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Evaporators 101

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“Return air” becomes “supply air” as it moves through the evaporator as seen in this penthouse.

Picking which evaporator is best for the job isn’t as easy as picking a number out of a catalogue

By Greg Scrivener

This year has brought a lot of changes to the commercial refrigeration industry. Both refrigerant environmental regulations, which we discussed in a recent article, and energy efficiency regulations have been introduced that have substantially changed how we design, sell and install refrigeration systems.

An article in the near future is going to dive deep into the new Canadian energy efficiency regulations and the annual walk-in energy factor (AWEF), which we briefly discussed a couple of years ago as the introduction of these regulations was underway in Canada. This month, however, I want to go back to basics a bit and talk about what makes an evaporator “efficient” and why we choose the evaporators we do.

Where the magic happens

Evaporators are often described as a place where work really happens. For people who don’t understand refrigeration, they can be something like magic and, truthfully, I have always been fascinated by evaporators. Frost pattern differences between flooded, liquid overfeed and DX ammonia evaporators, the complexities of the proper nozzle and distributor selection for proper liquid distribution halocarbon evaporators, and how some evaporators just “seem” to work better have given me a lot to think about over the years.

Evaporators remove heat from a space by being colder than the space they are trying to cool; the temperature difference between the evaporator and the space is colloquially called the TD (much more on this below). As air moves through the evaporator it changes temperature from the “return air” temperature to the “supply air” temperature. This relationship can be described by the following equation:

This equation shows that for a given cooling capacity we can choose between coils with higher airflows and lower temperature changers, and coils with lower air flows and higher temperature changes; there are reasons we might choose one situation over the other. For example, in some pharmaceutical and blood storage applications it is important to maintain relatively consistent temperatures in a cooler or freezer and choosing a coil with a higher airflow rate can help achieve this.

Need to know

The term TD itself is often a point of confusion. In an evaporator rating sense, TD usually means the difference between the refrigerant saturated suction temperature (SST) and the return air temperature because it is this temperature that affects coil performance. Recall, however, that we often use TD to mean the difference between the room temperature setpoint and the refrigerant SST. These are not the same thing.

The relationship between TD and coil performance is close to linear.

Consider a walk-in cooler with a required average box temperature of 35F and a design SST of 20F. Something many of us would say has a 15F TD. If the temperature sensor is placed in the middle of the room and assumed to be measuring the average room temperature, the temperature of the air returning to the coil is actually going to be higher than 35F. If an evaporator manufacturer uses “TD” this way, then they are rating their coils using the average or mean temperature difference and not the actual temperature difference (this is called DTM rating method).

Assuming that our example cooler has a cooling load of 11,400 Btu/hour, I chose a coil that has exactly that capacity at a 10F TD and an airflow of 1,410 CFM. Using this information and the previous formula, we can estimate that the air temperature across the coil will change as follows:

This means that if the average temperature in the box is 35F, then the supply air temperature must be colder than that and the return air temperature must be warmer. The result might then be a return air temperature of 38.7F and a supply air temperature of 31.3F. If you are designing your system for a 10F TD in order to increase your operating efficiency you should actually be choosing a coil rating at 38.7F and expecting your refrigeration system to operate at 28.7F SST, even though the box is at 35F but if you do this and the manufacture is using DTM ratings, then this coil will end up underperforming. If your temperature sensor is at the ceiling behind the coil and it’s set to 35F, then the system’s average air temperature in the room will be lower.

The truth is that there are several other complications to this type of analysis that include things like how well the air mixes and the evaporator placement relative to the load. If you are interested in further reading on this subject, Colmac Coil Manufacturing has published two white papers on the topic. The point I wanted to get across here is that we need to be careful considering evaporator performance during design and it is not always as easy as picking a number out of a catalogue.

Temperature sensor location plays an important role.

Normal operating range

So far we’ve talked about the TD in a lot of detail but haven’t really discussed how we choose a TD if we are picking an evaporator coil. Within the normal operating range of approximately 5F – 20F TDs, the relationship between coil performance and TD is close to linear, which means that if you are given an evaporator rating of 11,400 Btu/hour at a 10F TD, the same coil is going to have a capacity of approximately 22,800 Btu/hour at a 20F TD (it isn’t quite linear and manufactures often provide some small correction factors but it is usually close in this range). Why would we choose a coil with a 20F TD over one with a 10F TD or visa versa? There are several considerations. First and foremost, as we have discussed in previous articles, increasing the suction pressure increases the efficiency of the refrigeration system significantly; it also decreases the size of the compressor or condensing unit. This would lead us to minimize the TD. However, you likely noticed that in our example we could get double the capacity out the same sized evaporator by doubling the TD. Ultimately, there is an optimal balance between capital and operating cost that can help determine the ideal TD but if there are space constraints (reach in coolers, VRFs, etc…), then you are far more likely to use coils with higher TDs.

Figure 1: An excerpt from the Heatcraft Engineering Manual H-EngM048, April 2008 available at heatcraftrpd.com/PDF/Misc/EM.pdf.

Controlling humidity

The other reason we care a lot about TDs is controlling the humidity in the space. So far in this article, we have completely ignored the latent cooling capacity of evaporator coils but if they are operating below the dew point of the air then water will be removed from the air. All of the coil ratings we’ve discussed so far the only factor in sensible heat; while this is typical, you must be cautious if you have significant latent heat removal requirements in your space. Conversely, if you are using a coil rating that does include latent heat removal, you could find yourself with a dramatically undersized coil unless you have adequality considered the latent heat requirement of your load. Since the TD of the coil determines how much moisture is removed, it also determines the relative humidity in the space. Figure 1 above provides some guidance in selecting appropriate TDs for several applications.

There are many factors that should go into the design and operation of evaporator coils and it’s not really a surprise that many of them do not function as well as we intend.

The TD determines how much moisture is removed from the system.

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