Refrigerating Specialties Division
4
pressure. (Normally this is known as a “Reverse Acting”
thermostat-controller.) The decrease in air pressure will lower the regulator
set-point and produce a lower inlet pressure and lower evaporator or
condenser temperature. Conversely, a drop in temperature at the
thermostat must cause an increase in air pressure, with a resultant
increase in evaporator or condenser temperature. Usually, the controller
modulates the air pressure from 0.21 to 1.0 bar (3 to 15 psig) throughout
its control range.
The A4WP can be used as a Differential Pressure Regulator if the
regulator outlet pressure is connected to the bonnet.
Principles of Operation
The Type A4WP operation is the same as the A4W described above
except that the set-point is changed (compensated) on a 1:1 basis by
the pressure that is fed into the bonnet.
If the A4WP is part of a pneumatically operated control system, the air
must be clean, dry and oil free. To avoid the possibility of moisture from
compressed air freezing in the bonnet or in other parts of the control
system, a dehydrated air system must be used whenever it may come in
contact with sub-freezing temperatures. This is particularly true if the
regulator is controlling suction gas flow at temperatures below freezing.
Adjustment
If the A4WP is part of a pneumatically operated control system. the
controller must be adjusted according to the manufacturers instructions
to obtain the optimum system performance. To adjust the pressure
regulator. disconnect the air line and follow the instructions, above listed
for the Type A4W. This setting represents the lowest inlet pressure the
regulator will allow, thus providing a low limit feature to the regulator.
Next, connect the air line. From this point the regulator’s set-point will be
increased on a 1:1 ratio with the air pressure increase.
If the A4WP is compensated by a remote refrigerant pressure, the set-
point of the regulator (the inlet pressure it will be controlling) will be
equal to the sum of the remote pressure and the pressure equivalent to
that made by the range spring adjustment. If the remote pressure is the
regulator outlet pressure, then the valve will control an inlet pressure
equal to the outlet pressure plus the Range Spring setting. Or, said
another way, the Regulator will be a Differential Pressure Regulator
because the set-point will be the difference in pressure across the
regulator.
Type A4WOE (See Fig. 2-2,2A)
Description
The Type A4WOE is an Outlet Pressure Regulator with an external, field
installed connection to the downstream (outlet) pipe of the regulator. This
connection is not furnished with the regulator. The regulator will tend to
open on a drop in outlet pressure below set-point and will tend to close on
a rise in outlet pressure above set-point.
Although not described in this bulletin, the A4WOE is also available in
combination with the “S”, “B”, “D” and “P” features described elsewhere.
Purpose
The Type A4WOE will modulate flow of refrigerant fluid to maintain a
constant downstream pressure as set-for, despite fluctuations in load.
However, once closed, the regulator can do nothing further to reduce
downstream pressure. This reduction must come from system capacity.
The regulator cannot maintain set-for pressure if uncontrolled blanch lines
feed into the main pipeline downstream of the A4WOE Regulator.
Typical applications are as a hold back or crankcase pressure regulator to
prevent pressure rise in a suction main, or to prevent too low a plant
suction pressure by putting an artificial load on the main from a higher
pressure source.
Principles of Operation
The outlet pressure is sensed under the Diaphragm through the external
tube which has been installed in the field to the outlet pipe downstream of
the main valve. When the force created by the outlet pressure acting
under the Diaphragm is less than the force of the Range Spring, the
pilot is open, allowing upstream pressure from passage N to flow to the
top of the Piston. This causes the Piston to force the Modulating Plug to
open to maintain a constant outlet pressure. A decrease in the outlet
pressure allows the Range Spring to open the pilot further, allowing
more pressure on top of the piston and opening the Modulating Plug
further.
An increase in outlet pressure will lift the Diaphragm against the force of
the Range Spring, allowing the Pilot Plug to start to close. The pressure
on top of the Piston is decreased and the Closing Spring acts to reduce
the opening of the Modulating Plug and the flow of fluid through the
regulator.
The pressure on top of the Piston is controlled by the flow through the
Pilot Plug and the bleed through the bleed hole in the Piston. Due to the
Seal Ring fitted to the Piston, the amount of bleed to the outlet of the
valve, as a result of blow-by of the Piston, is very small. A minimum pressure
drop of 0.14 bar (2 psi) across the valve is required to open it fully.
The Type A4WOE Outlet Pressure Regulator opens on a drop in outlet
pressure below set-point and closes on a rise in outlet pressure above its
set-point. The outlet pressure set-point is not appreciably affected by
variations in the inlet pressure.
Adjustment
See Outlet Pressure Regulator Adjustment on page 2.
Type A4WR (See Fig. 6)
Description
The Type A4WR is a main valve only intended for pilot operation by one or
more remote piloting devices, such as Type S6N Solenoid Valve, Type
A2B (inlet) or Type A2BO4E (outlet) Pilot Regulators. The valve is
complete with a Manual Opening Stem but has no integrally mounted
pilot devices to provide any type of control function. A plain Cover with
a 3/8" FPT connection is in the position otherwise occupied by the pilot
devices.
The Type A4WR uses upstream pressure which is externally fed to the
A4WR through pipes (not included with valve) field installed to provide
the moving force for the valve operation. A minimum 0.14 bar (2 psi)
pressure drop is required to fully open the valve. The upstream pressure
must be fed from a connection field installed in the main valve inlet pipe.
There is no connection on the valve body for this purpose.
A small flow of refrigerant passes through the line connected to the
upstream side of the valve so care must be used not to create a pressure
drop by undersizing the pipe or using too great a length. Normally
standard 3/8" US pipe size no longer than 6m (20 ft.) will be adequate.
Consult the factory if a greater length is required.
Purpose
The Type A4WR should be used where there is an advantage to install
all of the control modules at some remote location. Frequently this is
done to make adjustment and servicing easier, or to fit a large valve in
a space that would be too small for a complete valve.
Principles of Operation
Since the Type A4WR is a main valve only and may be used with an
assortment of remote devices: the reader is referred to the most recent
editions of the bulletins describing each of those devices. The most
common of these are: Type S6N Solenoid Valve - Bulletin DN30-90;
Type A2B and Type A2BO4E Regulators - Bulletin DN21-02.
Fig. 2-2 – A4WOE
