Handbook of air conditioning and refrigeration / Shan K

10.44 CHAPTER TEN
electric power input to the tower fans is approximately 10 percent of the power input to the centrifugal
compressors; and the power input to the circulating pumps for condenser water is about 2 to
5 percent of the power input to the centrifugal compressors.
One earlier strategy for capacity control of cooling towers during part-load operation is to maintain
a fixed temperature T tl for condenser water leaving the tower. This strategy does not minimize
the sum of power input to the compressors, tower fans, and circulating pumps when both T tl and the
condensing temperature can be lowered. When T o� falls, a lower condenser water temperature leaving
the tower T tl and entering the water-cooled condenser is always energy-saving, except when low
T tl causes a low condensing pressure which cannot provide effective operation of the refrigeration
system refrigerant flow from the condenser and evaporator and the required evaporation (refrigeration
effect) in the evaporator.
In ASHRAE Journal (no. 3, 1997), “The HVAC&R ‘Evolution’ Continues,” it was reported that
a centrifugal chiller operated with an entering condenser water temperature of 55°F (12.8°C), a
minimum entering condenser water temperature. A better strategy of capacity control of cooling
towers for water-cooled condensers in a centrifugal chiller is to lower T tl until T tl falls to a limit, the
minimum entering condenser water temperature T con, m. If T tl falls below this limit, the refrigeration
system cannot be effectively operated.
Most tower fans in current use are two- or three-speed fans. The key points for a nearly optimum
control of cooling towers, according to Braun and Diderrich (1990), are as follows:
● Use sequencing of tower fans to maintain a possibly lowered Ttl � Tcon, min during part-load operation
to minimize the sum of power input to the compressors and tower fans.
● Heat rejection Qrej, p determines the sequencing of tower fans, and Qrej, p, is measured by the product
of the range and the water circulation rate.
● Establish a simple relationship between the tower capacity and the sequencing of the tower fan.
● Use an open-loop control (see Chap. 5) for a stable operation.
● When an increase of tower capacity is required, the speed of tower fan operated at the lowest
speed should be raised first. Similarly, during a decrease of tower capacity, the highest tower fan
speed should be the first one to be reduced.
● The speed of the tower fan should not be reduced in sequence to maintain a T tl lower than T con, min.
Example 10.2. A counterflow induced-draft cooling tower is required to cool the condenser water
from 95 to 85°F (35 to 29.4°C) at a design outdoor wet-bulb temperature of 78°F (25.6°C). If the
water-air ratio of this tower is 1.2, do the following:
1. Calculate the tower coefficient at this design wet-bulb temperature.
2. If the outdoor wet-bulb temperature, water circulation rate, and airflow rate remain the same as
in the design conditions, calculate the condenser water entering and leaving the tower if the rate
of heat rejection at the water-cooled condenser has fallen to 50 percent of its design value.
Take the specific heat for water as c pw � 1 Btu/lb�°F (4187 J/kg�°C).
Solution. To find the enthalpy of saturated air film h s, the enthalpy of ambient air h a, and the
driving potentials h s � h a, do these calculations:
● Divide the temperature range of condenser water into 1°F (0.56°C) divisions such as 85, 86, ...,
93, 94, 95°F. From App. Table B.1, the enthalpy of the saturated air film that surrounds the water
droplets h s at various temperatures can be determined.
● Find the enthalpies of the air that contacts with the condenser water at various temperatures h a by
means of Eq. (10.22), starting with incoming air at a wet-bulb T o��78°F (25.6°C) in contact
with the coldest condenser water at temperature 85°F (29.4°C).
Based on Eq. (10.26), the tower coefficient or fill surface area required to cool the condenser
water from 86 to 85°F and raise the air enthalpy from that at T o��78°F to h a78 � 1.2 is