The use of piston chiller compressors is becoming less common in high-efficiency chilled water plants. However, these chillers can still be found, particularly in low-capacity systems and older installations with medium thermal loads.
How Does a Piston Chiller Work?
A piston chiller operates using a conventional refrigeration compression cycle.
- Compression Process:
The refrigerant vapor is compressed by a piston located within a cylinder. A thin layer of oil prevents refrigerant vapor from escaping the compression chamber, lubricates the system, and reduces friction. The piston is connected to the crankshaft via a connecting rod. As the crankshaft rotates, the piston moves back and forth within the cylinder. This reciprocating motion draws refrigerant vapor into the cylinder, compresses it, and discharges it to the chiller’s condenser. - Valve Operation:
The piston compressor has two valves: a suction valve and a discharge valve.- During the suction stroke, the piston moves away from the discharge valve, creating a vacuum and reducing the cylinder pressure below that of the evaporator. This pressure difference forces the suction valve to open, allowing refrigerant vapor to enter the cylinder.
- During the compression stroke, the piston reverses direction and compresses the refrigerant vapor, increasing the cylinder pressure. As the piston continues its stroke, the refrigerant is compressed to a pressure higher than that of the condenser, causing the discharge valve to open and releasing the compressed refrigerant to the condenser.
Capacity Control in Piston Chillers
To maintain optimal operation, it is essential to regulate the capacity of the piston compressor, especially when the cooling load decreases or outdoor temperatures are low.
- Challenges During Partial Load Conditions:
Without proper capacity control, the piston compressor operates at a lower evaporation temperature, which can lead to freezing of the water passing through the chiller’s evaporator. The evaporator temperature decreases during partial load conditions because the compressor’s vapor suction rate exceeds the evaporation rate in the evaporator. - Hot Gas Bypass Regulation:
Some piston chillers use a hot gas bypass valve located in the compressor’s discharge zone. When the evaporator pressure falls below a predetermined value, this valve opens, redirecting some discharge refrigerant back to the suction side. This bypass simulates a thermal load to maintain the compressor’s nominal design capacity.- Advantages and Limitations:
This method allows precise capacity control but offers little to no energy consumption reduction during reduced cooling demand. Additionally, improper application may result in excessive suction gas overheating, causing compressor overheating. Sending the hot bypass gas to the evaporator inlet can help reduce compressor overheating and maintain sufficient refrigerant velocity to return oil to the compressor.
- Advantages and Limitations:
- Cylinder Unloading:
Larger piston compressors, typically above 10 tons (35 kW), are equipped with cylinder unloaders. These devices adjust refrigerant flow capacity to match the application requirements.- In response to decreasing load, an electronic controller activates a solenoid valve, diverting pressurized discharge refrigerant to the top of specific cylinders. This action prevents the refrigerant vapor from entering the suction side of those cylinders. While the pistons continue to move, they no longer perform compression.
