Handbook of air conditioning and refrigeration / Shan K

13.12 CHAPTER THIRTEEN
System Characteristics
P6 or standby pump P7 and to the interior zone through zone pump P4 or standby pump P5. The
chilled water supply temperature T ws is usually between 42 and 45°F (5.6 and 7.2°C). Chilled water
pump P1 and hot water pump P2 are both constant-speed, constant-flow pumps. Zone pumps P5,
P6, P7, and P8 are all variable-speed, variable-flow pumps.
Unoccupied Cooling Mode. When T r, p � 75°F (23.9°C) and the space is unoccupied, as
determined by a timer, the DDC unit controller shuts down the entire centrifugal chiller and heat
recovery system.
Occupied Heating Mode. When T r, p � 70°F (21.1°C) and the conditioned space is occupied, the
system is under occupied heating mode operation. The centrifugal chiller is energized, and pump
P1 supplies chilled water to the interior zone through zone pumps P4 and P5. At the same time, the
DDC unit controller actuates the two-position valve V2 to block passage of chilled water to
the perimeter zone. Hot water pump P2 is energized so that hot water from the heating bundle in
the condenser is forced through the deenergized supplementary heater and supplied to heating coils
in the perimeter zone through zone pump P6 or P7 at a temperature T sup between 105 and 110°F
(40.6 and 43.3°C). If the rejected heat in the heating bundle is more than that required in the heating
coils in the perimeter zone, the return water temperature T ret increases before the inlet of the
heating bundle and pump P2. When sensor T2 detects such a rise in temperature, the DDC unit
controller modulates valve V1, which permits some hot water to flow to the storage tank. When
the storage tank is full of hot water and sensor T2 detects a further increase in water temperature,
the modulation valve is positioned so as to allow all the hot water to bypass the storage tank. Tower
pump P3 starts, and valve V3 is modulated to control the flow rate of condenser water to the
cooling tower.
If the outdoor temperature T o is so low that heat rejection at the heating bundle is not sufficient
to meet the heating requirement at the perimeter zone, then the hot water temperature detected by
sensor T2 is less than a preset value. The tower pump is then stopped. Hot water bypasses the storage
tank, and the supplementary heater is gradually energized so as to maintain the water temperature
sensed by T2 within a predetermined range.
Unoccupied Heating Mode. When T o � 70°F (21.1°C) and the conditioned space is unoccupied,
the DDC unit controller commands the following actions:
1. The hot water temperature detected by sensor T2 is reset to a value according to outdoor temperature
T o to maintain a night setback indoor temperature T r � 55°F (12.8°C) and lower.
2. Only hot water pump P2 and zone pump P6 or P7 are in operation; all other equipment ceases to
operate.
3. Valve V1 is modulated so that hot water from the storage tank alone is supplied to the heating
coils in the perimeter zone.
If T2 senses a water temperature lower than the preset value, the supplementary heater is then gradually
energized. The modulating valve V1 allows the hot water to bypass the storage tank and then
enter the heaters.
First, compared with a cooling-only centrifugal chiller, the refrigeration cycle of a centrifugal
chiller incorporating a heat recovery system has a greater pressure differential between condensing
and evaporating pressure �p e,c, in psi (kPa). This is because a cooling-only chiller usually has
water leaving the condenser at a temperature at 90 to 95°F (32.2 to 35°C) during summer design
conditions, whereas a chiller incorporating heat recovery needs a higher condenser leaving temperature
T cl of 105 to 110°F (40.6 to 43.3°C) for the hot water supply in winter heating. Although the
evaporating temperature can be slightly higher in winter than in summer, an increase of 10 to 12°F