Trane VAV-SVX02B-EN User Manual
Installation/
Operation
Maintenance
VariTrane
™
Pneumatic Controls
March 2002
VAV-SVX02B-EN
Thermostat
20
(137.9)
(103.4)
Thermostat Selection
and Location
In general, both one- and two-pipe
thermostats have a gain of 2.5 psi per
degree Fahrenheit, unless special
thermostats are used that provide a
different gain. The major difference
between them is that a one-pipe
thermostat is considered a lowcapacity thermostat and a two-pipe
thermostat is considered a highcapacity thermostat.
When selecting between one-pipe and
two-pipe thermostats consider the
following:
Use one-pipe thermostat when:
tubing runs are less than 50 feet from
thermostat to controller.
a single thermostat will control no
more than three controllers.
slower response is desired from the
thermostat.
Use two-pipe thermostat when:
tubing runs exceed 50 feet from
thermostat to controller.
a single thermostat will control four or
more controllers.
high capacity air is required due to
restrictions in the air line.
Thermostat Piping
The two-pipe thermostat (Figure 1)
includes a restrictor (internally). Attach
supply air line and output pressure
(branch) line to the thermostat.
The one-pipe thermostat (Figure 2)
requires an external restrictor tee.
Connect main air supply to restricted
leg of restrictor tee. Connect branch line
from restrictor tee to the thermostat.
Figure 1–Two-Pipe Remote Thermostat (Reverse-Acting)
Tee
S
Figure 2–Typical One-Pipe Thermostat (Reverse-Acting)
Restrictor
Tee
Tee
Restricted Leg
S
20
Fan
(137.9)
P.E.
Switch–1
(N.O.)
9 PSI
(62.06 kPa)
Two-Pipe
Remote-Mounted
T-Stat
(Reverse-Acting)
One-Pipe
Remote-Mounted
T-Stat
(Reverse-Acting)
2
VAV-SVX02B-EN
Thermostat
Specifications
TP970 Series Pneumatic T-stats
Maximum Safe Air Pressure:
25psi (172 kPa)
Maximum Safe Air Temperature:
150°F (66°C)
Throttling Range:
2–10°F (1–5°C)
Gain:
2.5 psi/°F Adjustable
Operation
Pneumatic thermostats modulate
output air pressure in response to room
air temperature. Pneumatic thermostats fall into two categories: directacting or reverse-acting. This section
describes how thermostat output
pressure responds to changes in room
temperature.
Figure 3 graphically illustrates the
relationship between room
temperature and output pressure of a
direct-acting thermostat. The graph in
Figure 3 shows that as the room
temperature increases, so does the
output pressure of the thermostat.
Figure 3–Direct-Acting
Thermostat Response
Output Pressure
Figure 4 illustrates the pressure/
temperature characteristics for a
reverse-acting thermostat. As room
temperature increases, the thermostat
output pressure decreases.
Figure 4 –Reverse-Acting
Thermostat Response
Output Pressure
Input Temperature
A thermostat in proper working order
will be able to output a pressure
ranging from approximately .5 psi to
supply air pressure depending on
distance from indicated setpoint.
Input Temperature
VAV-SVX02B-EN
3
Reversing Relay
Installation and Operation
The reversing relay is a proportional,
non-bleeding device for use in pneumatic control systems where a proportional signal from a controlling device
must be reversed (see Figure 5). The
most common use of the reversing
relay is to change the action of a
pneumatic thermostat.
Figure 5 –Pneumatic Reversing Relay
Trane Part No. RLY-755
The reversing relay requires three
piping connections: main/supply air,
input signal air, and output branch air
(see Figure 6). The branch line pressure
decreases in direct proportion to an
increase in the input signal pressure
and vice-versa. The input signal is also
amplified, thereby minimizing any
pneumatic line transmission lag
(see Figure 7).
Input pressure to the relay and output
pressure to the PR should always add
up to a constant, which is usually set
between 16 and 25 psi. For example,
with a constant of 18, six-pound input
signal will result in a 12 pound branch
signal. A bias adjustment screw is
provided to retard or advance the out
signal.
Figure 6 –Reversing Relay Connections
Output
S
Branch
20
(137.9)
Input
Branch
Figure 7 – Reversing Relay Characteristics
15
Inc.
12
3
Output Branch Pressure
3
Input Pressure Signal
B
Reversing
M
S
6
Relay
9 PSI In
(62.06 kPa)
9 PSI Out
(62.06 kPa)
15
Inc.
4
VAV-SVX02B-EN
Reversing Relay
Calibration
By using the “bias” adjustment on the
relay, the factory-set 8.0–9.0 “crossover” can be changed. To recalibrate,
the relay should be connected as
illustrated in Figure 8.
Pressure
Regulator
M
If the new desired “cross-over”
pressure is 11.0 psi, this must be applied
to port #3 by adjusting the pressure
regulator. The “bias” adjustment is then
turned until 11.0 psi is obtained on the
output gage. Turning the “bias” adjustment “CW” increases the out put;
“CCW” decreases it.
This 11.0 psi “cross-over” would yield
the following operational chart:
Chart
Figure 8 – Recalibrating Reversing Relay
Gage
M
1
C
3
New Cross-over pressure
Bias
Adjustment
Screw
Input: 1234567891011121314151617181920
Output: 2019181716151413121110987654321
Specifications
Maximum Pressure:
30 psig
Connections:
3/16" (4.& mm) nipples for ¼" (6.4 mm) OD
polyethylene tubing
Air Consumption:
0.5 SCFH (.236 L/M) @ 20 psig
Material:
ABS
Ambient Limits:
+40°F/+120°F operating (+4°C/+49°C)
Mounting:
In-line via air connections
Factory Settings:
9 psig in./9 psig out and 8 psig in./8 psig
out
Bias Adjustment:
+/-15 psi
Weight:
3 ½ oz
VAV-SVX02B-EN
5
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