Handbook of air conditioning and refrigeration / Shan K

Part-Load Operation
At cooling mode part-load operation as the zone sensible cooling load decreases, the zone temperature
decreases accordingly. In a four-pipe fan-coil system, when the zone temperature sensor senses
this temperature decrease, a DDC controller closes the two-way valve in a fan coil, reducing the
chilled water flow rate entering the coil. As the chilled water flow reduces, the temperature of
chilled water leaving the fan coil Twfl rises accordingly, which raises both the temperature and humidity
ratio of the conditioned (cooled and dehumidified) air leaving the fan coil Tfcp and wfcp as
well as the humidity ratio of the zone air wrp, as shown in Fig. 28.1b. As soon as the reduction in
the sensible cooling effect of the conditioned air due to the increase of Tfcp in Qs,fcp � 60V˙ fc�scpa
(Tr � Tfcp) is just equal to the reduced sensible zone cooling load, a new balance is formed and the
zone temperature is maintained at the indoor design temperture Tr. At cooling mode part-load operation, the chilled water temperature entering the fan coil and the
cooling coil of the outdoor air AHU Twfe is better reset 5 to 7°F (2.8 to 3.9°C) higher, according to
either the outdoor air temperature or the system load. Chilled water temperature reset can save compression
energy and reduces the sensible cooling effect of the conditioned air. On the contrary, it
also increases the zone relative humidty. Analyses should be done to select an appropriate value.
During cooling mode part-load operation, reducing the chilled water flow rate entering a cooling
coil to match the reduction in zone or space load always causes a higher zone relative humidty. But
reducing the supply air volume flow rate to match the reduction in zone or space load in VAV systems
causes the zone relative humidity to remain approximately the same.
Zone Temperature Control and Sequence of Operations
AIR CONDITIONING SYSTEMS: SPACE CONDITIONING SYSTEMS 28.13
The zone temperature control of a four-pipe fan-coil system includes a fan-coil unit FC-1, using a
separate cooling coil and a heating coil that serve a control zone rx1 in the perimeter zone, and the
corresponding outdoor air AHU which supplies the conditioned outdoor ventilation air to the fan coil
FC-1. The operating mode can be divided into cooling mode, deadband mode, and heating mode.
1. When the time-of-day clock signals the four-pipe fan-coil system in the off position, and the
outdoor air AHU and fan-coil unit serving control zone rx1 in the perimeter zone is shut off, the
two-way valves for both cooling and heating coils in the fan coil FC-1 should be closed. Also the
outdoor air damper and the supply fan in the outdoor air AHU (OAAHU) are shut off.
2. When the time-of-day clock signals the four-pipe fan-coil system to the on position, and the
perimeter zone temperature sensor senses a zone temperature T rx � 75°F (23.9°C), both the outdoor
air AHU and fan coil FC-1 call for cooling. A DDC system controller then opens the two-way
valves, resets the discharge air temperature T dis, and starts the supply fan motor of the OAAHU with
the outdoor damper closed. After a time delay, such as 30 s, the outdoor damper of the OAAHU is
fully opened.
3. A DDC terminal controller next opens the two-way valve of the cooling coil of the fan coil,
starts the fan motor and the small fan, and modulates the valve opening to maintain a zone temperature
of 75°F (23.9°C). The two-way valve of the heating coil remains closed.
4. During cooling mode part-load operation, the zone sensible cooling load reduces. When the
drop in zone temperature T rx1p is sensed by the temperature sensor of control zone rx1, a DDC terminal
controller reduces and modulates the opening of the two-way valve of the cooling coil in the
fan coil, to maintain a preset zone temperature of 75°F (23.9°C).
Because of the reduction in the chilled water flow in the cooling coil, the result is an increase in
the temperature of chilled water leaving the cooling coil as well as in the temperature and humidity
ratio of the cooled conditioned air leaving the coil, which results in a higher zone relative humidity
� rp, as shown in Fig. 28.1b.
Usually, the sensible heat ratio of the space conditioning line (SHR sp) at part load is smaller than
that at summer design conditions. This further increases the zone relative humidity.