Thermostatic Expansion Valves

The reliability and performance of the refrigeration system is heavily dependent upon proper expansion valve adjust- ment. Therefore, the importance of maintaining the proper superheat cannot be over emphasized. Accurate measure- ments of superheat will provide the following information.

1.How well the expansion valve is controlling the refriger- ant flow.

2.The efficiency of the evaporator coil.

3.The amount of protection the compressor is receiving against flooding or overheating.The recommended range for superheat is 10 to 16 degrees at the evaporator. Sys- tems operating with less than 10 degrees of superheat:

a.Could cause serious compressor damage due to refrigerant floodback.

b.Removes working surface from the evaporator normally used for heat transfer.

Unit Start-Up (Continued)

With the unit operating at "Full Circuit Capacity", acceptable subcooling ranges between 14 F to 22 F.

Measuring Subcooling

1.At the liquid line service valve, measure the liquid line pressure. Using a Refrigerant 22 pressure/temperature chart, convert the pressure reading into the correspond- ing saturated temperature.

2.Measure the actual liquid line temperature as close to the liquid line service valve as possible. To ensure an accu- rate reading, clean the line thoroughly where the tem- perature sensor will be attached. After securing the sen- sor to the line, insulate the sensor and line to isolate it from the ambient air.

Note: Glass thermometers do not have sufficient contact area to give an accurate reading.

3. Determine the system subcooling by subtracting the ac- tual liquid line temperature (measured in step 2) from the saturated liquid temperature (converted in step 1).

Systems operating with superheat in excess of 16 degrees:

a. Could cause excessive compressor cycling on internal winding thermostat which leads to compressor motor failure.

b. Lowers the efficiency of the evaporator by reducing the heat transfer capability.

Tables are based on outdoor ambient between 65 & 105 F, relative humidity above 40 percent. Measuring the operat- ing pressures can be meaningless outside of these ranges.

Measuring Superheat

1.Measure the suction pressure at the suction line gauge access port located near the compressor.

2.Using a Refrigerant/Temperature chart, convert the pres- sure reading to a corresponding saturated vapor tem- perature.

3.Measured the suction line temperature as close to the expansion valve bulb, as possible.

4.Subtract the saturated vapor temperature obtained in step 2 from the actual suction line temperature obtained in step 3. The difference between the two temperatures is known as "superheat".

When adjusting superheat, recheck the system subcooling before shutting the system "Off".

Charging by Subcooling

The outdoor ambient temperature must be between 65 and 105 F and the relative humidity of the air entering the evaporator must be above 40 percent. When the tempera- tures are outside of these ranges, measuring the operating pressures can be meaningless.

Do not attempt to charge the system with the low ambient dampers and/or hot gas bypass operating (if applicable). Disable the low ambient dampers in the "Open" position (re- fer to the "Low Ambient Damper" section) and de-energize the hot gas bypass valves before taking performance mea- surements.

Low Ambient Dampers

Operation

Low Ambient Dampers are available as a factory installed option on 20 - 75 Ton units or can be field-installed. Damp- ers are used to extend the operation of these units from the standard operational temperatures to a minimum of 0 F without hot gas bypass or 10 F with hot gas bypass. (These values apply when wind speed across the condenser coil is less than 5 m.p.h. If typical wind speeds are higher than 5 m.p.h., a wind screen around the unit may be required.) By restricting the airflow across the condenser coils, saturated condensing temperatures can be maintained as the ambient temperatures change.

The low ambient modulating output(s) on the compressor module controls the low ambient damper actuator for each refrigerant circuit in response to saturated condensing tem- perature.

When the control has staged up to it's highest stage (stage 2 or 3 depending on unit size), the modulating output will be at 100% (10 VDC). When the control is at stage 1, the modulating output (0 to 10 VDC) will control the saturated condensing temperature to within the programmable "con- densing temperature low ambient control point".

The following Table gives the minimum starting tempera- tures for both "Standard" & "Low" Ambient units. Do not start the unit in the cooling mode if the ambient temperature is below the recommended operating temperatures.

	Minimum Starting Ambient (1)
	Standard (2)		Low Ambient
		with	without
Unit Size		HGBP	HGBP
20 & 40	55°	10°	0°
25 & 30	50°	10°	0°
70 - 130	45°	10°	0°
55	40°	10°	0°
50	35°	10°	0°
60	30°	10°	0°

Notes:

1.Min. starting ambients in degrees F and is based on unit operating at min. step of unloading & unloading & 5 mph wind across condenser.

2.With or Without HGBP