The Professor: Low Ambient Controls Reduce Head Pressure
Control Is a Style of Head-Pressure Control Valve
When refrigeration systems are exposed to low ambient conditions, the condensing pressure (head pressure) will fall. If the head pressure gets too low, the metering device will not have enough pressure drop across it to operate properly. The metering device will underfeed the evaporator and a low pressure will result in the evaporator. This can cause refrigeration systems to short cycle from the low-pressure control, opening prematurely. Also, once the refrigeration system is off, it may never turn back on again because the evaporator pressure may never reach the cut-in pressure of the low-pressure control. Condensing units that are exposed to low ambient conditions must have some sort of low ambient control installed on them to prevent their head pressures from falling too low.
Low ambient control (Figure 1) is a style of head-pressure control valve for low ambient conditions that has a round pressurized dome at its top that is pressure charged. This valve is often referred to as a low ambient control (LAC) valve. It is used in a refrigeration system. The dome charge of the LAC is independent of the refrigerant charge in the actual refrigeration system. The dome charge will expand and contract in volume and act on an internal diaphragm as the outside ambient changes. This happens because the entire valve is located in the condensing unit, which is exposed to an outside ambient temperature.
This expanding and contracting of the pressure charge will move a diaphragm, which in turn will move a piston in the valve and modulate the valve either in a more open or closed position. The dome charges can also be custom-ordered from the manufacturer to meet specific system requirements. This often allows these head-pressure control valves to be used on “floating head-pressure” systems. Floating head pressure means letting the condenser pressure follow the ambient temperature to a minimum low pressure while still having enough pressure drop across the metering device to properly feed the evaporator.
When the condensing unit is exposed to an ambient temperature above 70˚F, the refrigerant flow from the compressor’s discharge is directed by the mixing valve through the condenser and into the receiver. However, if the outside ambient temperature drops below 70˚, the condensing pressure will fall and the pressure of the liquid coming from the condenser will also fall to a point below that of the bellows in the dome of the valve. This causes a piston to move in the valve and partially restrict the flow of refrigerant leaving the condenser. The condenser will now partially flood with liquid refrigerant at its bottom to maintain a condensing pressure for that certain ambient. The colder the ambient, the more condenser flooding.
At the same time, some hot discharge gas from the compressor will bypass the condenser and flow directly to the receiver through the LAC valve. This hot superheated discharge gas going to the receiver mixes with liquid refrigerant coming from the bottom of the condenser.
This mixing is done by the LAC valve. This keeps the receiver pressure up and helps keep metering devices fed with refrigerant. The receivers on systems having head-pressure control valves must be sized large enough to hold the normal operating charge plus the additional charge that is necessary to totally flood the condenser in wintertime operations.
In fact, the receiver should be sized so that they are about 80 percent full when they contain the entire system charge. This allows for a 20 percent vapor head for safety when pumping the system down. Any refrigeration system’s receiver should be able to hold all of the system’s refrigerant charge and still have a 25 percent vapor head for safety. Some manufacturers publish charts and tables on system charge recommendations showing the service technician how much refrigerant to add to systems having head-pressure control valves designed to flood the condenser in colder weather. Today, these tables and charts can be accessed on the Internet by a service technician with a personal computer.
If the receivers of a refrigeration or air conditioning system haven’t been oversized to accommodate the extra refrigerant for flooding the condenser during low-ambient conditions, the service technician will have to remove refrigerant from every spring to prevent high head pressures at a design ambient, only to add it back again in the fall when it will be required for flooding the condenser for head pressure control. This technique is often referred to as a winter/summer charge procedure. However, over-sizing the receiver can save this added labor charge.
Troubleshooting a low ambient head-pressure control valve with a pressurized dome can be easy if the proper steps are taken. If the outdoor ambient is below 70˚ and the head pressure is low, feel the line between the LAC valve and the receiver. If this line is cold, the valve is not bypassing the compressor’s discharge gas into the receiver. The valve is defective and should be replaced. This line should be warm at an outdoor ambient temperature below 70˚.
If the head pressure is low, the ambient is above 70˚, and the line between the valve and the receiver is hot, the valve is defective and should be replaced. The line should be a little warmer than the ambient because of the subcooled liquid coming from the condenser’s outlet to the receiver at the warmer outside ambient. Discharge gas flow from the receiver should be shut off by the valve. However, the refrigeration system could also be low on charge causing a low condensing pressure and fooling the valve. This would cause the valve to bypass compressor discharge gas to the receiver. So, after verifying that the system charge is correct, only then replace the defective valve.
LAC Valve Replacement
To replace a defective low-ambient control valve with a pressurized dome charge, follow the steps below:
1. Recover the refrigeration system’s refrigerant;
2. Cut the process tube on the valve’s charged dome to remove the pressure charge (wear safety glasses);
3. With a torch, heat the valve until the solder melts;
4. Remove the valve;
5. Wrap the replacement valve’s body with a heat sink to prevent damage;
6. Silver solder a new valve in place;
7. Install a new filter drier;
8. Leak check the system and evacuate to a deep vacuum (500 microns);
9. Charge the system with refrigerant; and
10. Leak check again.
Notice the LAC valve has marking C, D, and R. These stamped markings on the LAC valve will help the service technician identify the ports on the valve so no mistakes can be made when installing a new valve. C stands for condenser, D stands for discharge line of the compressor, and R stands for receiver.
Publication date: 1/6/2014