Heat Pump Overview - Bosch FHP RT-20 Installation, Operation And Maintenance Manual

Heat Pump Overview

Every RT has a complete, sealed refrigeration system
that is ready for connection to a water source.
The heat pump has all the typical DX components
and also has refrigerant reversing valve(s) to enable
the system to work in both heating and cooling
modes.
Factory installed Heat Pump System Components:
Thermostatic Expansion Valve (TXV)
Each compressor is equipped with a thermal
expansion valve. The valve controls the flow of liquid
refrigerant entering the evaporator coil by maintaining
a constant, factory set superheat of 10ºF.
Refrigerant Distributor
Attached to the TXV is a refrigerant distributor. The
refrigerant distributor evenly distributes the refrigerant
to each circuit of the airside coil to provide optimum
performance.
Airside Coil
Each unit uses a single refrigerant coil known as an
airside coil. If two compressors are used in the unit,
then the airside coil will be a split configuration so
that each compressor has a dedicated portion of
the airside coil. Depending on whether the unit is in
cooling or heating mode, the airside coil will function
as either a condensing coil or an evaporator coil. See
also Subassemblies/Coils.
Coaxial Refrigerant-to-Water Heat Exchanger
The RT uses one coaxial heat exchanger per
compressor, essentially a tube inside a tube. Water
flows through the inner copper tube and compressed
refrigerant is forced through the spaces between the
inner and outer tubes. Depending on whether the unit
is functioning in a cooling or a heating capacity, heat
is rejected from one tube to the other.
Low Limit Pressure Switch
The unit includes a low limit pressure switch (located
in the compressor compartment). The switch is
installed in the suction line and turns off the DX
system when the suction pressure drops to 40 psi.
The switch has an auto-reset which closes the circuit
and allows the system to run when the pressure
increases back to 60 psi.
Compressors
Each unit includes one or two high efficiency scroll
type compressors depending on needed capacity.
Scroll type compressors are essentially maintenance-
free since they are a self-contained, self-cooling
design. The scroll compresses the refrigerant in the
gaseous state to a high temperature, high pressure
gas.
Compressor Protective Devices:
Thermal Overload - each compressor is equipped
with an auto reset thermal overload
High Temp Protection - internal devices within
the compressor protect it against excessively high
discharge gas temperatures (only on compressors
above 6 hp)
Crankcase Heater - liquid refrigerant is
incompressible. Therefore, a crankcase heater
is installed around the base of each compressor
in the unit to boil-off any liquid refrigerant
that may be absorbed into the oil during idle
periods. System power energizes the heater; it is
recommended the heater operate 24 hours prior to
the compressors being started.
High Limit Pressure Switch
To safely shutdown the DX system in case of
an increase in refrigerant pressure, a high limit
pressure switch is included (located in compressor
compartment). It trips when the refrigerant pressure
increases to 600 psi in the liquid line and closes when
the pressure drops to 420 psi. Typically if the high
limit switch trips, a failure in the system has occurred
and more investigation is needed to determine the
underlying problem.
Liquid Line Filter Drier
The liquid line filter drier prevents moisture and
foreign matter from entering the thermal expansion
valve. It is located in the compressor compartment.
Hot Gas Bypass Valve
On units equipped with hot bypass, hot gas from
the compressor is injected into the liquid line of the
airside coil after the TXV. This process starts when
suction gas temperatures drop below 28ºF, which is
32º–34ºF coil surface temperature. Hot gas helps the
airside coil from freezing up and the compressor from
cycling. The valve is factory set, but should be field
adjusted to maintain a suction pressure of 90 psi.
Valve Adjustment - to adjust the valve, connect a
pressure gauge to the suction line and block the
entering air to the coil. The valve should begin
to open when the suction pressure drops to
approximately 58 psig (the valve will feel warm to
the touch). Adjustments are made by first removing
the cap on the valve and then turning the adjusting
stem counterclockwise to decrease the pressure.
Allow several minutes between adjustments for
system to stabilize. When adjustment is complete,
replace the cap on the valve.
Reversing Valve
Each compressor is equipped with a reversing valve
to reverse the direction of refrigerant flow. The valve
is electrically actuated.
Access Ports
For easy measurement and charging access, several
ports are provided throughout the system. These
can be used to measure system pressures and also
charge or evacuate the system.
Model RT Energy Recovery Unit
31