Handbook of air conditioning and refrigeration / Shan K

Controls
Capacity and Selection
HEAT PUMPS, HEAT RECOVERY, GAS COOLING, AND COGENERATION SYSTEMS 12.13
capacity of the unit or by measuring the temperature differential between the refrigerant inside the
outdoor coil and the ambient air. Defrosting terminates when the temperature of liquid refrigerant
leaving the outdoor coil (or the coil temperature) rises above 60°F (15.6°C).
Having or not having a suction line accumulator also affects the performance of the heat pump.
Nutter et al. (1996) showed that the refrigerant flow rate averaged 18 percent higher and had a
7 percent shorter defrost cycle for heat pumps without an accumulator than those with an accumulator.
For reciprocating heat pumps, on/off, speed modulation, and cylinder unloading (as described in
Sec. 11.5) capacity controls are generally used. For scroll heat pumps, on/off, variable-speed, and
variable-displacement modulation capacity control are usually used.
Either the discharge air temperature or the return temperature can be used as the criterion to
change automatically from cooling mode to heating mode and vice versa. A dead band of 2 to 3°F
(1.1 to 1.7°C) and a time delay are always required between cooling and heating mode operations
to prevent short cycling.
Most of the packaged heat pumps provide specific safety controls of high pressure, low pressure,
head pressure, or low ambient control; freezing protection of indoor coil; protection from
overloading; and supplementary heating. The principle and operation of these controls are the same
as described in Sec. 11.5. A microprocessor-based DDC system controller may be used to integrate
all the controls in one package and to add time delay, compressor lockout, loss of refrigerant
charge, and short-cycling protection controls to the sequence control of heat pump and gas furnace
in heating mode operation, and air economizer and refrigeration capacity control in cooling mode
operation.
Air-source heat pump capacity is selected according to its cooling capacity because supplementary
heating may be required under winter design conditions. Also, the rated cooling capacity
at summer design conditions is often greater than the rated heating capacity at winter design
conditions.
When an air-source heat pump is installed directly inside or above the conditioned space, the
noise generated by the heat pump must be taken into consideration. Attenuation remedies should be
provided if necessary to maintain an NC curve at an acceptable level in the conditioned space.
Sound control is discussed in Chap. 19. In 1992, air-source heat pump products were available
ranging in cooling capacity from a fraction of a ton to about 40 tons (few kilowatts to 140 kW) with
an indoor airflow of 16,000 cfm (7550 L/s).
12.3 GROUNDWATER HEAT PUMP SYSTEMS
Groundwater heat pump (GWHP) systems use well water as a heat source during heating and as a
heat sink during cooling. When the groundwater is more than 30 ft (9 m) deep, its year-round temperature
is fairy constant. Groundwater heat pump systems are usually open-loop systems. They are
mainly used in low-rise residences in northern climates such as New York or North Dakota. Sometimes
they are used for low-rise commercial buildings where groundwater is readily available and
local codes permit such usage.