Trane SYS-APM001-EN manual Three-way valve load control, Two-way valve load control

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 ΔT 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

Airflow

Three-Way

Modulating

Valve

Bypass Pipe

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.

8 Chiller System Design and Control SYS-APM001-EN