A heat pipe is a passive energy recovery heat exchanger that has the appearance of a common plate-finned water coil except the tubes are not interconnected. Additionally it is divided into two sections by a sealed partition. Hot air passes through one side (evaporator) and is cooled while cooler air passes through the other side (condenser). While heat pipes are sensible heat transfer exchangers, if the air conditions are such that condensation forms on the fins there can be some latent heat transfer and improved efficiency.
Heat pipes are tubes that have a capillary wick inside running the length of the tube, are evacuated and then filled with a refrigerant as the working fluid, and are permanently sealed. The working fluid is selected to meet the desired temperature conditions and is usually a Class I refrigerant. Fins are similar to conventional coils - corrugated plate, plain plate, spiral design. Tube and fin spacing are selected for appropriate pressure drop at design face velocity. HVAC systems typically use copper heat pipes with aluminum fins; other materials are available.
- passive heat exchange with no moving parts,
- relatively space efficient,
- the cooling or heating equipment size can be reduced in some cases,
- the moisture removal capacity of existing cooling equipment can be improved,
- no cross-contamination between air streams.
The use of the heat pipe
- adds to the first cost and to the fan power to overcome its resistance,
- requires that the two air streams be adjacent to each other,
- requires that the air streams must be relatively clean and may require filtration.
Heat pipe heat exchanger enhancement can improve system latent capacity. For example, a 1°F dry bulb drop in air entering a cooling coil can increase the latent capacity by about 3%. Both cooling and reheating energy is saved by the heat pipe's transfer of heat directly from the entering air to the low-temperature air leaving the cooling coil. It can also be used to precool or preheat incoming outdoor air with exhaust air from the conditioned spaces.
- Where lower relative humidity is an advantage for comfort or process reasons, the use of a heat pipe can help. A heat pipe used between the warm air entering the cooling coil and the cool air leaving the coil transfers sensible heat to the cold exiting air, thereby reducing or even eliminating the reheat needs. Also the heat pipe precools the air before it reaches the cooling coil, increasing the latent capacity and possibly lowering the system cooling energy use.
- Projects that require a large percentage of outdoor air and has the exhaust air duct in close proximity to the intake, can increase system efficiency by transferring heat in the exhaust to either precool or preheat the incoming air.
Technology types (resource)
Hot air is the heat source, flows over the evaporator side, is cooled, and evaporates the working fluid. Cooler air is the heat sink, flows over the condenser side, is heated, and condenses the working fluid. Vapor pressure difference drives the evaporated vapor to the condenser end and the condensed liquid is wicked back to the evaporator by capillary action. Performance is affected by the orientation from horizontal. Operating the heat pipe on a slope with the hot (evaporator) end below horizontal improves the liquid flow back to the evaporator. Heat pipes can be applied in parallel or series.
Heat pipes are typically applied with air face velocities in the 450 to 550 feet per minute range, with 4 to 8 rows deep and 14 fins per inch and have an effectiveness of 45% to 65%. For example, if entering air at 77°F is cooled by the heat pipe evaporator to 70°F and the air off the cooling coil is reheated from 55°F to 65°F by the condenser section, the effectiveness is 45 % [=(65-55)/(77-55) = 45%]. As the number of rows increases, effectiveness increases but at a declining rate. For example, doubling the rows of a 48% effective heat pipe increases the effectiveness to 65%.
Tilt control can be used to:
- change operation for seasonal changeover,
- modulate capacity to prevent overheating or overcooling of supply air,
- decrease effectiveness to prevent frost formation at low outdoor air temperatures.
Tilt control (6° maximum) involves pivoting the exchanger about its base at the center with a temperature-actuated tilt controller at one end. Face and bypass dampers can also be used.
Contact us for a detailed list of manufacturers for this equipment.