Some specialized process loads do not involve cooling air. Instead, they may
involve heat transfer directly within a piece of process equipment, such as the
cooling jacket of an injection-molding machine. Heat transferred from the loads
can be controlled in a number of ways:
Three-way valve load control
A three-way control valve 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 cause of so-called "low
T syndrome" discussed on page 63 in the section Chilled-Water-System
Issues. Three-way valves are used in many existing systems.
Figure 2 Valve-controlled loads
Two-way valve load control
A two-way, water modulating valve 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.
Heat transferred from the loads can be controlled in a number of ways: n Three-way valve n Two-way valve n Variable-speed pump n Uncontrolled coils Three-way valve load control A three-way control valve regulates the amount of water passing through a coil in response to loads.