In order to understand how to make your refrigeration system more efficient, it’s important to understand how the system works. In a commercial setting, a vapor-compression cycle is most commonly used. In a vapor-compression cycle:
- A refrigerant in vapor form is pressurized by a compressor, which in turn heats the refrigerant.
- This vapor then flows through condenser coils on the outside of the fridge.
- The hot refrigerant gives off heat to the outside air, which cools down the vapor into a liquid form. However, it is still at high pressure.
- The liquid refrigerant then travels through an expansion valve which converts some of the liquid back into a vapor by lowering the pressure.
- Finally, the vapor/liquid refrigerant passes through an evaporator. The refrigerant picks up heat from inside the fridge, which turns any remaining liquid into a vapor.
- This vapor travels back to the compressor to start the cycle over.
Capacity and Performance:
- Refrigeration capacity, or the measure of its effective cooling power, is most often expressed in Btu/hr, however some commercial systems use the kilowatt (kW) as the basic unit of refrigeration. In North America, refrigerators are often rated in “tons of refrigeration”
- Ton of Refrigeration (TR) is equal to the rate of energy removal need to freeze one ton of water in one day
- One TR of refrigeration is defined at 12,000 Btu/hr, or 3.517 kW.
- A refrigeration system’s coefficient of performance (CoP) is one way to determine the overall efficiency of a system.
- CoP is defined as the refrigeration capacity in kW divided by energy input in kW
- However, CoP isn’t frequently used as a rating system in the US
- US systems typically rate refrigeration using a performance factor (PF)
- PF is defined as the system’s energy input in horsepower divided by its refrigeration capacity in TR.
- PF can be used to rate the entire system, or just a part of the system
In order to make your refrigeration system run more efficiently, measures can be taken at each step described above to help the process run more efficiently. Below are some options you may want to consider:
- Heat reclamation
- Cool reclamation
- Subcooling/Super heating
- Control Systems
One of the most cost-effective ways to reclaim excess heat from your refrigeration system is by tying it in with a hot water heater. The excess heat from refrigeration is absorbed by the water heater, which can then supply hot water to the buildling. The water heater doesn’t have to work as hard to heat up water and the excess heat from refrigeration isn’t just being wasted. This usually requires a major retrofit, but is one of the best energy saving options out there.
Cool reclamation uses the same idea as heat reclamation – capturing excess cold – and tying it in with another system, such as your AC unit. By supplying the AC system with precooled air drawn from the refrigeration system, the AC system doesn’t have to work as hard to cool air. In this process, cool air from refrigeration isn’t simply being given off to the outside world, but transferred to a system that can utilize it.
Subcooling is a process used to ensure that refrigerant has cooled to its liquid form when it reaches the expansion valve of the refrigeration system. It does this by expelling heat to refrigerant that is in the hyperheating phase of the cycle, which occurs in a gas form. If any refrigerant is still in its gas phase when it reaches the expansion valve, unwanted behaviors could occur, including unnecessary use of power, deterioration of the refrigeration system, and irregular performance. Furthermore, allowing the process to occur outside the condenser maximizes the condensers heat exchange capacity.
Superheating involves the same process of heat exchange from the liquid refrigerant to the gas refrigerant – this time to ensure that refrigerant is fully in its gas phase when it reaches the compressor system. It some liquid refrigerant remains, the compressor can be damaged as liquid is uncompressible. By coupling subcooling and superheating, heat transfer can occur more efficiently, thus saving energy.
Economizers can be installed in refrigeration units to increase the efficiency of the compression stage. Economizers are an attached unit that takes liquid refrigerant from the condenser and flashes it at a lower pressure to get a cool vapor. This vapor is then added to the compression process in between stages to lower the average temperature rise associated with with the compression. This in turn, lowers the overall work load of the compressor.
Variable Frequency Drives:
VFDs can be installed on various equipment within the refrigerator system including evaporator fans, condensor fans, and compressors. These involve a retrofit of the current system but can lead to significant savings over time.
Evaporator fan controllers:
Evaporator fan controllers work to increase efficiency of the refrigeration system by reducing air flow when the compressor shuts off as well as slowing fan motor speeds during an off cycle. Essentially, this helps the refrigerator reduce power consumption when it’s not in use.
Quick Efficiency Tips:
- Clean condenser and evaporator coils. If coils accrue build up, it can block air flow. This can cause the system to use up to 90% more energy to stay cool!
- Insulate suction lines where appropriate. Suction lines transport refrigerant to the compressor. Keeping lines insulated prevents heat absorption by the refrigerant, meaning the whole system doesn’t have to work as hard to keep cool. Furthermore, insulation is inexpensive and easy to install.
- Replace old gaskets around doors. Old gaskets can lead to serious air leaks in a refrigeration system. These leaks can cause the system to work significantly harder in order to keep things cool.