Savings II: Outside Air

How Much Help from Outside Air? On the Worksheet this question is represented by the value for "days/year outside air takes over refrigeration" (line 67). The answer depends on the climate, the refrigeration load, and the number of outside air packages to be installed. Here is a graph showing the average number of days per year it is colder than a particular temperature in various cities in the United States. It is prepared from "bin data" that show the average annual amount of time the outside air within the city is at various temperatures. As you can see on the graph, there are 138 days a year where the average temperature is colder than 35°F in Burlington, Vermont. Let's assume that we want to maintain the walk-in at 40°F. The "TD" (for Temperature Difference) or "Delta T" (the Greek letter that looks like a little triangle, plus T) is the difference in temperature between the outside air and the air inside the walk-in. The higher the TD the higher the cooling power of the outside air. A TD of only 5 degrees (40° minus 35°) may not usually be sufficient to keep a larger cooler with a larger cooling load at 40°F without help from the compressor. If a larger TD is needed, say 10°F (with twice the cooling power of a 5°TD) so that a 30 °F or colder temperature is needed for outside air to entirely refrigerate the space, then there would only be 107 days per year that are cold enough in Burlington.

Your town may not be represented on this graph, but there would be a similar line for it if there were. Estimate where your location would be in relation to the cities shown.

Estimating the TD. You also need to estimate the TD you will need to handle your winter cooling load. Subtract this TD from the temperature that you want to maintain inside your walk-in. Use the graph to estimate how many days/year the outside air is colder than that temperature. On the Worksheet, enter this for the number of days per year the outside air takes over refrigeration (line 67).

You don't know what TD your system requires? You could just "guessimate", based on our experience. We've found that a 40 °F walk-in cooler, 8 ft. x 8 ft. x 30 ft. in size, with 10 reach-in glass doors, 2 outside walls, on a slab, an unheated attic space above and an average refrigeration load of beer, soda and milk can be considered a "typical" convenience store walk-in. For this average installation, a TD between 5° to 10°F is usually cold enough to allow a single Outside Air Package to take over the entire refrigeration load.

You probably have an idea of how your walk-in compares to an average walk-in, so just use this knowledge to estimate your cooler's TD. Increase the TD for a larger cooler, a larger product load, and more heat gained from more inside walls or reach-in doors. Decrease it if the opposite is true. Divide the TD in half if 2 Outside Air Packages are used, since twice the air moved through the cooler doubles the cooling power.

If this is just not accurate enough for you, you will have to calculate the needed TD by doing a refrigeration load estimate. We're warning you, however, that you'll have to get into great detail about things about which you may know very little. Just what temperature is your beer or soda when delivered? What is the temperature of the ground beneath that slab? How many hours are your cooler lights left on? It's a lot of trouble to go through just to end up estimating again.

Calculating the TD: The formula you need to use is:

Temperature Difference = Outside Air Cooling Requirement/Outside Air Cooling Capacity

Calculating the Outside Air Cooling Requirement: One can arrive at an outside air cooling requirement, or load estimate, by doing a BTU analysis of the cooler, in a similar way that the cooling requirement for a compressor system is done. Why not simply use the compressor system's load estimate?, you ask. Because the cooling requirement of a Freeaire's Outside Air Package is not determined in the same way a compressor system's is and are not comparable. You need to use figures that apply to the time when outside air is used. It is very likely that the BTU/Hour cooling requirement for the Freeaire System with Outside Air will be only 1/4 as large as the BTU/Hour rating for the compressor system with which it will be paired in your installation. There are several reasons for this:

  • Compressor systems are are designed for the hottest summer day. They are intentionally "oversized" to handle the maximum cooling load when operating the compressor only 16 to 18 hours a day in order to allow time for air circulation through the evaporator coils to defrost any ice which forms when the compressor operates. The Freeaire can operate continuously, because it doesn't have to stop for defrosting.
  • Outside air refrigeration always occurs when the outside temperature is cooler than the temperature inside the walk-in, so there is heat is lost through outside walls, not gained, and there is less heat gained from heated spaces than is the case in the middle of the hottest summer day, which is when the compressor-system is assumed to be at the upper range of its operation.
  • The Freeaire's outside air fan slightly pressurizes the walk-in cooler so there is very little infiltration of heated air.
  • When the compressor operates, the evaporator fans add a great amount of heat that needs to be removed by operating the compressor. When the outside air is used, the low-energy (36 watts) Circ fan takes over from the evaporator fans and very little heat is added to the walk-in. A Freeaire's 150-watt permanent split capacitor (PSC) outside air fan is very efficient at moving air with a minimum of watts and therefore only about 1 degree of heat is added to the cool air coming into the walk-in.
  • Often the product load for the outside air system is not great due to the fact that the product arrives pre-cooled on unheated delivery trucks.

Outside Air Cooling Requirement= Total Refrigeration Load/24 hours

Total Refrigeration Load= transmission loads (use appropriate TD for each exterior surface area, including the floor) + additional loads (motors, lights and people, including hours/day. Use formula: 1 watt = 3.41 BTU/hour) + sensible product load (the temperature drop required of the product) + the product load due to respiration (for unpackaged foods)+ infiltration load.

Let's assume that you come up with a total refrigeration load of 126,720 BTU/day. Your Outside Air Cooling Requirement = Total Refrigeration Load/24 hours = 5280 BTU/Hour. Now it 's time to move on to:

Calculating the Outside Air Cooling Capacity: The Cooling Capacity of a Cool Breeze Outside Air Package is determined by the formula:

BTU/hour = (specific heat of air ) x (density of air) x (volume of air/hour) x TD.

The specific heat of air with 50% relative humidity is .267 BTU/lb. The density of that air is .075 lbs./cubic foot. This make the equation:

BTU/hour =(.267 BTU/lb.) x (.075 lbs/cu.ft.) x (60 minutes/hr x CFM) x °FTD = 1.2 x CFM x °FTD

The CFM rating for the intake fan is 440 cubic feet/minute when the static pressure is .4" of mercury, a typical value for a Freeaire System with a slightly dirty filter. This gives us:

Cooling Capacity for each Cool Breeze Outside Air Package= 528 BTU/Hour x °FTD

This means, for every degree(°F) that the incoming outside air is cooler than the inside air being exhausted, the Freeaire 500 removes 528 BTU/Hour from the walk-in.

Lastly, we use our formula:

TD = Outside Air Cooling Requirement/Outside Air Cooling Capacity = 5280/528 = 10°F

Hey, isn't 10°F what the folks from Freeaire told us to use about two hours ago? Yes, but you had to see for yourself, didn't you? Satisfied now?

Duty Cycles: Outside Air Packages are usually assumed to have a duty cycle of 50% (line 72) for the period of time they take over the refrigeration of the space. This can vary, however, with undersized systems operating more than half the time and oversized systems running less than half. After an Outside Air Package is installed, the condensing unit and evaporator fans are assumed to have a duty cycle of 0% for the number of days per year outside air takes over the refrigeration (line 67).