The hidden secrets of heat pipes

April 10, 2000
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Heat pipes offer a cost-conscious way of designing air conditioning systems for dehumidification. Their ability to greatly reduce frost formation on coils in supermarket settings makes them a valuable, yet economic asset.

What's a heat pipe?

A heat pipe is a device that quickly transfers heat from one point to another without the use of external energy. The heat pipe’s basic design consists of a metal tube filled with a phase-change medium (PCM) which is sealed at both ends.

To transfer heat, place the tube in the path of two airstreams. One airstream should be cooler than the other. As warmer air moves across the evaporator side, heat is absorbed, causing the internal medium to boil. The vapor pressure created from the boiling medium then pushes the gas toward the cooler airstream on the condenser side of the tube where the heat is then emitted to the environment.

The PCM then condenses back into a liquid and falls back to the evaporator side by either a natural gravitational pull or a manufactured capillary system, which may or may not include an inside “wick.” The cycle then repeats itself.

Heat pipe construction

The outside of a heat pipe generally is constructed of copper and/or aluminum fin material. The tube normally is copper which has been mechanically expanded into the aluminum fins.

Internally, a wick is used to enhance the capillary action of the PCM to the evaporator side and helps the PCM adhere to the internal walls of the tube to increase surface area.

In recent years, enhanced internal tube surfaces have been introduced to replace the wick and have reduced the cost of manufacturing heat pipes, making them an economical choice for various hvacr applications.

A typical cooling system without a heat pipe will provide sensible and latent cooling. On average, 75% to 80% of the system’s cooling capacity is used to cool the air (sensible heat) and 20% to 25% to condense the moisture from the air (latent heat).

However, condensation will take place only if the coil temperature is lower than the dewpoint of the air passing through the coil. Since a conventional air conditioner uses most of its cooling capacity to cool the air to its dewpoint, it has little capacity left for dehumidification or latent cooling.

In order to remove the humidity from the air, the thermostat must be lowered, thereby causing the air to be overcooled and resulting in a need to reheat the air to raise the temperature before it is delivered to the conditioned space. This causes an air conditioner to operate longer and consume more energy.

Today, some residential units have been modified from the normal control scheme and humidistats have been added. With a humidity sensor, a two-speed fan motor, and a two-speed compressor in the control sequence, dehumidification is the result of essentially lower air velocity and higher evaporator temperature.

Here again, we are using the system to overcool the air and discharge saturated air to the conditioned space.

It stands to reason that this system could become extremely costly to operate in high-humidity situations. For example, many public facilities such as hospitals, hotels, supermarkets, libraries, and museums usually maintain their humidity levels around 50% (ASHRAE Standard 62-89).

The traditional system, called “active overcooling and reheating,” would overcool and reheat the air to satisfy these requirements, thereby increasing operating costs.

Do they fit with heat transfer coils?

Heat pipes should “wrap around” the cooling coil. The precool (evaporator) section of the heat pipe is located on the entering air side of the evaporator coil, and the reheat (condenser) section of the heat pipe is located on the leaving air side.

This type of setup is also known as a thermosiphon.

As the entering air moves across the first section of the heat pipe, it precools to approximately 8°F, reducing the sensible cooling load requirement for the evaporator coil.

The evaporator coil can now do more latent cooling as the air enters the coil with its dewpoint nearer the coil temperature, thus reducing the moisture content of the air leaving the evaporator coil. Once again, the overcooled and saturated air must be reheated to obtain more comfortable conditions.

As the air leaves the evaporator coil, it enters the second section of the heat pipe (condenser) and the air is reheated to approximately 8°, discharging drier air to the conditioned space. This method of passively precooling and reheating the air can save up to 50% of energy use over the method of active overcooling and reheating.

Heat pipes are the most beneficial when the system is installed in a hot, humid climate.

Cost reduction

Studies by utility companies in the southern U.S. states show that using a heat pipe system achieves energy savings of more than 40%.

The savings are attributed to the alleviation of the reheat methods used by traditional air conditioning systems, and user comfort conditions obtained at higher thermostat settings.

Since heat pipes simply move heat from one place to another without adding any extra heat to the building, it is possible to maintain optimum comfort levels at higher thermostat levels.

Internal air quality

Since heat pipes reduce the moisture in an indoor environment, problems with mold and mildew are reduced, improving indoor air quality (IAQ).

This, in turn, can alleviate many human irritants, such as burning eyes, itchy skin, and allergies stemming from “Sick Building Syndrome.”

Additionally, problem areas such as supermarkets, which must constantly fight the battle of frost formation on the evaporator coils, can benefit from the reduction of humidity in the store. Drier, conditioned space in the store saves the need for defrost in the display cases and development of “fog” on the glass door units.

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