Trane SYS-APM001-EN Coil response to decreased entering water temperature, System Design Options

System Design Options

Figure 21. Chilled water system performance at part load

350
300
	Base
250
200	Low Flow*
kW
150
100
50
0	25% Load	50% Load	75% Load	Full Load

*Low-flow conditions in Figure 21 are 1.5 gpm/ton [0.027 L/s/kW] chilled water and 2.0 gpm/ton [0.036 L/s/kW] condenser water.

While the magnitude of the benefit of low-flow changes depends on the chiller type used (centrifugal, absorption, helical-rotary, scroll), all chilled- water systems can benefit from judicious use of reduced flow rates as recommended by the ASHRAE GreenGuide8.

If coil performance data is not available from the original manufacturer, its performance could be approximated using current selection programs and known details about the coil, such as fins per foot, number of rows, tube diameter, etc. Some designers use the following approximation instead. For each 1.5 to 2.5°F [0.8°C to 1.4°C] the water temperature entering the coil is reduced, the coil returns the water 1°F [0.6°C] warmer and gives approximately the same sensible and total capacities. This is a rough approximation and a coil’s actual performance depends on its design.

SYS-APM001-EN

Coil response to decreased entering water temperature

A coil is a simple heat exchanger. To deliver the same sensible and latent capacity when supplied with colder water, the coil’s controls respond by reducing the flow rate of the water passing through it. Because the amount of water decreases while the amount of heat exchanged remains constant, the leaving water temperature increases. Thus, by supplying colder water to the coils, a low-flow system can be applied to an existing building. In a retrofit application, it is wise to reselect the coil, using the manufacturer’s selection program, at a new chilled-water temperature to ensure its performance will meet the requirements.

One possible concern of low supply-water temperatures is the ability of the valve to control flow properly at low-load conditions. A properly-sized valve with good range can work well in low-flow systems. In existing systems, valves may need to be replaced if they cannot operate with the new range of flows, but the coils do not need to be replaced.

Example of coil reselection at colder temperature/reduced flow rate

Water temperatures and flow rates are variables. They should be selected to achieve an efficient and flexible water distribution system. Consider the following example of a six-row coil in an existing air handling unit.

Table 9 shows an example of selecting a chilled-water cooling coil in a 13,000-cfm (6.1-m3/s) VAV air-handling unit. The left-hand column shows the

Chiller System Design and Control 33