Heat pipes are the most effective passive method of transferring heat available today. In their simplest form, a sealed tube (usually copper), is evacuated and charged with a phase change liquid. In the case of heat pipes for HVAC purposes, a range of refrigerants such as R22 or R134A is currently used. Heat transfer occurs with the need for energy input.
A. BASIC OPERATION
Figure 1 shows the basic structure of a heat pipe and identifies the major steps in the heat pipe process. Heat is absorbed from the incoming warm air stream in the evaporator section, boiling the refrigerant. The vapour moves rapidly to the cooler condenser section of the heat pipe, carrying with it the absorbed heat.
As the vapour reaches the condensing area of the heat pipe, heat is released to the cooler air and the vapour condenses. The liquid returns by gravity or capillary action by means of a “wick” to repeat the process almost immediately.
The entire heat transfer process occurs with a very small temperature difference along the pipe.
A traditional heat pipe is a hollow cylinder charged with a suitable liquid.
A. Heat is absorbed in the evaporator section.
B. Liquid boils to vapour phase.
C. Heat is released from the upper part of cylinder to the environment; vapour
condensed to liquid phase.
D. Liquid returns by gravity to the lower part of cylinder (evaporator section).
Heat pipes have been used in many applications, including the cooling of casting dies, electronic circuity, nuclear powered generators, energy conservation, defrosting applications and in the food industry. Click here to download
B. RECENT DEVELOPMENTS
Modern heat pipes are able to transfer heat several thousand times faster than a solid copper rod, but applications for heat pipes have been limited in the past, largely due to the expense of their construction. The requirement for separate pipes and interior capillary “wick” has traditionally kept the cost of heat pipes too high for all but the most exotic applications. Specialised wick materials often did not last very long.
Khanh Dinh, the founder of Heat Pipe Technology Inc., revolutionized the construction of heat pipes in the early 1980’s and was granted the first of several heat pipe patents in 1986. The methods which he developed brought the cost of the company’s patented heat pipes down to a level that was commercially affordable.
His first patent also disclosed a new and previously undiscovered use for heat pipes, using them to increase the efficiency and dehumidification capability of air conditioning systems. This is achieved by wrapping heat pipes around a conventional cooling coil to pre-cool and reheat the air stream.(figure 2)
2. HEAT PIPES FOR DEHUMIDIFICATION
When applied to HVAC, heat pipes provide significant dehumidification enhancement and improved indoor air quality. The additional benefit of energy savings occurs in many situations, especially where reheat is required. Depending upon conditions, heat pipes can double the amount of moisture removed by an air conditioner’s cooling coil
A. SUPERIOR DEHUMIDIFICATION ABILITY
By circuiting heat pipes in sections before and after the cooling coil refer to the above figure, heat is removed from the air stream before it encounters the cooling coil. This passively pre-cooled air means less sensible cooling is required by the coil, providing more latent capacity and superior dehumidification ability. The now “over cooled” air passes across the reheat section of the heat pipe, bringing the air temperature to a comfortable indoor condition. This free reheat is provided by the same heat energy which was absorbed in the incoming air stream. In the case of fresh air treatment, heat pipes can be used to pre-cool the incoming outside air, with dehumidification similarly performed by the enhanced air conditioner.
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B. HEAT PIPES AS APPLIED TO AIR CONDITIONING
The cooling coil in the standard air conditioner provides both sensible and latent cooling. When warm incoming air passes through the cooling coil, its temperature is lowered. The ability of the coil to remove moisture from the air depends upon the condensation of the air’s water vapour on the cooling coil surface. Condensation will only take place if the coil temperature is lower than the dew point of the air. The pre-cooling function of the heat pipes can be represented on a psychrometric chart as in Figure 4. Humidity problems can occur because the air conditioner utilises most of the cooling capacity to bring the air temperature down to the dew point (sensible cooling) with little or no capacity allowed for dehumidification or latent cooling.
By using heat pipes, a higher degree of latent cooling can be provided by the cooling system at a substantially reduced cost.
Air conditioners without heat pipes (or other, more inefficient forms of dehumidification) often cannot cope with humid conditions. The standard AC system will work harder and longer, and still be unable to effectively reduce humidity levels. In the case of outside air introduction, there is little chance of ever meeting a healthy relative humidity level without additional dehumidification. Comfort is further lessened as moist indoor air is usually over cooled, producing a “clammy” indoor air environment. Click here to download