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30 TRG-TRC016-EN
notes
period two
Chilled-Water System Design
A two-way modulating valve is similar to a three-way valve in that the water
flow through the coil is modulated proportionately to the load. The primary
difference is that the two-way valve does not bypass any unused water, it
simply throttles the amount of water passing through the coil.
The coil and the air being conditioned experience no difference in the cooling
effect of using a two-way versus a three-way valve. The chilled-water system,
however, sees a great difference. Recall that with a three-way valve, the
terminal water flow (water through the coil plus the water bypassing the coil)
is constant at all loads. With a two-way valve, the terminal water flow varies
proportionately with the load. Because there is no mixing of coil and bypassed
water, the temperature of the water leaving the load terminal remains relatively
constant at all conditions. In fact, this return-water temperature may actually
rise slightly as the load decreases, due to coil heat-transfer characteristics.
Systems that use two-way valves have the following characteristics:
n The temperature of the water returning from the system is constant
(or increases) as the cooling load decreases. This increases the effectiveness
of options such as heat recovery, free cooling, and base-loading, which will
be discussed further in Period Three.
n The water flow through each load terminal varies proportionately to the
load, resulting in pump energy savings at part load.
n A variable-flow system is less sensitive to water balance than most
constant-flow systems.
A variable-flow, chilled-water distribution system, however, may require
another method to provide constant water flow though the chillers, or else
the chillers must be equipped to handle variable water flow.
Two-Way Valve Control
two
two
-
-
way
way
modulating valve
modulating valve
airflow
airflow
Figure 35