. This is because the conditioned air leaves the system slightly warmer than it would have been in the absence of the heat pipes. To cool air to the same temperature while also removing more latent heat would require some additional energy of the system. HPT bases its claim of net energy savings on the argument that with lower humidity, building occupants will be comfortable at a higher temperature, so not as much sensible cooling will be needed. There is little doubt that raising the thermostat setpoint will save energy--the company cites Florida Power Corporation data showing that raising the setpoint from 78deg.F (25.6deg.C) to 80deg.F (26.7deg.C) saves 15% in cooling costs. But there is substantial disagreement on the more subjective question of what temperatures occupants will find comfortable at various humidity levels. While the company provides data suggesting that a 10% difference in relative humidity feels like a temperature difference of 2 to 4 degrees Fahrenheit, ASHRAE standard guidelines suggest that the difference is less than 1deg.F, according to Don Shirey of the Florida Solar Energy Center.

There is also the matter of increased fan power needed to blow the air past the heat pipes, in addition to the standard cooling coils. With residential systems, the increased air flow resistance can be up to 0.3 inches of water column (75 Pa), according to Yount. This additional static pressure increases the fan load, and care must be taken not to cause problems with the airflow through the system, according to engineer Marc Rosenbaum of Meriden, New Hampshire. Rosenbaum also notes that when high humidity in the incoming fresh air is a problem, it makes sense to keep it from entering the building in the first place. He suggests an enthalpic wheel, which transfers moisture from incoming air to outgoing air, as one such option.

One environmental concern with HPT's products is their use of the R-22 refrigerant, an ozone-depleting hydrochlorofluorocarbon (HCFC). For the Z-Coil, which replaces the coil in a standard AC, HPT has no choice but to go with the refrigerant in the system, and "99.5% of the market is still using R-22," according to Yount. Yount also points out that their products have no moving parts, so there are no valves, gaskets, or seals that might leak refrigerant, and that they contain relatively small amounts--a typical residential system contains less than two pounds (0.9 kg) of fluid. For the DHP and Wrap-Around coils HPT could use any refrigerant they choose. While they are preparing for the eventual switchover to chlorine-free refrigerants, they don't seem to be interested in leading the pack.

Contrary to what some people selling these systems would have us believe, there is no such thing as a free lunch--unless someone has been throwing your lunch away! Compared with inadequate dehumidification, HPT's system will likely increase energy use, though in some cases surprisingly little. If excess latent heat is only an occasional problem, it may be hard to justify the cost of heat pipes. If conventional reheat systems would otherwise be used regularly, however, heat pipes can save a bundle. They represent just one example among several options for low-energy humidity control. Although best known for specialized applications, these enhanced dehumidification systems have been around for a while, and engineers interested in energy efficiency should know about them.

For more information:
Jerry Thomas
Heat Pipe Technologies, Inc.
P.O. Box 999
Alachua, FL 32616
904/462-3464, 904/462-2041 (fax)
http://www.heatpipe.com/

Don B. Shirey III
Principal Research Engineer
Florida Solar Energy Center
1679 Clearlake Road
Cocoa, FL 32922-5703
407/638-1451, 407/638-1010 (fax)
shirey@fsec.ucf.edu (e-mail)