Walk-in coolers and freezers currently operate outside of any significant energy regulation in Canada, but this doesn’t mean that you shouldn’t pay attention to energy. The refrigeration costs in restaurants and convenience stores can be significant.
I like to split the energy efficiency of refrigeration system into two categories, which can be thought of as primary factors and secondary factors. Primary factors are those that, once chosen, set the maximum possible efficiency of the system. Secondary factors are those that affect the efficiency through parameters not directly related to the maximum possible theoretical efficiency. In this article we are going to look at the primary factors.
It turns out there are only two factors that affect the maximum theoretical efficiency of a refrigeration system – the temperature you choose to evaporate the refrigerant at to provide cooling and the temperature used to reject heat from the system. If you remember some early thermodynamics lessons, these factors characterize the Carnot efficiency of a thermodynamic cycle and are represented by the following formula which determines the maximum possible Coefficient of Performance (COP) – remember that you must use absolute temperature scales like Rankine or Kelvin. The COP is simply the net refrigerating effect divided by the amount of work required to get that effect.
Of course, it is not possible to achieve the Carnot efficiency in the real world, but it does give us a way of understanding how big of an impact the temperatures have on the efficiency of a refrigeration cycle.
There are a couple of things that may jump out at you from the above formula. The first is that if you have no difference between the hot and cold temperature, then the maximum COP goes to infinity. The corollary is that if the difference between the hot and cold temperatures is large, the COP goes to 0. Figure 1 shows the result of applying this formula for a few different typical condensing temperatures.
Have you ever heard the rule of thumb that the efficiency of a refrigeration system is decreased by two percent for every degree Fahrenheit that you decrease the saturated suction temperature? Or perhaps, the efficiency of a refrigeration system decreases by one percent for every degree Fahrenheit you increase the saturated condensing temperature?
It is apparent from Figure 1 that these rules of thumb are not really accurate for many operating conditions. That said, they do provide a good way to show the drastic changes in efficiency that happen with differing evaporating and condensing temperatures.
Making the right choices
This information leads to the importance of choosing the correct equipment when you are designing or installing a refrigeration system; we have choices when it comes to choosing evaporators and condensing units. Evaporators obviously must be colder than the temperature we are required to reach in the cooler or freezer. If we made them only 1°F colder, the surface area would have to be enormous and the evaporators would be extremely expensive. Conversely, we could use small inexpensive evaporators and make them really cold. The optimum, like it is for most things, lies somewhere in the middle. There are additional considerations for evaporators because they control the moisture removal from a space.
For general storage type walk-in coolers, a typical design will keep the evaporator 8°F-15°F colder than the box temperature (This difference is usually referred to as TD). The lower TDs will result in higher humidity and are suitable for some applications like flower and some vegetable/fruit coolers, but low TDs also increase the chances of mould and introduce risk in meat coolers.
Our condenser obviously must be warmer than the ambient in which it’s located to reject heat. How much warmer? As with evaporators, we have a competing interest between size, cost and efficiency. If you were around during the switch from 10 SEER to 13 SEER condensing units, the size increase was very noticeable.
The typical condenser TDs are 10°F to 25°F for commercial walk-in equipment. Consider a kitchen walk-in cooler in Calgary where the outdoor design temperature is about 85°F. Even being conservative (as one usually is for food related and critical refrigeration systems), it is rare to see a system in Calgary designed for an outdoor ambient temperature higher than 95°F.
This means that the condenser will be operating between 105°F and 120°F on a hot day.
What if the unit was installed in an unventilated or poorly ventilated space above the walk-in (as so many are)? It’s common to see ambient temperatures of 110°F or higher in these spaces, so in this system, the condenser would operate in the 120°F to 135°F range.
If this was a 35°F cooler operating with an evaporator TD of 10°F, the resultant Carnot efficiency would change from 6.9 in the case of the outdoor unit to 5.2 for the unit in the poorly ventilated space, a 25 percent reduction in the maximum theoretical efficiency! There are several complicated considerations when determining the actual change in efficiency, but the change will usually be in the same order as this estimate.
What does this have to do with walk-in coolers and freezers specifically? They do not have many energy regulations like smaller commercial and domestic appliances do and they are often ‘field designed’. However, they are not large enough that significant engineering is devoted to equipment selection; inefficient installations run rampant. Condensing unit manufacturers provide you with tables that look like Figure 2. This table is the capacity for four different units (horizontal rows) at different evaporating temperatures.
Notice that they do not provide the condensing temperature; instead they provide the ambient temperature, which in this case 95°F. This is for a good reason – the TD will change depending on the ambient and this information would complicate the table. The downside, however, is that you have no idea how efficient the unit will operate.
The manufacturer likely uses a few different techniques to determine the condenser size. One might be that they want to provide an even range of condensing unit capacities. There has been pressure from manufacturers to avoid this in the industrial evaporator industry and industry rating standards are important to prevent this type of design because it offers no real benefit.
The main thing the manufacturers will consider is cost. There are fixed sizes of compressors available and if increasing the condenser size decreases the cost of compression enough to switch to a smaller compressor, it is likely to happen.
Conversely, while increasing the condensing temperature decreases the cost of the condenser, it will increase the compression requirements. There is a balance between the two and if you want energy efficiency, the cost increases. Have you ever wondered why you can find condensing units with the same compressors that have 10-20 percent different published capacities?
In the next issue, we will compare a couple of selection options and look at some of the secondary parameters that affect efficiency.