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ENGINEERING MANUAL OF AUTOMATIC CONTROL

CHILLER, BOILER, AND DISTRIBUTION SYSTEM CONTROL APPLICATIONS

372

Oversized Valve

In Figure 114A a large valve is used to ensure 128 kPa in the

coil. When the valve is full open, the coil is full of steam and
the output is 30 kW. The return header and trap are also full of
steam keeping the trap closed much of the time. The trap opens
slightly when water accumulates around the thermostatic
element allowing the condensate into the return mains. The
pressure drop through the valve and coil is small, but large across
the trap.

Figure 114B shows the pressure relationship and heat output

as the modulating valve closes to the half-open position. The
smaller opening in the valve requires 128 kPa to get the steam
through the valve, called a 128 kPa drop across the valve. This
leaves 107 kPa in the steam coil header. Although the thermostat
has signaled to the valve to cut heat output in half, the coil is
still full of steam and the heat output is only reduced by 5
percent. The reduction is because of the difference in
temperature of 128 kPa steam compared to 107 kPa steam,
assuming 10

C air temperature entering the coil. The trap drop

is 5 kPa. Most of the pressure drop between the supply and the
return mains is now across the steam valve. The portion of the
modulating valve stroke between half open and full open
reduces the heat output only 5 percent.

Figure 114C shows the quarter-open valve position. Half of

the coil surface is starved of steam, the heat output is reduced
to about half of the original value and the trap is full open. All
of the steam has been condensed in the coil before reaching the
trap. Virtually all of the drop between the supply and return
mains is dissipated through the control valve.

The conclusions reached from Figure 114 are:

1. The sum of the individual pressure drops across the valve,

coil, and trap equals the pressure difference between the
supply and return mains.

2. Heat output changes little due to pressure change within

the coil, as long as the coil is full of steam.

3. Heat output changes little until the valve assumes most

of the pressure drop between supply and return mains.

4. Heat output from the coil is reduced by starving a part

of the coil surface of steam and allowing the surface to
cool off.

NOTE: Steam distributing coils allow reduced output without

the return end becoming cold.

Correctly Sized Valve

The valve in Figure 115A is sized for a 28 kPa pressure drop

when it is full open leaving 6 kPa of steam for the coil and the
trap. Full heat output of the coil is 28 kW and most of the drop
between supply and return mains occurs across the valve.

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