Why would you want to do that?!
The topic of ventilation in refrigerated spaces doesn’t get a lot of attention; this is for a relatively obvious reason. We usually try hard to seal our refrigerated spaces and rarely want any ‘intentional’ infiltration.
That said, the question comes up from time to time on what the code requirements are for ventilation of refrigerated spaces. This question is usually followed by “okay, that’s what the requirements are, but what should they be?” The first thing to address, then, is when do we actually want ventilation in refrigerated spaces?
We need ventilation when we are growing something. Grow rooms typically have plants that are using the CO2 in the air and this CO2 needs to be replaced. In some cases, CO2 is injected into the room and a corresponding amount of air is exhausted. However, in most cases, simple ventilation is used.
We need ventilation when people are working in the room. Ventilation becomes important in large production areas that are refrigerated. If you spend time in large meat facilities, you will have an appreciation for the increased ventilation used in newer facilities.
We need ventilation when we have a contaminant in the room. One of the most common contaminants is propane forklift exhaust (or ice-resurfacer exhaust in the case of an ice rink). While these types of forklifts aren’t terribly common in refrigerated spaces, some facilities do use them. There are also some products or production techniques that off-gas dangerous or noxious substances which are typically removed by using a form of dilution ventilation.
When don’t we want ventilation? Any time we can’t think of a very good reason to have it. Ventilation costs a lot of money and can contribute significantly to the refrigeration load, especially in freezer applications.
What do the codes say?
Historically, refrigerated spaces were ignored by the ventilation codes and standards. ASHRAE 62.1 2010, for example, makes no mention of refrigerated spaces except to exempt them from some humidity requirements (ASHRAE 62.1 is the ventilation standard that is most often applied to commercial and industrial construction in North America). If you open the most recent edition, published in 2016, you will find that this is no longer the case.
ASHRAE 62.1 2016 provides required ventilation rates for both a “per person” parameter and a “per square foot” parameter. Most types of spaces require some amount of ventilation based on their floor area regardless of whether there are people in the space and then additional ventilation if the space is occupied.
ASHRAE 62.1 also provides a default occupant density to use in calculations if the actual occupant density is unknown. The 2016 version of 62.1 requires a ventilation rate of 10 cfm/person in refrigerated spaces but has no requirements for floor area and the default occupancy is zero. We don’t have the time here to get into the details of the calculations required by ASHRAE 62.1; however, it is important to recognize this issue because we are starting to see enforcement in some jurisdictions.
Requirements depend on application
Different types of refrigerated spaces require different solutions. Let’s look at a few different examples:
Restaurant/small general cooler and freezer
If you consider the refrigerated space to be a storage area, it likely wouldn’t be considered an ‘occupiable space’ since access is occasional and for short periods of time. It is most likely that no ventilation would be required.
Meat cutting cooler
Coolers and freezers where activities such as meat cutting are performed are clearly occupied spaces and some requirement for ventilation likely exists in your jurisdiction. The requirement would be based on the number of occupants and, if you were following ASHRAE 62.1 2016, the ventilation rate would be 10 CFM per person. This can be further complicated in large production areas where it may be desirable to have ventilation based on the floor area of the facility in order to provide additional comfort and odour control.
Large coolers/freezers and docks
These types of freezers and coolers present the biggest challenge and uncertainty. They often have several people inside, including forklift drivers, palletizing workers, order pickers, and other packaging type operations. They also usually have a significant infiltration load from the doors and docks and the requirement to have ventilation is often seen as an unnecessary waste of energy and refrigerating capacity.
Consider a 100 x 100 ft. freezer with two doors that are opened six times each hour. The doors are fast opening and closing and are open for 20 seconds each opening and in any given hour the door is open three minutes.
There are several ways to calculate the infiltration that vary in accuracy and complexity, but if we use the Infiltration by Air Exchange method published in the ASHRAE Refrigeration handbook, this freezer has approximately 150 CFM of infiltration when the doors are in use. A load diagram for the freezer is shown in Figure 1.
If, on average, there are five people in this freezer picking orders and driving forklifts, the ventilation requirements would be 50 CFM. In comparison, recall that the door infiltration in our example would be almost 150 CFM any time that the door traffic was active. Figure 2 shows the same freezer with the added ventilation load.
Notice that this small amount of additional ventilation becomes five percent of the total load. This phenomenon is especially striking in freezers where unconditioned air is used for ventilation; the argument can be made that this ventilation is an unnecessary waste of energy.
If the refrigeration system in question used R507 with a -20°F evaporating temperature and a 100°F condensing temperature, it would use approximately 3.2 HP/TR (Tons Refrigeration). The freezer in our example is 13.1 TR with the ventilation and 12.5 TR without. The additional 0.6 TR of refrigeration results in an increased energy usage of five percent. There are several variables and assumptions in this example and the point here is not to justify these or get into specifics. However, it is important to identify that in freezers and coolers, such as the one described for the example, it is almost always the case that the door infiltration exceeds the ventilation requirements.
Infiltration not enough
Unfortunately, from a cost and energy perspective, the fact that infiltration exceeds the ventilation requirements is not usually an acceptable method to meet code requirements. One option to mitigate the energy use that has been employed is to measure the CO2 in the freezer or cooler and modulate ventilation to maintain the CO2 level at some fixed amount above the ambient concentration. While this too will no longer meet current code requirements, many jurisdictions will find it acceptable. To be completely honest, when code required ventilation ends up installed, owners and operators often choose to shut it off.
Engineering and installing ventilation in coolers and freezers can be challenging. There are humidity and load issues that can cause significant problems during operation and it can be challenging to justify the expense when no changes in human comfort are expected. That said, work with your local authority having jurisdiction to make sure you don’t end up in a situation where they require that you include ventilation in your refrigerated space and your refrigeration system is not sized big enough. That’s a problem you don’t want to have.