Parallel Compressor Systems
Liquid Drain Control Method
This method is ideal for large capacity systems since a smaller regulator is required for liquid line than for discharge line.
During warm ambient temperature conditions valves A and C will be open and Valve B will be closed. When the ambient temperature at the condenser drops, the condenser pressure will tend to become lower. As this pressure is reduced, when the pressure becomes as low as its setting, Valve A will begin to close, thus causing the refrigerant to back up inside the condenser tubes, reducing the condensing surface and allowing the pressure to be maintained. As Valve A closes, the receiver pressure may be reduced by the cold entering liquid to a level below the setting of Valve B, which will begin to open to bypass sufficient gas to maintain the receiver pressure at the set point of Valve B. Check valve C will prevent the high pressure from backing up to the condenser when the receiver pressure is higher than the condenser pressure, as would be the case during shutdown in a system with a warm receiver.
When the condenser pressure builds up to the setting of Valve A, it will open allowing liquid to flow to the receiver.
To describe it again, during cold ambient temperature conditions this liquid will be considerably subcooled and will tend to lower the receiver pressure. Valve B will sense the drop in pressure and open to admit hot gas into the drain line, thus pressurizing and warming the liquid and maintaining the receiver pressure.
Recommended Valve Settings
Regulators ordered for a Condenser Pressure Control system for use with common refrigerants will be furnished with the ranges and factory settings shown in Table 29. Turning the adjusting stem in (clockwise) will raise the set point; turning the stem out (counterclockwise) will lower the set point. See Table 29 for ranges and amount of pressure change per turn.
Table 29. Pressure Range, Set Point & Charge Per Turn
Valve |
| Range | Factory | Change Per | ||||
| Set Point |
| Turn | |||||
Type |
|
|
|
| ||||
| psig | kg/cm2 | psig | kg/cm2 | psig |
| kg/cm2 | |
|
|
| ||||||
A4A | D | 75 to | 5.2 to | 140 | 9.8 | 53 |
| 3.7 |
ARA0 |
| 280 | 19.7 |
|
|
|
|
|
A7A |
| 75 to | 5.3 to |
|
|
|
|
|
A7A1 | D | 120 | 8.4 | 75 |
| 5.3 | ||
A72 |
| 280 | 19.7 |
|
|
|
|
|
|
|
|
|
|
|
|
| |
A9 | B | 80 to | 5.6 to | 120 | 8.4 | 25 |
| 1.8 |
|
| 220 | 15.5 |
|
|
|
|
|
Field Adjustment
Before final field adjustment of regulators for Condenser Pressure Control, the following should be done:
1.Install gauges to read compressor discharge, condenser and receiver pressures.
2.Fully charge the system.
3.Have other controls and components functioning properly.
4.Have the system as fully loaded as possible.
5.Final adjustments should be made when the outdoor ambient is below 65°F. Ideally the ambient should be near minimum system outdoor temperature to allow the system pressures to drop below the regulator pressure settings. Before adjusting make sure that all manual opening stems on the regulators are set for automatic operation. It may be necessary to temporarily deactivate the low pressure cut out controls to keep the compressors running during adjustment.
6.The regulators should be preset using information in Table 29.
7.When the valves are adjusted with the system operating, enough time must be allowed for the system to stabilize. Check the sight glass to make sure sufficient liquid is supplied to the evaporators.
8.Turn the condenser fans off to allow the discharge pressure to build up. The regulator A should be adjusted to open when the pressure reaches the desired control point. Listen for flow through regulator A while watching the pressure gauge.
To determine the final setting of regulator B, allow the condenser fans to run long enough to subcool the liquid supplied to the receiver. Adjust the regulator until the desired receiver pressure is obtained. Flow through the valve can be determined by listening at the valve for gas flow or by feeling the outlet for change in temperature. The setting of regulator B should be at least 10 psi lower than the setting of regulator A.
Hot Gas Bypass Regulator Adjustment
Discharge Bypass Valves (DVB) respond to changes in downstream or suction pressure. When the evaporating pressure is above the valve setting, the valve remains closed. As the suction pressure drops below the valve setting, the valve responds and begins to open. As with all modulating type valves, the amount of opening is proportional to the change in the variable being controlled - in this case the suction pressure. As the suction pressure continues to drop, the valve continues to open farther until the limit of the valve stoke is reached.
On refrigeration systems discharge bypass valves are used to prevent the suction pressure from going below the minimum value determined by the job requirements.
Sporlan Valve Company
Valve Setting and Adjustment
A complete discussion on valve settings is given in Sporlan Application Bulletin
To adjust these valves, remove the cap an turn the adjustment nut with a 5/16” hex wrench for fully adjustable models
Parallel Compressor Systems Installation & Operations Manual, October 2004 | 31 |
