Occasionally while servicing refrigeration equipment, technicians may come across a system utilizing a water-cooled condenser instead of an air-cooled condenser. Water is a much better heat transfer fluid than air, as its specific heat value is 1 Btu/lb. compared to 0.24 Btu/lb. for air. However, the cost of operating a water-cooled system is generally higher, making it an uncommon choice in many refrigeration systems.
Water-cooled condensers are generally piped using a counterflow arrangement. The water entering the condenser first comes in contact with the liquid refrigerant leaving the condenser, and the water leaving the condenser comes in contact with the refrigerant entering the condenser. This allows the cooler refrigerant to be exposed to the coldest water and the warmer refrigerant to the warmer water. This provides a greater mean temperature difference between the two fluids and allows for a higher heat transfer rate.
The capacity of a water-cooled condenser to remove heat energy from the refrigerant is based on several operating characteristics. The greater the mean (average) temperature difference between the water and refrigerant, the more heat energy will be transferred from the refrigerant to the water. The higher the water velocity, the greater the heat transfer. At very low velocities, a very thin but relatively stagnant film of water will remain on the tube surface, reducing heat transfer. Higher velocities result in better scrubbing action of the water on the tube surfaces, increasing the heat transfer. Cleanliness of the tubes is also a factor. Foreign material such as corrosion, scale, and mud will act as an insulator, reducing the heat transfer ability of the condenser.
When the capacity of the condenser to remove heat energy is reduced by either a reduction in water flow or dirty tubes, the system’s head pressure will increase, which will likely lead to a system failure. If the system has a high-pressure switch, it would eventually cause the switch to open and the compressor to shut down. Systems without a high-pressure switch could cause the relief valve to open or the compressor to cycle on its overload.
To identify the root cause of the increase in head pressure, technicians should analyze the condenser’s approach temperature, as well as the temperature difference between the water entering and the water leaving the condenser. The approach temperature is the temperature difference between the fluids leaving the condenser. The slower the water travels through the condenser, the longer the cooling water remains in the condenser and the more closely its leaving temperature approaches the condensing temperature of the refrigerant. A reduction in the water flow through the condenser will likely cause a reduction in the approach temperature and an increase in the temperature difference of the water entering and the water leaving the condenser. Condensers operating with fouled (dirty) tubes will cause an increase in the approach temperature, as well as a reduction in the temperature difference between the water entering and leaving the condenser.
Determining if low water flow or dirty tubes are causing the elevated high-side pressure can be tricky at times. But analyzing the correct operating characteristics of the system can help technicians identify the root cause of the system failure.