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Oil Temperature Regulating Valves
The purpose of this specification is to describe our preferred method of oil temperature control for
screw compressors.
1.
Introduction
Oil temperature control is necessary to keep the compressor operating temperature within
the design range as the suction, discharge and capacity conditions vary.
Our standard screw compressor pack oil cooling system uses a pumped glycol loop through
the condenser and oil cooler. In this case we normally allow the glycol temperature to vary,
depending on the condenser operating condition. We then use a self-regulating 3-port
mixing valve to control the oil feed temperature to the compressor. The same oil temperature
control system can also be used with a water cooled oil cooler or a thermosyphon (refrigerant
cooled) oil cooler which has a continuous uncontrolled liquid supply.
2.
Valve Details
The 3-port mixing valve consists of a cast steel body with an internal thermostatic element
(Figure 1). The compressor oil supply temperature (leaving the valve at port XY) is regulated
by varying the proportion of oil bypassing the oil cooler (entering the valve at port Y) and
flowing through the oil cooler (entering the valve at port X).
The thermostatic fluid inside the thermostatic element expands as it is heated, thus tending
to close port Y and open port X. This gives good stable temperature control across a
specified temperature range of, typically, 10K. Thus a valve with a nominal design control
setting of 48C will bypass all the oil around the cooler when the compressor supply
temperature falls to 44C and will cause all the oil to flow through the oil cooler when the
compressor supply temperature rises to 54C. Different control temperatures are available
(typically 100F, 110F, 120F, 130F and 140F) but this must be specified when ordering.
The 120F (control range 44C to 54C) valve works well on most applications. Selection
programs tend to predict higher discharge temperatures than we actually see. If it is
necessary to change the control temperature, then the thermostatic element can be changed.
This type of valve is particularly suitable for refrigeration applications as it has no external
moving parts and therefore no external dynamic seals.
3.
Arrangement and Selection
The 3-port valve will be used as a mixing valve, with the piping arranged as shown in Figure
2. A single valve has been used, successfully, to control the supply oil temperature to the
compressors on triplex compressor packs. This is against valve manufacturer’s
recommendations as the control is not as good (at less than half the design flow rate),
however it has proved successful in our application, probably due to slow or infrequent
changes in the operating conditions.
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It has not been necessary to fit a regulating valve or orifice in the oil cooler bypass line
because the pressure drop through the (shell and tube) oil cooler is typically less than the
pressure drop through the valve so good control can be maintained. The valve should be
selected for each application as the correct valve will often be smaller than the oil line size.
Star have used both Thermoreg and Amot valves, but Thermoreg is our preferred option.
We are currently testing Hansen Technologies HOTV valves and may use them again in the
future.
Thermoreg Valve Selection Table
Valve
Size
20NB
25NB
40NB*
40NB
50NB
65NB
80NB
Order
Code
M20S3D-TEMP-HA
M25S3D-TEMP-HA
M40TS6D-TEMP-HA
M40TS6D-TEMP-HA
M50TS8L-TEMP-24
M65S8L-TEMP-24
M80S8L-TEMP-24
Maximum Oil Flow
(m3/hr)
5.2
5.5
12.5
15.0
27.0
48.0
55.0
* Includes reducing bush – must be noted on order.
20NB and 25NB series have ¾” and 1” NPT screwed ports respectively. All others are
PN40 flanged.
The above oil flows are based on a maximum allowable pressure drop across the valve of
0.4 Bar. Reduce the allowable flow by 14% for an allowable pressure drop of 0.3 Bar and by
29% for an allowable pressure drop of 0.2 Bar.
The oil flow figures are valid across the normal oil viscosity range for screw compressors.
The above valves have a design pressure of 25 Bar G and a maximum test pressure of 40
Bar G.
They are suitable for use with both ammonia and fluorinated refrigerants.
3.1
Valve Port Designation
Different manufacturers use different port designations, even within their own range, so the
Design Engineer must check the port arrangement and label the valve on the Diagram of
Connections or Lubrication diagram.
3.2
High Evaporating Temperature and Low Superheat Due to Compression
A special case exists with high evaporating temperature with certain refrigerants (such as
R404A and R507) in that low discharge superheat causes large amounts of refrigerant to be
absorbed into the oil due to miscibility. In this case the compressor selection must be
checked to ascertain whether or not the rotor oil feed (as well as the bearing and shaft seal
oil feed) should be cooled. Figure 3 shows an oil circuit for a screw compressor using R507
at an evaporating temperature of +5C.
Date
6.1.00
21.3.00
Issue
DRAFT
1
March 2000
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AMG
AMG
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