When we think of refrigeration, we usually think of a cold beer, yummy food, and all those miscellaneous science experiments hiding in the back. I’d like to temporarily change all that and focus on how it works, why some commercial refrigeration wastes tons of water, and a fantastic example of a retrofit project at UHN. Then we can go back to trying to identify that smelly mystery jar that you came across while cleaning out the bottom shelf.
Refrigeration works by removing heat from one area (the refrigerated compartment) and rejecting it to another medium. This transfer of heat is accomplished through the refrigeration cycle, which essentially takes advantage of the fact that the boiling point of a substance (known as a refrigerant) changes at different pressures. At low pressure, the refrigerant evaporates at a low temperature, allowing it to remove heat at cold and even freezing temperatures. Evaporation of the refrigerant occurs at a low temperature in a coil inside the refrigerator, removing heat from the air. The refrigerant is then mechanically compressed to a much higher pressure. At higher pressure, the temperature of the refrigerant increases and it also has a higher evaporation/condensation temperature. The high pressure refrigerant is condensed into a liquid in a coil outside of the refrigerator, rejecting heat. The liquid refrigerant then passes through an expansion valve, reducing the pressure. With the abrupt decrease in pressure, the refrigerant cools (similar to the way an aerosol can cools when the pressure in the can decreases as the product is being sprayed out) and returns to the evaporator coils in the refrigerator. There are many variations to this cycle, but this is the general operating principle of a refrigerator.
In your refrigerator at home, the heat is rejected to the air through the condensing coils on the back side of the unit:
In a commercial operation, such as a hospital or grocery store with large walk-in refrigeration units, there is too much heat generated to simply expel it into the local interior air. There are several common mechanisms used to remove heat from commercial refrigeration. Refrigerant can be piped to an outdoor condenser, where the heat is rejected to ambient air, usually with fans blowing over the condensing coils to increase heat transfer. If the building has a central chilled water supply, the condenser can be configured to reject heat to the chilled water loop. In buildings with large year round heating and cooling loads heat from the condenser can even be used as an energy source. For example, in hockey rinks heat generated by ice-making equipment can be used to heat domestic water for showers.
In many older buildings that were designed before resource conservation was a major consideration, refrigeration systems were designed to run domestic cold water over the condenser coils and send the water down the drain. As a matter of fact, this configuration was used at Toronto Rehab’s University Centre up until this summer! In a constant year round operation such as a refrigerator, water continuously running down the drain adds up quickly. This opportunity to reduce water consumption at TRI-UC was identified during a City of Toronto sponsored water audit and the work to convert the system over to Enwave chilled water (cooled by low temperature deep-lake water from Lake Ontario) was carried out by facilities manager Gord Wood and his trusty team of building operators. In the new configuration, two walk-in freezers and two walk-in refrigerators have been retrofitted to use Enwave chilled water to cool the refrigerant in the condensers. Through building piping, the resulting warmed water is eventually returned to Enwave to be cooled again and reused. The savings of the project outlined below are astounding:
- Water Savings: 2,365 m3 (almost one Olympic sized swimming pool!!!!) per year
- Cost Savings: $5,650 per year
- Payback: 3 months
We are currently working to eliminate once-through domestic water cooling at all Toronto Rehab sites. And with those kind of savings, we can definitely afford a forensic analysis of those leftovers from 3 weeks ago that you forgot about!