Handbook of air conditioning and refrigeration / Shan K

7.38 CHAPTER SEVEN
The wire-to-water efficiency of a water system � ww, expressed either in dimensionless form or as
a percentage, is defined as the ratio of energy output from water to the energy input to the electric
wire connected to the motor. It can be calculated as
� ww � � p� dr� mot
(7.15)
The total efficiency of the centrifugal pump � p can be obtained from the pump manufacturer or calculated
from Eq. (7.8). The pump efficiency � p depends on the type and size of pump as well as the
percentage of design volume flow rate during operation. Pump efficiency usually varies from 0.7 to
0.85 at the design volume flow rate. Drive efficiency � dr indicates the efficiency of a direct drive,
belt drive, and various types of variable-speed drives. For direct drive, � dr � 1. Among variablespeed
drives, an adjustable-frequency alternating-current (ac) drive has the highest drive efficiency.
For a 25-hp (18.7-kW) motor, � dr often varies from 0.96 at design flow to 0.94 at 30 percent design
flow to 0.80 at 20 percent design flow. Motor efficiency � mot depends on the type and size of motor.
It normally varies from 0.91 for a 10-hp (7.5-kW) high-efficiency motor to 0.96 for a 250-hp (187kW)
motor as listed in Table 6.2.
7.9 OPERATING CHARACTERISTICS OF CHILLED
WATER SYSTEM
Many chilled and hot water systems used in commercial central hydronic air conditioning systems
often have their central plant located in the basement, rooftop, or equipment floors of the building.
The hot/chilled water from the boiler/chiller in the central plant is then supplied to the coils and terminals
of various zones in one building or in adjacent buildings by means of supply main pipes.
Water returns from the coils and terminals to the central plant via the return mains.
Coil Load and Chilled Water Volume Flow
In AHUs or fan coils, two-way control valves are currently widely used to modulate the water
volume flow rate so as to maintain a predetermined air discharge temperature or space temperature
at reduced system loads. Coils, especially oversized coils, operate at design load usually less than 5
percent of their total operating time. For a typical coil, nearly 60 percent of the operating time may
correspond to a coil load of 35 to 65 percent of the design value.
During part-load operation, the required fraction of design volume flow rate of chilled water
flowing through a coil V˙
w is not equal to the fraction of design sensible coil load Qcs Btu/h (W)
which is the sensible heat transfer from the coil to the conditioned air, as shown in Fig. 7.17a. In
Fig. 7.17a, indicates the design chilled water volume flow rate, gpm (m 3 /min), and Q cs,d the
V˙ w,d
design sensible coil load, Btu/h (W). This is because of the characteristics of sensible heat transfer
described by
Qcs � AoUo�Tm where Ao � outer surface area of coil, ft2 (m2 )
Uo � overall heat-transfer coefficient based on outer area, Btu/h�ft2�°F (W/m2�°C) �Tm � logarithmic temperature between conditioned air and chilled water,°F (°C)
(7.16)
When the volume flow rate of chilled water V˙
w is reduced, the decrease in the product of AoUo�Tm is not the same as the reduction in the chilled water volume flow rate V˙
w.
When V˙ w drops, the outer
surface area Ao remains the same and Uo is slightly reduced. Only a considerable rise in chilled
water temperature across the coil �Tw,c � Twl � Twe, as shown in Fig. 7.17b, can reduce �Tm suffi-