Condenser-System Variations
SYS-APM001-EN
75
Retrofit opportunities
A tremendous retrofit opportunity can be realized if the low-flow concepts
discussed earlier in the section Chilled-Water-System Options, page 20,
areutilized. Building owners may need to increase the capacity of an existing
system, for example in response to a building addition. In many of these
buildings, the condenser water system (piping, pump, and tower) is in good
condition, but is considered to be too small. By changing from traditional
design conditions, the existing infrastructure can often be used while still
providing additional capacity.
Keep in mind that a cooling tower is not limited to a specific tonnage. A cooling
tower is a heat exchanger that exchanges heat between the entering water
temperature and the ambient wet bulb. By varying the flow or the temperature,
the tower capacity can be changed--often increased. An example can easily
show this.
Example
A hospital presently has a 450-ton [1,580-kW] refrigeration] chiller
that needs to be replaced. The condenser water flow is 1,350 gpm [85.2 lps].
The cooling-tower selection conditions are
n
entering water temperature: 95°F[35°C]
n
leaving water temperature: 85°F[29.4°C]
n
ambient wet bulb: 78°F [25.6°C]
Recently, the cooling-tower fill was replaced. The tower, condenser water
piping, and pump are in good condition. The hospital is planning an addition
with 50 percent more load for a total of 675-tons [2,370-kW]. Must the hospital
replace the condenser water system? The answer is "no", as long as the chiller
is selected properly.
How is this possible? As long as the new chiller's condenser-water pressure
drop is at or below that of the present chiller, the same amount of water can still
be pumped. With the same flow rate, a 675-ton [2,370-kW] chiller may be
selected with a condenser-water-temperature rise of approximately15°F [8.3°C].
Using the cooling- tower manufacturer's selection software, the same cooling
tower can be selected at the elevated temperature difference. As shown in
Figure 31, the new selection point will be:
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entering water temperature: 103°F [39.4°C]
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leaving water temperature: 88°F [31.1°C]
n
ambient wet bulb: 78°F [25.6°C]