Handbook of air conditioning and refrigeration / Shan K

available to perform the control actions at part-load if the variations in pressure drop of various
branches are small and the difference in main pipe pressure drop between the farthest and nearest
branches is within 60 ft WC (18 m WC).
Common Pipe and Thermal Contamination
For a plant-building loop in a chilled water system, if there is a backflow of a portion of the return
chilled water from the building loop that enters the common pipe at junction L and is mixed with
the supply chilled water from the plant loop at junction B, as shown in Fig. 7.20a, then the thermal
contamination of building return chilled water occurs. Lizardos (1995) suggested that the length
of the common pipe (expressed as the number of diameters of the common pipe) should be as
follows:
The diameter of the common pipe should be equal to or greater than the diameter of the return main
of the building loop.
Example 7.1. Consider a chilled water system using a plant-building loop. The design chilled water
flow rate is 1000 gpm (63.1 L/s) with a chilled water temperature rise across the coils of
�Tw,c � 20°F (11.1°C). There are three chillers in the central plant, each equipped with a constantspeed
chiller pump that provides 350 gpm (22 L/s) at 40-ft (12-m) total head. In the building loop,
there are two variable-speed building pumps, each with a volume flow rate of 1000 gpm (63.1 L/s)
at 60-ft (18-m) head. One of these building pumps is a standby pump. At design conditions, chilled
water leaves the chiller at a temperature Tel � 40°F (4.4°C) and returns to the chiller at 60°F
(15.6°C). Chilled water leaving the chiller is controlled at 40°F (4.4°C) for both design and partload
operation. Once the fouling and inefficiency of the coils have been taken into account, the frac-
tions of design volume flow rate V˙ bg / V˙ bg,d required to absorb the coil load at various fractions of the
sensible load Q cs/Q cs,d are listed in Table 7.8.
When the system load drops, plant chiller 1 will turn off when Q cs/Q cs.d equals 0.65 and chiller 2
will turn off when Q cs/Q cs,d equals 0.30. When the system load increases, chiller 2 turns on at
Q cs/Q cs,d � 0.35, and chiller 1 turns on at Q cs/Q cs,d � 0.7. Plant chiller 3 operates continuously.
Calculate the following based on the chillers’ on/off schedule at various fractions of the sensible
coil load:
1. Mean chilled water temperature rise across the coil
2. Water flow in the common pipe
3. Temperature of water returning to the water chiller
Solution
Chilled water velocity in return main, ft/s (m/s) Minimum length of common pipe
� 5 (1.5) 3 diameters, or � 2 ft (0.6 m)
� 5 (1.5) 10 diameters
1. From the given information in Table 7.8, and Eq. (7.1), the mean water temperature rise across
the coil for Q s,c/Q sc,d � 0.9 is
�T w,c � 20Q csV˙ bg,d / Q cs,dV˙ bg �
20 � 0.9
0.8
� 22.5�F (12.5�C)
WATER SYSTEMS 7.51
Values of �T w,c at other values Q sc/Q sc,d can be similarly calculated and are listed in Table 7.8.