2.5 Access Valves
A temporary access valve can be used to service or evaluate the system. The access
valve can be installed on the compressor’s process tube (this will also be a low-pressure
side). Be sure to cap off access valve while servicing. This will prevent contamination
of the system and/or refrigerant from leaking.
After servicing, the access valve should
be removed.
A pinch-off tool can be used to close the system to remove the access valve
and then braze the hole for the access valve to seal the system. Be sure to leak check
after brazing.
2.6 Pressures and Temperatures
There are three ways to measure the temperature of the evaporator plate.
1. Use a thermocouple to measure the temperature of the evaporator. The thermocouple
must be secured to the evaporator. The best and accurate location for the
thermocouple end is where the thermistor is secured to the evaporator. Allow
anywhere from 5
°
-8
°
F. of variation in measured temperatures if you measure on the
plate because of the thermal conductivity of stainless steel thermistor in comparison
to the thermocouple.
2. If it has been determined that there is proper contact between the thermistor and the
evaporator plate, the thermistor resistance value can be corresponded to the
temperature (see section 5.2 for values).
3. If it has been determined that there is proper contact between the thermistor and the
evaporator plate, the voltage at the thermistor terminals can be read. The voltage can
be read while the unit is running be attaching test leads to the thermistor terminals at
the electronic control. The temperature to switch into the harvest cycle is 11.0
°
F. and
the corresponding voltage is 3.0 – 3.1 V. The temperature to switch into the
production cycle is 45
°
F. and the corresponding voltage is 1.9 – 2.0 V. Slight
variations in actual measurements can occur due to input voltage variations.
Note: The temperature and pressure readings must be taken with unit on and water
flowing over the evaporator plate (in the production cycle). Use gage pressure
readings from the compressor’s process tube (low side) access valve.
Refrigeration Temperature-Pressure Chart for R-134A
Degree F.
Pressure (psi) R-134A
Degree F.
Pressure (psi) R-134A
-12
1.1
36
31.3
-8
2.8
38
33.2
-4
4.5
40
35.1
0
6.5
42
37
2
7.5
44
39.1
4
8.5
46
41.1
6
9.6
48
43.3
8
10.8
50
45.5
10
12
52
47.7
13
