VALVES AND
REFRIGERANT
CONTROLS
11.1 TYPES OF VALVES
All devices considered in this chapter, even those called controls, are valves in
that they are placed in refrigerant lines and can restrict or even completley
block the flow of refrigerant. Two-position shutoff valyes are expected to operate
either in a completely open or a completely closed position. Other valves
modulate the flow rate of refrigerant in response to some variables, such as
temperature, pressure, or liquid level.
The specific types of valves explored in this chapter are:
manual shutoff valves
manual expansion valves
check valves
solenoid valves
level controls
pressure-regulating valves
superheat controlling expansion valves
A discussion of safety relief valves appears in Chapter 13, Safety
11.2 MANUAL SHUTOFF VALVES
A basic valve type distributed liberally throughout an industrial refrigeration
system is a manual shutoff valve. In the completely open position, this valve
should allow a free flow of refrigerant and when closed completely block the
flow. The usual function of the shutoff valve is to isolate a component or a
section of the system. Some major categories of manual shutoff valves are globe,
angle, inline, and ball valves, as shown schematically in Fig. 11.1. Three
desirable characteristics of manual shutoff valves are:
that they permit no pasغير مجاز مي باشدe of refrigerant when closed
that they cause only a low-pressure drop of refrigerant flowing through
them when they are open
that they do not leak to atmosphere
Several other types of valves are gate and butterfly valves which meet the lowpressure-
drop requirement, but in general do not seal as well as other valves
when closed. Consequently they are not widely used in industrial refrigeration
service. All valves shown in Fig. 11.1 have accessible handles, but in recent
years a strong preference has developed for capped valves when the valve does
not need to be opened and closed often.
Shutoff valves are oriented so that they close against the flow which usually
means that they close against the high pressure. With this orientation the
upstream pressure assists in the opening of the valve. If the valve must open
against high pressure, cases have been reported where the pressure holds the
disc with such force that the stem may pull away from the disc in an attempted
opening. When the valve closes against the flow the highest pressure is kept off
the stem and bonnet when the valve is shut off. Globe valves should be mounted
with the stem horizontal so that any vapor in the line cannot form a pocket at the
valve inlet which would periodically release, causing noise and unsteady flow.
Ball valves have become very popular in the past few years, primarily because
of the low pressure drop that they cause in their completely open position. A
further advantage of ball valves in certain situations is that they are quarterturn
valves so that a quarter turn of the handle permits quick opening or closing
of the valve. An undesirable characteristic of the basic ball valve is that of
trapping liquid within the ball when the valve is shut off. A ball valve in a cold
liquid line traps cold liquid inside the ball when the valve is closed, and this
liquid is likely to warm up when the flow is interrupted. The trapped liquid
expands which could blow out the valve seat or even rupture the valve body.
Two methods1 used most commonly to relieve pressure of trapped liquid in the
ball and prevent damage are upstream-venting and self-relieving seats. In upstream
venting a small hole is drilled through one side of the ball, connecting the upstream
line with the cavity when the valve is in its closed position. This configuration
bypasses the upstream seat, and provides a continuous vent path for cavity pressure.
In the self-relieving seat design, the seats act as internal relief valves to open a
vent path from the valve body cavity to the line. Self-relieving valve seats serve
as normal valve seats unless the pressure within the ball rises to an extreme
level, in which case they permit leakage of a few drops of liquid.
Judgment should be used in whether and what type of valves should be
incorporated in the lines. Even valves that are rarely shut off may be invaluable in
isolating a certain component or even another valve on rare occasions. On the
other hand, extra valves, particularly those placed in vapor lines, may represent a
persistent demand for extra compressor power when ever the system operates.
One estimate2 calculated that a fully open valve in the liquid/vapor line between
the evaporator and the low-pressure receiver causing 7.5-kPa (1.1-psi) pressure
drop could add ,400 to the annual operating غير مجاز مي باشدt of a 2100 kW (600 ton) system.
A fully open ball valve would add only to the annual operating غير مجاز مي باشدt.
Some pressure drop is expected when refrigerant flows through open valves,
and this pressure drop adds to the pressure drop occurring in straight sections of
pipe. Methods for calculating the pressure drop in straight pipes were presented
in Chapter 9, and Table 11.1 provides data for computing the pressure drop in
fittings and valves. Table 11.1 gives the values of the c-terms in the equation
An important observation that can be made from Table 11.1 is the relative
pressure drops of globe and angle valves. An angle valve causes anywhere from
1/2 to 1/8 the pressure drop of a globe valve (depending upon the valve and pipe
size) and should be considered if the physical arrangement permits. The
pressure-drop coefficients for ball valves are likely to be approximately the
same as gate valves.
11.3 MANUAL EXPANSION OR
BALANCING VALVES
Manual regulating valves are designed to adjust the flow rate through their entire
stem travel. Shutoff valves, on the other hand, are not intended for use as regulating
valves since they provide most of their regulation in the first turn of the valve from
its closed position. Two frequent applications of manual expansion or balancing
valves are at the evaporator coils of liquid-recirculation systems and in conjunction
with on-off liquid level control valves, as illustrated schematically in Fig. 11.2. In
the liquid recirculation system of Fig. 11.2a the function of the valves is to throttle
the flow rate to coils whose unthrottled coil-and-piping circuit has a lower pressure
drop than others. The liquid supply pressure ahead of the valves is increased,
which diverts liquid to those coils that otherwise might be inadequately fed. In
liquid recirculation systems, the drop in pressure through the valve is small
relative to that occurring when an expansion valve separates condensing and
evaporating pressures.
Valves regulating liquid flow into vessels where the level is controlled are
often electrically operated on-off valves. Such solenoid valves are combined
with manual control valves, as illustrated in Fig. 11.2b, to prevent wild
pressure fluctuations in the vessel as the solenoid valve opens and closes.
Pressure drop occurs in both the manual valve and the solenoid valve, even
when it is open, but approximately 2/3 of the pressure drop should be taken in
the manual valve.
11.4 CHECK VALVES
Check valves allow the flow of refrigerant in only one direction by automatically
closing when fluid attempts to flow in the opposite direction. The moving element
may be assisted by gravity so that the valve closes unless a slight pressure drop
in the permitted flow direction opens the valve, as in Fig. 11.3a. This swing
type of check valve is not as reliable as the spring-actuated valve of Fig. 11.3b,
but this check valve imposes a pressure drop because of the spring pressure. A
more sophisticated check valve is the gas-powered type in which the valve is
normally open and held open by spring force. When the downstream pressure
rises above the entering pressure to the valve, vapor from a high-pressure source
closes the valve, which remains closed as long as the downsteam pressure exceeds
the upstream pressure.
11.5 SOLENOID
VALVES-DIRECT-ACTING
Solenoid valves are electrically operated shutoff valves. Probably the most
common is the normally closed (NC) valve, but normally open (NO) valves are
also available4. With both types, system pressure works to keep the valve closed
when that position is desired. Solenoid valves thus can hold against high
upstream pressures, but will not restrain much pressure in the reverse direction.
Two classifications of solenoid valves are:
1. direct-acting
2. pilot-operated (to be discussed in the next section)
In the direct-acting solenoid valve, as shown in Fig. 11.4, the magnetic force
developed by the electric coil draws the stem and connected plunger off the valve
port when the coil is energized. Some solenoids are designed to allow the stem to
start its motion before engaging the plunger which is seated against the system
pressure. The momentum of the stem thereby helps open the valve. When the
coil is de-energized, the plunger either drops into the closed position by gravity,
and/or a light spring assists the closing.
A solenoid valve must be selected to be able to open against the maximum
operating pressure differential (MOPD), a characteristic of the valve listed in
the manufacturer’s catalog. Because direct-acting valves must generate sufficient
force in the coil to open the valve against the system pressure, they are limited
to line sizes of perhaps 6 to 25 mm (1/4 to 1 in).
11.6 SOLENOID
VALVES—PILOT-OPERATED AND
GAS-PRESSURE-OPERATED
To make the solenoid valve practical for large pipe diameters, concepts must be
applied other than direct force on the valve stem from the magnetic coil. Two
approaches are pilot-operated and gas-pressure-operated valves. In the pilotoperated
valve, a small solenoid opens to apply high upstream pressure on a
power piston. The power piston has a greater area than that of the valve plug,
which is also subjected to the upstream pressure, so the force of the power
piston opens the valve. One design of a pilot-operated solenoid5 is shown in Fig.
11.5 where the opening of the pilot solenoid permits upstream pressure at M to
pass through the pasغير مجاز مي باشدe N to the power piston.
The pilot-operated solenoid valve requires a minimum pressure drop across
it, even when completely open. There are situations where the pressure drop
through the open valve is to be kept as low as possible, such as in low-pressure
vapor lines, or in the liquid leg or the return line of a flooded evaporator. In
other situations, viscous oil may be coating the moving parts of the valve, which
requires more positive action to open and close the valve. The gas-pressureoperated
valve5, such as ahown in Fig. 11.6, may be considered in such cases.
The construction is similar to the pilot-operated solenoid, except that pressure
from the condenser or another high-pressure source is applied to the power
piston. Usually a pressure of the high-pressure gas of 69 kPa (10 psi) higher
then the inlet pressure to the valve is adequate to open the main valve. When
the valve is to be opened, the pressure pilot solenoid opens to allow the highpressure
gas access to the power piston. To close the valve, the pressure pilot
solenoid valve closes and the bleed pilot solenoid valve opens. With this valve
status, the pressure above the power piston vents to the low-pressure side,
permitting the spring to close the main valve.
11.7 GAS-POWERED SUCTION STOP
VALVES
In low-temperature installations the shutoff valve between the evaporator and
the suction line should offer as little flow resistance as possible during
refrigeration. A normally-open valve meets this requirement, but in addition
the valve must be capable of closing positively, for example, during a hot-gas
defrost. A valve designed to meet these requirements, and first shown in Fig.
6.49 among the defrost piping arrangements, is the gas- powered suction valve,
as shown in Fig. 11.7. If the external gas pressure is removed, the pressure
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