Page 187 - Boiler_Operators_Handbook,_Second_Edition

172 Boiler Operator’s Handbook

the compressor. Don’t forget to put the bulb back in its from the air or a water source and pumps that heat into
thermowell after testing it. a swimming pool. Systems such as the pool heater incor-
Note what appear to be grooves around the dam- porate a simple refrigeration cycle.
aged chiller evaporator tube shown in Figure 5-30. Note that label of “pool heater.” A piece of refriger-
They’re not exactly fins but they do increase the surface ation equipment that’s only used for heating is normally
available for heat transfer. called a heater. The label of “heat pump” is primarily re-
When all else fails, refrigerant systems are fit- served for systems that can cool or heat, they’re revers-
ted with safety relief valves or rupture discs that will ible. Reversible heat pumps operate to switch the con-
open and dump the refrigerant to atmosphere should denser and evaporator operation. Figure 5-31 is a photo-
the system pressure get too high threatening an explo- graph of the most common one, a residential heat pump.
sive rupture of some part of the system. A safety relief The outdoor coil that forms almost three sides of the unit
will vent refrigerant until the pressure drops below the is a condenser in the summer and an evaporator in the
setting of the valve. A rupture disc, once activated, will winter. The enclosure also contains the compressor and
remain open completely draining the system of refriger- the reversing valve. The solenoid operated reversing
ant. With all modern refrigerants that are mostly heavier valve switches the suction and discharge between the
than air and, therefore capable of displacing the air in the evaporator and condenser to control the route of the re-
equipment room a safety relief or a rupture disc should frigerant. It fails to the heating mode because, normally,
have connecting piping discharging to atmosphere high loss of heat can result in more damage to a facility than
above the plant. loss of cooling. Heat pumps can utilize a single capillary
connecting the evaporator and condenser to handle the
Heat pumps reversed flow. The unit shown uses two TXVs and check
In truth every refrigeration system is a heat pump. valves to control liquid refrigerant flow.
They all absorb heat at a lower temperature and pump Whenever outdoor air temperatures are less than
it up to a higher temperature. However certain pieces of about 42°F a heat pump will start forming ice on the
equipment are reversible and others are simply used to outdoor (evaporator) coil and that ice can block air flow
pump heat up. Perhaps you, a family member, a friend, as well as insulate the coils to reduce heat transfer. The
or a neighbor have a heat pump serving their house. It’s control systems for heat pumps that can be subjected to
reversible so it can pump heat from the outside air into colder temperatures monitor the ice buildup by sensing
the house in the winter time as well as pumping heat out air pressure drop across the coil or below freezing tem-
of the house into the outside air in the summer. There are perature at the coil surface to initiate a defrost cycle. The
also heat pumps that operate only in the heating mode. condenser fan is shut down, the reversing valve switch-
The most common of those is a pump that extracts heat es to cooling mode, and the compressor is operated to
dump hot gas into the outdoor coil. For the comfort of
occupants electric strip heaters in the indoor unit are
powered to heat the air that is cooled during the defrost
cycle.
Most residential heat pumps, and air conditioners
are arranged the same so it’s difficult to tell if the out-
door unit is a heat pump or simply a condensing unit
for air conditioning only. When a residence has a gas or
oil fired heating system a heat pump is only provided
for operation at outdoor temperatures well above 42°F
to produce heat more economically than firing fuel.
It’s important to note that residential style units,
whether heat pumps or simple condensing units, usu-
ally have a vertical discharge. While this dramatically
reduces the potential for recirculating the cooling air it
also provides a catch basin for pollen, leaves, and other
debris. It’s not easy but I typically shut down and dis-
connect my unit, temporarily remove the discharge grill,
and clean out the bottom of the unit twice a year so the
Figure 5-31. Residential heat pump