What Are Heat Pipes?
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.
Heat Pipe Benefits
- 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 Pipe Technology
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.
Applications/Benefits
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.
Dehumidifier Heat
Pipes
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.
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