In an earlier blog, I spoke about why coolers and freezers need a defrost cycle so that the systems work properly. So how does the refrigeration system know when to go into defrost and, just as importantly, when to come out?
The least sophisticated way to control an electric defrost is to have a mechanical time clock that is programmed by a refrigeration technician to start and stop defrost cycles at one to several definite times of the day. During each cycle, the system goes into defrost mode. Every cycle is “time-initiated” and “time-terminated”. The timing and duration of each defrost cycle is determined largely by trial and error. Since it is especially important to proper functioning of a refrigeration system for all ice or frost to be completely removed from the coils by the end of a defrost cycle, there is a tendency to err on the side of too much defrost time.
The unfortunate reality is that the amount of frost to be removed varies widely from day to day, due to variations in cooling loads and the humidity of infiltrating air and other sources of moisture introduced into the walk-in. This means that the defrost time needed will vary as well. A dry January day when a business might not even be open will require much less defrost time than a warm and humid day in July when the compressor system is working overtime. However, generally, the technician will select the total time based on the worst-case scenario. This means that a technician will general choose a program of cycles long and often enough to avoid trouble on that hottest summer day. Consequently, a typical system using a time-terminated time clock for electric defrost will use much more energy than necessary much of the time.
A much more efficient way to control electric defrost is to use an electronic controller like the Freeaire® Cooler Controllertm. Defrost cycles can be programmed to occur at specific times of the day or after a specified number of hours of compressor operation. A record of exactly when each defrost cycle happens and the temperature of the evaporator at each point in the process is a powerful diagnostic tool to help verify proper functioning. Further, this means that a refrigeration technician can use the web enabled control features to initiate a defrost cycle remotely, eliminating the immediate need for a site visit.
Most importantly and unlike the crudest type of time clock, a defrost cycle controlled by an electronic controller is temperature terminated: the defrost cycle ends when the probe sensor in the evaporator reaches a temperature that guarantees the ice is gone. On dry winter nights, that may be very quick, whereas on a humid July night, it may be much longer. Only the time needed to get the job done (and consequently the energy needed) is used. Let’s look at an electric defrost to understand all the ways that this matters.
As a reminder, electric defrost is generally used in walk-in freezers and many coolers that are kept very close to freezing because the “off-cycle defrost” and “air defrost” do not add heat and it does not get warm enough to reliably remove the frost. With electric defrost, the compressor and the evaporator fans are switched off, and the electric heaters interspersed between the evaporator coils are energized (you can almost see the line of heaters in the photo above). These warm coils melt the ice and the water is drained away.
However, the bad news is that those electric coils are powered with electricity. Further, the heat generated increases the temperature in the box. This means that after the defrost is complete, the compressor needs to work hard to remove the heat that was just added to the box. The more heat added, the more work it has to do.
By running the electric defrost less often and for a shorter duration on most defrost cycles, an intelligent defrost system uses less energy to melt the ice and, as a consequence, less energy to remove that heat when the job is complete.