chillerman2006
10-10-2011, 09:36 AM
Air-source heat pumps
Heat pumps use the same vapor-compression cycle as the refrigeration systems
described above, but they have additional components that enable them to pump
heat in either direction, such that the same equipment unit can provide cooling
or heating. The two added components that make heat pumps fully reversible are:
A 4-way reversing valve, which can reverse the direction of flow
around the refrigerant loop while maintaining the same direction of flow through
the compressor (the next two diagrams show how this is done).
A bi-directional expansion valve, which is able to meter the flow of
liquid refrigerant in either direction (for purposes of illustration, the
diagrams below show two expansion valves and two bypass valves, but modern heat
pumps often incorporate all of this functionality in a single valve).
An air-source heat pump in cooling mode acts
identically to an air conditioner. Refrigerant vapor exits the compressor at a
temperature in the range of 120-140°F, which is warmer than the outside air
temperature. As a result, it spontaneously loses heat when it enters the outdoor
coil, causing it to condense. The refrigerant leaves the condenser as a liquid,
still under high pressure. The expansion valve lets through just as much
refrigerant liquid as can be completely vaporized by the indoor coil. The
pressure drop through the expansion valve vaporizes some refrigerant and lowers
its temperature to 40-50°F. As a result, it spontaneously gains more heat, which
vaporizes the rest of the refrigerant liquid. The low-pressure refrigerant vapor
leaves the indoor coil, goes through a U-bend in the reversing valve, and
returns to the compressor, where the cycle begins again.
The reversing valve can be switched to heating mode
such that the high-pressure output of the compressor is directed toward the
indoor coil, which now acts as a condenser where the refrigerant gives up its
latent heat to the room. It is then expanded in the reverse direction (compare
with Figure 6) and vaporized in the outdoor coil, where it gains latent heat
from the outside air. The refrigerant vapor then goes through a U-bend now on
the other side of the reversing valve, and returns to the compressor where the
cycle begins again.
To summarize, the indoor and outdoor coils are where the refrigerant changes
phase, gaining or losing latent heat through evaporating and condensing. The
compressor drives the refrigerant around the loop and creates the high-pressure
and high-temperature conditions that enable it to condense as a liquid. The
expansion valve meters the flow of liquid refrigerant from the
high-pressure/temperature side to the low-pressure/temperature side of the loop,
such that it all will be vaporized in whichever coil is acting as the
evaporator. The reversing valve determines which coil is on the ""high side"
(condensing) or "low side" (evaporating) of the loop. Following the natural
tendency of spontaneous heat flow, the high side loses heat to its surrounding
environment, and the low side gains heat from its surrounding environment.
An air-source heat pump can be installed either as a "packaged" unit, where
both coils are in a weatherproof enclosure, typically mounted on flat roofs
(Figure 8), where they supply warmed or cooled air to the rooms immediately
below them, or as a "split" system, where the indoor coil is contained within
the building's air handling system (Figure 9). Packaged units are more common in
commercial and institutional buildings, while split systems are more common in
residential applications.
Packaged terminal heat pumps also can be installed as "through the-wall"
units, where they are fitted into a sleeve that passes through an exterior
building wall. These typically are noisier than rooftop packaged units, since
the compressor and fan are located in the room; they are commonly found in
hotels and motels.
During the winter heating season, air-source heat pumps cease to be effective
when the outside air temperature falls below 25-35°F. To handle such conditions,
they are supplied with a supplemental heating system - usually electric
resistance strips to further warm the building supply air after it leaves the
indoor coil (Figure 9).
During the heating season, moisture in the air outside may freeze on the
outdoor coil if its surface temperature drops below 32°F. Therefore, when
outside temperatures fall below about 40°F the heat pump will periodically enter
a defrost cycle, during which the reversing valve intermittenly sends hot
refrigerant through the outdoor coils for periods lasting anywhere from two to
ten minutes. During a defrost cycle, the electric heater is used to warm the
indoor supply air, but its temperature still may fall below skin temperature,
causing a "cold blow" sensation. This is not a problem with geothermal heat pump
units, which do not require defrost cycles even in cold weather, due to the
stable ground loop temperature.
Water-source heat pumps
Water is a much more efficient heat energy transfer medium than air, due to
its much higher specific heat. Using pumps or fans with comparable efficiencies,
it takes four times less energy to move a given quantity of heat with water than
with air. Furthermore, due to the higher density of water, a piping conduit
takes up less space than an air duct with the same heat moving capacity.
Therefore water is the preferred heat distribution medium for large, multi-story
buildings.
Conventional water-source heat pumps use a fossil-fuel-fired boiler as a heat
source during the winter and an evaporative cooling tower to reject heat during
the summer. This is sometimes referred to as a boiler/tower system; it is also
known as a "California system", since this concept is thought to have originated
in that state. The water loop temperature is maintained between 60 and 90°F.
When the loop temperature falls below 60°F, the boiler adds heat, and when the
loop temperature exceeds 90°F, the cooling tower rejects heat.
As shown in Figure 10, there is a common water loop connected to all the heat
pump units distributed throughout the building, which themselves can have a
variety of configurations, including horizontal, vertical, or console. The water
loop can be integrated with the sprinkler system to reduce cost.
Heat pumps use the same vapor-compression cycle as the refrigeration systems
described above, but they have additional components that enable them to pump
heat in either direction, such that the same equipment unit can provide cooling
or heating. The two added components that make heat pumps fully reversible are:
A 4-way reversing valve, which can reverse the direction of flow
around the refrigerant loop while maintaining the same direction of flow through
the compressor (the next two diagrams show how this is done).
A bi-directional expansion valve, which is able to meter the flow of
liquid refrigerant in either direction (for purposes of illustration, the
diagrams below show two expansion valves and two bypass valves, but modern heat
pumps often incorporate all of this functionality in a single valve).
An air-source heat pump in cooling mode acts
identically to an air conditioner. Refrigerant vapor exits the compressor at a
temperature in the range of 120-140°F, which is warmer than the outside air
temperature. As a result, it spontaneously loses heat when it enters the outdoor
coil, causing it to condense. The refrigerant leaves the condenser as a liquid,
still under high pressure. The expansion valve lets through just as much
refrigerant liquid as can be completely vaporized by the indoor coil. The
pressure drop through the expansion valve vaporizes some refrigerant and lowers
its temperature to 40-50°F. As a result, it spontaneously gains more heat, which
vaporizes the rest of the refrigerant liquid. The low-pressure refrigerant vapor
leaves the indoor coil, goes through a U-bend in the reversing valve, and
returns to the compressor, where the cycle begins again.
The reversing valve can be switched to heating mode
such that the high-pressure output of the compressor is directed toward the
indoor coil, which now acts as a condenser where the refrigerant gives up its
latent heat to the room. It is then expanded in the reverse direction (compare
with Figure 6) and vaporized in the outdoor coil, where it gains latent heat
from the outside air. The refrigerant vapor then goes through a U-bend now on
the other side of the reversing valve, and returns to the compressor where the
cycle begins again.
To summarize, the indoor and outdoor coils are where the refrigerant changes
phase, gaining or losing latent heat through evaporating and condensing. The
compressor drives the refrigerant around the loop and creates the high-pressure
and high-temperature conditions that enable it to condense as a liquid. The
expansion valve meters the flow of liquid refrigerant from the
high-pressure/temperature side to the low-pressure/temperature side of the loop,
such that it all will be vaporized in whichever coil is acting as the
evaporator. The reversing valve determines which coil is on the ""high side"
(condensing) or "low side" (evaporating) of the loop. Following the natural
tendency of spontaneous heat flow, the high side loses heat to its surrounding
environment, and the low side gains heat from its surrounding environment.
An air-source heat pump can be installed either as a "packaged" unit, where
both coils are in a weatherproof enclosure, typically mounted on flat roofs
(Figure 8), where they supply warmed or cooled air to the rooms immediately
below them, or as a "split" system, where the indoor coil is contained within
the building's air handling system (Figure 9). Packaged units are more common in
commercial and institutional buildings, while split systems are more common in
residential applications.
Packaged terminal heat pumps also can be installed as "through the-wall"
units, where they are fitted into a sleeve that passes through an exterior
building wall. These typically are noisier than rooftop packaged units, since
the compressor and fan are located in the room; they are commonly found in
hotels and motels.
During the winter heating season, air-source heat pumps cease to be effective
when the outside air temperature falls below 25-35°F. To handle such conditions,
they are supplied with a supplemental heating system - usually electric
resistance strips to further warm the building supply air after it leaves the
indoor coil (Figure 9).
During the heating season, moisture in the air outside may freeze on the
outdoor coil if its surface temperature drops below 32°F. Therefore, when
outside temperatures fall below about 40°F the heat pump will periodically enter
a defrost cycle, during which the reversing valve intermittenly sends hot
refrigerant through the outdoor coils for periods lasting anywhere from two to
ten minutes. During a defrost cycle, the electric heater is used to warm the
indoor supply air, but its temperature still may fall below skin temperature,
causing a "cold blow" sensation. This is not a problem with geothermal heat pump
units, which do not require defrost cycles even in cold weather, due to the
stable ground loop temperature.
Water-source heat pumps
Water is a much more efficient heat energy transfer medium than air, due to
its much higher specific heat. Using pumps or fans with comparable efficiencies,
it takes four times less energy to move a given quantity of heat with water than
with air. Furthermore, due to the higher density of water, a piping conduit
takes up less space than an air duct with the same heat moving capacity.
Therefore water is the preferred heat distribution medium for large, multi-story
buildings.
Conventional water-source heat pumps use a fossil-fuel-fired boiler as a heat
source during the winter and an evaporative cooling tower to reject heat during
the summer. This is sometimes referred to as a boiler/tower system; it is also
known as a "California system", since this concept is thought to have originated
in that state. The water loop temperature is maintained between 60 and 90°F.
When the loop temperature falls below 60°F, the boiler adds heat, and when the
loop temperature exceeds 90°F, the cooling tower rejects heat.
As shown in Figure 10, there is a common water loop connected to all the heat
pump units distributed throughout the building, which themselves can have a
variety of configurations, including horizontal, vertical, or console. The water
loop can be integrated with the sprinkler system to reduce cost.