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8 Chiller System Design and Control SYS-APM001-EN
Primary System Components
Heat transferred from the loads can be controlled in a number of ways:
• Three-way valve
•Two-way valve
• Variable-speed pump
• Face-and-bypass dampers
Three-way valve load control
A three-way control valve (Figure 5) regulates the amount of water passing
through a coil in response to loads. The valve bypasses unused water around
the coil and requires a constant flow of water in the system, regardless of
load. A drawback of this bypass is that the temperature of the water leaving
the three-way valve is reduced at part-load conditions. This can be a major
contributor to so-called “low T syndrome” discussed on page 79. Three-way
valves are used in many existing systems, especially in those with constant-
volume pumping.
Figure 5. Three-way valve
Two-way valve load control
A two-way, water modulating valve (Figure 6) at the coil performs the same
water throttling function as the three-way valve. The coil sees no difference
between these two methods. The chilled-water system, however, sees a great
difference. In the case of the two-way valve, all flow in the coil circuit is
throttled. No water is bypassed. Consequently, a system using two-way
valves is a variable-flow chilled-water system. The temperature of the water
leaving the coil is not diluted by bypass water so at part-load conditions, the
system return-water temperature is higher than with three-way valve control.
Airflow
Bypass
Pipe
Three-Way
Modulating
Valve