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HVAC Fundamentals: Refrigeration
Figure 2.10 Capillary tube and short-tube restrictor expansion devices.
Figure 2.11 Heat pump four-way reversing valve.
flow from the compressor discharge to the indoor (heating)
coil, as shown in the “Heating Mode” position of
Figure 2.11. Thus, the hot gas from the compressor goes
to the indoor coil, which is the condenser in this mode.
The refrigerant flows from the indoor coil through the
expansion device into the outdoor coil, which is the
evaporator in this mode. When the coil is activated electrically,
the U-shaped port is set so that gas flows from
the indoor coil to the compressor suction. Hot compressor
gas is routed to the outdoor coil, which is the condenser
in the cooling mode. Refrigerant moves through
the expansion device through the indoor coil, which is
now the evaporator (cooling coil). Except for the reversing
valve, heat pump operation is identical in this mode
to a conventional air conditioner.
REFRIGERANTS
Refrigerants for vapor compression systems are a
rapidly evolving topic due to a multitude of constraints
including performance, application operating conditions,
environment, safety, lubricant compatibility, and materials
of construction compatibility. In the early development
of refrigeration, flammable and hazardous fluids
were common. More stable and inert refrigerants were
developed by substituting fluorine and chlorine for some
or all of the hydrogen in light hydrocarbons (methane,
ethane, propane). Chlorofluorocarbons (CFCs) were
widely used for low-temperature refrigeration, mobile air
conditioning, and centrifugal chillers. Hydrochlorofluorocarbons
(HCFCs) were commonly used in air-conditioning,
heat pumps, and screw compressors. Azeotropic
blends (mixtures that behave similarly to pure refrigerants)
of CFCs and HCFCs were also common.
The chemical stability and low cost of CFCs that
made them attractive refrigerants have resulted in unintended
environmental consequences. For many years,
CFCs from systems that needed repair were frequently
vented to the atmosphere. Systems with minor leaks
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