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How does a heat pipe air conditioning system work?
June 09, 2008 3:46AM
A heat pipe is a simple device that can transfer heat from one point to another without having to use an external power supply. It is a sealed tube that has been partially filled with a working fluid. In HVAC applications, this fluid is refrigerant. The sealed refrigerant - which will boil under low-grade heat - absorbs heat from the warm return air such as in an air-conditioning system and vaporizes inside the tube. The vapor then travels to the other end of the heat pipe (the high end), which is placed in the stream of cold air that is produced by the air conditioner. The heat that was absorbed from the warm air at the low end is now transferred from the refrigerant's vapor through the pipe's wall into the cool supply air. This loss of heat causes the vapor inside the tube to condense back into a fluid. The condensed refrigerant the travels by gravity to the low end of the heat pipe where it begins the cycle all over again. * Improved Comfort Level * Moisture Reduced * Improved Air Quality * Existing Systems Easily Retrofitted * No Moving Parts * No Additional Energy Required To Operate * Reduces A/C Energy By 15% * Reduces HVAC Load By 22% * Annual Savings Of Up To $1,000 * New Installations Pay Back In Less Than 2 Years * Documented Tests in ALA, FLA, GA & MICH Heat pipes can dramatically improve the moisture removal capabilities of many air-conditioning systems - yet actually lower power bills at the same time. Air can be precooled by simply transferring heat from the warm incoming air to the cool supply air. This "bypassing" can be accomplished by placing the low end of a heat pipe in the return air and the high end in the supply air. Heat is removed from the warm upstream air and rerouted to the cool downstream air. This heat, in effect, bypasses the evaporator -- although the air that contained the heat does indeed pass through the A-C coil. The total amount of cooling required is slightly reduced and some of the air conditioner's sensible capacity is therefore exchanged for additional latent capacity. Now the unit can cope with high-moisture air more efficiently. To accomplish a heat transfer around a cooling coil through utilization of heat pipe technology, different configurations may be used. One method is to arrange several heat pipes in parallel banks with the evaporator coil separating the pipes' evaporator ends and condenser ends. Fins (much like those found in air conditioner coils) may then be attached to the outside surface of the heat pipes to improve the heat transfer between the tubes and the air. After a heat pipe system has been installed, most vendors will use a vacuum pump, evacuate the heat pipes to less than 50 microns of air, then partially fill them with a working fluid, usually HCFC22. The pipes then will be hermetically sealed and affixed to your air-conditioning unit. It is important that no service valves be left on the pipes. At least one vendor, however, supplies prepackaged heat pipe systems with the pipes already partially filled and sealed. A highly effective air-to-air heat exchanger has now been created. The result is that the air conditioner's latent cooling capacity has now been increased. The air supplied to the building is drier than that provided by the air conditioner alone. The kinds of businesses that can benefit the most from heat-pipe technology include libraries, restaurants, storage facilities and supermarkets -- any type of business that needs moisture-controlled air to preserve goods and products kept inside, to prevent the increased wear and tear associated with high humidity, or to increase occupant comfort. Any air-conditioning system that uses reheat, desiccants, or mechanical dehumidification is a good candidate for heat-pipe assistance. When reheat is used, the energy savings that can be accomplished through heat-pipe dehumidification assistance can be substantial. While the percentage of energy savings may vary greatly from customer to customer due to a number of variables, one of the best examples reported so far involves a chain restaurant that was retrofitted with heat pipes. A restaurant was selected for the test because restaurants have traditionally been victimized by extremely humid interior conditions. High humidity causes interior fixtures and building materials to deteriorate at an accelerated rate due to water condensation. High humidity also results in increased energy and equipment repair/replacement costs. In addition to the geographic location, the elements that contribute to high humidity in restaurants include customer loads, cooking loads and code requirements concerning the rate of movement of outside air to the building's interior. Analysis of data from the test site indicated that outside air requirements can be a key causative factor of extremely high interior humidity. A team of engineers retrofitted the existing air conditioners at the test site. Additionally, recording equipment was used to track temperature and relative humidity both inside and outside the restaurant. The condensate flow was metered through use of a standard flow pump and water meter. Power consumption information was gathered by metering equipment that isolated and monitored the air-conditioning load. The project objective was to research ways to reduce the extremely high humidity generally found in restaurants and similar businesses. Analysis of the project data showed the heat pipes enhanced the moisture removal capacity of the air-conditioning units by 30 percent and lowered the relative inside humidity by 10 percent while at the same time significantly reducing the energy units consumed. Based on the project analysis, the addition of the heat pipes reduced A-C- energy consumption by an average of 17 percent. Cooling demand (tonnage) requirements also were reduced by 15-20%. Furthermore, no evidence was found to indicate water condensation on ventilation registers or lighting fixtures. As a result of the test, the restaurant chain expects to install heat pipes at its other facilities throughout the southeastern United States and will further test their effectiveness in northern climates. A good question to ask at this point is: What about the cost of heat pipes versus reheat and desiccant systems? The answers point to heat pipes in virtually every instance. A heat pipe installation is the most cost-effective system you can utilize for dehumidification purposes. How They Work
The Cooling coil of an air-conditioner removes moisture from the air in much the same way that a cold glass "sweats". The colder the cooling coil, the more moisture it removes. Old energy-inefficient air-conditioners had very cold cooling coils, which removed sufficient moisture from the air. Today's high efficiency machines have much warmer coils as the coil is generally larger, and less energy is used to cool it, but they save energy at the expense of not removing as much moisture. The problem, up until now, has been how to run a cooling coil cold enough to remove plenty of moisture, while not having to use extra energy to do so. The solution is found in HEAT PIE TECHNOLOGY, INC. dehumidifier heat pipes which "wrap around" the cooling coil. One section of the heat pipe is located in the return air and the other section in the supply air. The cool supply air chills one section while the warm return air heats the other. Heat is transferred from the warm return air to the cool supply air. The reheat as it is called, taken from the return air, is free. The effect of precooling the air going to the cooling coil brings it very close to the dew point and moisture begins to condense very early in the cooling coil. Because the coil does not have to perform the precooling function, more of its thickness is used to condense moisture and condensate flow is increased by a factor of 1 ½ to 2. The result is lower relative humidity. We also feel equally comfortable at a higher thermostat setting when the humidity is reduced. One degree higher setting represents approximately 8% savings in energy. Typically 20% to 30% energy savings are possible. Indoor air quality is improved, creating a situation of enhanced comfort, greater health, elimination of mold and mildew, and reduction of building deterioration. Energy savings through the use of heat pipes are achieved in the following ways: # By the elimination of reheat and the additional air conditioning load imposed by the reheat. # By setting the thermostat a few degrees higher to achieve the same comfort level due to lower relative humidity.
BENEFICIAL EFFECTS OF HEAT PIPE EXCHANGERS IN HVAC SYSTEMS
Increases moisture removal in HVAC system: Through heat-pipe heat-exchanger (HPHX) passive precooling, air entering the cooling coil is cooler and has a higher relative humidity. This allows the cooling coil to remove up to twice as much moisture. Control of mold growth in air ducts: Through HPHX passive reheat, air leaving the cooling coil and entering the ducts has a lower relative humidity. Mold can not grow in this drier environment. Allows for higher thermostat settings By maintaining the proper humidity level, thermostats can be adjusted to a higher setting while maintaining comfort. This results in energy savings of 10-30%. Allows for increase in chilled water temperature: By increasing moisture removal at the cooling coil, chilled water temperature can be raised several degrees while maintaining proper indoor conditions of temperature and humidity. This increases the energy efficiency of the HVAC system by reducing chiller run time and/or increasing the chiller evaporating temperature. Replaces heat system: Heat-pipe heat-exchanger are utilized as a direct replacement of electric, hot water, steam or hot-gas bypass reheat systems. Because HPHX reheat is passive, there is no operating cost to produce it. Because the reheat is removed in the HPHX precooling step, there is no additional load placed on the compressors as there would be with any other form of reheat. This results in capital cost savings and operating cost savings. LONG TERM BENEFITS FOR PEOPLE AND BUILDINGS
Comfortable healthy indoor environment: By controlling humidity in the air ducts and conditioned space, heat-pipe heat exchangers provide the following: Longevity of books, records, building materials, and furniture.
Better operation of copy machines, computer printers, and other office equipment.
Higher productivity of employees.
Reduction in absenteeism.
Fewer medical insurance claims.
Reduction of threat of litigation resulting from indoor air quality problems.
Economical addition of fresh air into buildings.
Assurance that the HVAC system can easily and economically adapt to higher latent loads. For example: increasing fresh air flow rate to meet the new ASHRAE standards for fresh air per person.