How to size refrigeration correctly for the box and application

The key to a properly operating walk-in box system starts with a good design and the basis of a good design is to accurately determine what the refrigeration heat load is.

To do this one has to realize that heat enters the refrigerated space in several different ways:
•    Transmission load (heat transferring through the physical barrier of the structure).
•    Air change load (air entering the refrigerated space as doors are opened for product loading, people entering, etc).
•    Product load (unless the new product to be refrigerated is brought in at the same temperature that it is to be stored at, the product will add heat to the refrigerated space).
•    Miscellaneous loads (any other means of adding heat to the refrigerated space, such as the electric motors in the evaporators or process machinery, people, lighting, forklifts, and so on).
As can be imagined, there will be no consistency to any of these various heat loads. As the temperature outside the refrigerated box changes, so will the transmission load. The air change load will vary with how often the box door(s) are opened, the product load will vary with the quantity and temperature of new product coming in, and the miscellaneous load will vary with how often lights are on, process equipment is operating, how often/how long a fork lift might be in use, etc.

Sizing refrigeration for walk-ins is two parts good design one part guesstimate.

So, it becomes an educated guesstimate as to what the worst-case scenario is. That way, even on the most miserably hot day in the middle of summer, when production is high and extra personnel are working inside the refrigerated box, the doors are opening at an accelerated rate, and a high quantity of new product at higher than normal temperature is constantly being brought into the space, there is sufficient refrigeration equipment capacity to maintain the desired space temperature.
So, it is a logical step to put together a survey of the application. The following Information is necessary to properly determine the refrigeration heat load:

Design Ambient Temperature
To determine what the peak transmission load will be. This may be the outdoor temperature in the case of an outdoor refrigerated box, or a warehouse temperature in the case of an indoor refrigerated box. The heat gain through the walls and glass doors (if used) will be directly proportional to the TD (temperature difference between the ambient temperature and the refrigerated space temperature). The other factors determining the heat load will be the size of the walls and the type/thickness of insulation used in the box walls.
It is also important to consider the heat gain through the floor, and in freezer applications it is recommended that the floor be insulated as well.

Refrigerated Space Temperature
This will be dependent on the product to be stored in the refrigerated space. In addition to temperature, the required humidity will need to be considered.
Some products (such as meat, product, flowers, etc) are especially sensitive to low humidity. Storage in a low humidity environment will dry the product out, resulting in a loss of product integrity as well as weight (which translates into value…$$/lb) The system design will need to consider humidity, and can be managed by the appropriate evaporator TD (difference between refrigerated space temperature and refrigeration saturation temperature in the evaporator). The greater the TD, the more moisture will be removed from the air as it passes through the fin-tube structure of the evaporator.

Refrigerated Box Dimensions
This is pretty straight forward, but the transmission load is dependent on the size of the walls through which heat will be transferred into the box.

Box wall construction
The type of walls (and the insulation that is used in them), and glass doors (if used) will have specific insulation K values (the relative ability to conduct heat). This value, along with the area and TD between the refrigerated space temperature and ambient temperature will be used to determine the transmission load.

Infiltration Load
Every time a box door is opened, warm air enters the refrigerated space. This results in both sensible and latent heat, which will need to be removed to maintain the design space temperature. Knowing the maximum temperature where the box is located should be a relatively simple thing to determine. It is the number of door openings/day that is the big variable, especially in a retail application where there are glass doors for customer access to product.  This is where the term “guesstimate” comes into play. In case you are not familiar with this scientific/engineering term:
Guess-ti-mate
Noun
•    an estimate based on a mixture of guesswork and calculation
Verb
•    to form an estimate based on guesswork and calculation

Product Load
Again, this is another item that falls under the category of guesstimate. While the box might be designed for a specific product like apples, it is sometimes difficult to determine how much product will be entering the box at any given time. Once that amount has been determined, the weight, entering temperature and expected pull-down time will also be required. The refrigeration equipment capacity will differ substantially in an application where the pull-down time is two hours versus 24 hours.

Miscellaneous Loads
How many lights, and how often are they on? How many evaporator fan motors, and what horsepower rating are they? Is there any process equipment such as meat grinders, saws, etc., in the refrigerated space? If so, what are the horsepower ratings of the motors? During peak production periods, how many people will be inside the refrigerated space? Will there be a forklift utilized inside the refrigerated space? If so, how many, and for how long?

Specifics of the Application
Is this a medium temperature or low temperature application, is it a storage application or a quick chill or blast freezer application? Are there any other specifics of the application that would influence the system design?
Now, it used to be that once this information was gathered, a long arduous process was required to determine the refrigeration heat load. It required having access to insulation K values for varying types of materials, specific heat of various types of product (to determine the product load), tables for lighting and motor heat loads, and so on.
Fortunately, all of that is in the past. There are many resources available online to easily determine the refrigerated space load for a walk-in box. The above information will still be required, but rather than manually performing the heat load calculations, the box load program will do the work for you.

Dave Demma holds a degree in refrigeration engineering and worked as a journeyman refrigeration technician before moving into the manufacturing sector where he regularly trains contractor and engineering groups. He can be reached at ddemma@uri.com.