One of the most important components of an HVAC system is the compressor. The compressor is a device that increases the pressure and temperature of the refrigerant, which is a fluid that transfers heat between the indoor and outdoor environments. The compressor has a crankcase, which is the lower part of the compressor that contains the crankshaft, pistons, connecting rods, and oil.
Crankcase pressure is the pressure inside the crankcase of the compressor. It is influenced by several factors, such as the speed of the compressor, the amount of oil in the crankcase, the temperature of the refrigerant, and the presence of any leaks or restrictions in the system.
Crankcase pressure is important for the performance and reliability of the compressor and the HVAC system. If the crankcase pressure is too high or too low, it can cause problems such as:
- Oil foaming: This occurs when the refrigerant mixes with the oil in the crankcase, creating bubbles that reduce the lubrication and cooling of the compressor parts. Oil foaming can lead to excessive wear, overheating, and damage to the compressor.
- Oil migration: This occurs when the oil in the crankcase moves to other parts of the system, such as the evaporator or the condenser, due to pressure differences. Oil migration can reduce the efficiency and capacity of the system, as well as cause oil slugging, which is the sudden return of oil to the compressor, resulting in noise, vibration, and possible damage to the compressor.
- Oil dilution: This occurs when the refrigerant dissolves in the oil in the crankcase, lowering the viscosity and lubricity of the oil. Oil dilution can also cause excessive wear, overheating, and damage to the compressor.
To prevent these problems, it is essential to maintain the proper crankcase pressure in the HVAC system. The ideal crankcase pressure depends on the type and size of the compressor, the refrigerant used, and the operating conditions of the system. However, a general rule of thumb is that the crankcase pressure should be about 10 to 15 psi above the suction pressure, which is the pressure at the inlet of the compressor.
To measure the crankcase pressure, a pressure gauge can be attached to the oil drain plug or the oil level sight glass of the compressor. The crankcase pressure can be adjusted by using a crankcase pressure regulator, which is a valve that controls the flow of refrigerant or gas into or out of the crankcase. The crankcase pressure regulator can be either internal or external to the compressor.
The following table summarizes some of the common causes and effects of high and low crankcase pressure in HVAC systems, as well as some possible solutions:
| Crankcase Pressure | Cause | Effect | Solution |
|---|---|---|---|
| High | High compressor speed | Oil foaming, oil migration, oil dilution | Reduce compressor speed, use a crankcase heater, install a suction accumulator |
| High | Low refrigerant charge | Oil foaming, oil migration, oil dilution | Check for leaks, recharge refrigerant, install a suction accumulator |
| High | High ambient temperature | Oil foaming, oil migration, oil dilution | Use a crankcase heater, install a suction accumulator |
| High | Restricted suction line | Oil foaming, oil migration, oil dilution | Check for kinks, bends, or blockages in the suction line, replace or repair the suction line |
| Low | Low compressor speed | Oil starvation, compressor damage | Increase compressor speed, use a crankcase heater, install an oil separator |
| Low | High refrigerant charge | Oil starvation, compressor damage | Check for overcharging, adjust refrigerant charge, install an oil separator |
| Low | Low ambient temperature | Oil starvation, compressor damage | Use a crankcase heater, install an oil separator |
| Low | Leaking crankcase | Oil loss, compressor damage | Check for cracks, holes, or loose fittings in the crankcase, replace or repair the crankcase |
