COMPUTERIZING REFRIGERATION LOAD CALCULATIONS©
by Bill Smith, president of Elite Software
HVAC load calculations have been computerized for many years now. In fact, there are numerous programs available on the market for calculating HVAC loads. These programs have proven so beneficial that most HVAC designers would not think of calculating loads without one. However, refrigeration loads are still often calculated by hand as few programs have been available for that purpose. Fortunately, this situation is improving, and refrigeration load calculation programs are becoming available from both refrigeration manufacturers and independent software vendors.
The benefits of using refrigeration load calculation software are exactly the same as with using HVAC load calculation software, time savings and improved accuracy. Software relieves the designer of the tedium of manual calculations, and lessens the chance of arithmetic mistakes. With software, the refrigeration designer can concentrate on system design and layout without having to worry so much about calculation details.
Most refrigeration load calculation programs are applicable to all types of refrigeration applications including freezers, chillers, walk-in coolers, display cases, and large refrigerated warehouses. To be able to handle the smallest to the largest applications, refrigeration load programs must have input provisions for a wide variety of load contributions.
Before a designer considers using a program for refrigeration load calculations, it is important to fully understand what such a program must be capable of doing. Besides knowing what a program should do, it is also important to be aware of any implicit assumptions made in the software. Even with software that makes few assumptions, it is important for designers to realize that engineering judgement must always be applied when interpreting results.
Refrigeration applications have many of the same loads as HVAC projects including roofs, walls, partitions, floors, people, lights, equipment, and outside air. Refrigeration also has several unique loads such as products (fruits, vegetables, meats, beverages, blood, medical supplies, etc.) and equipment defrost cycles. Although internal loads such as lights, people, and equipment are calculated exactly the same way as for HVAC applications, many loads such as outside air and envelope transmission loads (roofs, walls, floors, etc.) are calculated slightly different in refrigeration applications.
Transmission loads for refrigeration applications are calculated very simply using the traditional heat gain equation listed below.
Q = U x A x TD
Q = heat gain, Btuh (btu’s/hour)
U = overall heat transfer coefficient of the exposure
A = area in square feet of the exposure
TD = difference between outside air temperature and air temperature of the refrigerated air space, deg F
The only adjustments to this equation suggested by the 1989 ASHRAE Handbook of Fundamentals is for the allowance of sun effects from the color of the exposed surface and the direction it faces. This is in marked contrast to what ASHRAE requires for commercial HVAC load calculations. Commercial HVAC designers must deal with numerous construction material types, cooling load temperature differences, indoor-outdoor correction factors, latitude-month correction factors, color adjustments, and insulation considerations.
Although transmission loads are easier to calculate for refrigeration than for HVAC design, the reverse is true for outside air infiltration loads. Most commercial HVAC projects use mechanically added ventilation to completely negate infiltration. When infiltration does occur in HVAC design, it is generally a straightforward procedure to account for it. However, in refrigeration design, infiltration is complicated by air curtain barriers, number of doorways, doorway types, number of times the doors are open and closed, and the average time each door remains open when accessed. Not only are infiltration loads more difficult to calculate in refrigeration design, they are also more critical than in HVAC design.
Without a doubt, the greatest difference between HVAC load and refrigeration load calculations is the consideration of product loads. Commercial HVAC applications have no equivalent to product loads. These loads are unique to refrigeration system design, and they are special for several reasons. Most load factors, at least for calculation purposes, contribute heat in a straight forward uniform fashion. For example, lights contribute a constant rate of sensible heat according to the lighting wattage consumed and the hours of operation. Products, on the other hand, contribute heat in several ways, and there are an almost infinite number of products each with its own thermal properties.
The total load contributed by a product such as blueberries can have up to four components, especially if sub-freezing temperatures are reached. The first type of product load is often called the product cooling load, and it represents the amount of heat that must be removed to take a product to its freezing point temperature. The magnitude of this load depends on the specific heat of the product, the temperature of the product enters the refrigerated space, and the freezing point of the product.
The second type of product load is the freezing load. Once a product has been brought to the freezing point, the additional amount of heat that must be removed to actually freeze the product depends on the product's latent heat of fusion. To take a product below freezing, a quantity of heat must be removed according to the product's specific heat value and the desired final temperature. This third product load type is often called the sub-freezing load. Finally, if a product undergoes respiration (as in the ripening of fruits and vegetables), then the heat given off by this chemical reaction is called the respiration load. The sum of these four load components is considered the total product load.
Calculation of the total product load in a space can be quite tedious in that many different products, each with different specific heat and latent heat of fusion values, must be individually calculated for. This calculation is further complicated in that the various products may be entering the refrigerated space at different temperatures and at different times of day. In an effort to lessen the calculation burden, many refrigeration designers attempt to average the values for all products, and then calculate a total product load based on one average product for the space. This technique sometimes works well, but in many cases it causes the product load to be compromised and somewhat inaccurate.
With software, it is easy to perform very accurate product load calculations. Most programs maintain a library of complete product data so that all the designer has to do is enter the name of the product, quantity in pounds, and the entering temperature of the product. The software automatically looks up the specific heat, heat of fusion, respiration rate, and everything else about the product being calculated for. Such calculation ease encourages the designer to specifically enter all the unique products being refrigerated in the space.
Most refrigeration load programs have a built-in product library that contains all the products listed in the 1985 ASHRAE Refrigeration Handbook. However, it is still quite possible for a designer to encounter an unusual product not included in the ASHRAE list. Fortunately, most programs allow the designer to add products to the program library. With this capability, it is easy for the designer to maintain a comprehensive product library.
Related to product loads are container loads. Crates, cardboard boxes, plastic tubs, and other such containers have their own specific heat values that affect the total refrigeration load. Container loads can amount to an appreciable heat contribution, and it is important to consider them. Since most refrigeration software also has provisions for container loads, it is easy for the designer to accurately account for them.
Besides looking up product load data, software can also make things easier for designers by providing automatic access to design weather data for hundreds of cities. This relieves the designer of looking up summer outdoor design conditions for every location that he might design for. Instead of manually entering the design conditions, software makes it possible for the designer to just type the name of the desired city. Of course, most refrigeration load programs also allow the designer to enter his own conditions or modify those that are looked up.
Another difference between HVAC load calculations and refrigeration load calculations is the consideration of time. In conventional non-thermal storage HVAC design, it is important to know the load in btu's per hour at the peak hour of the summer design day so that equipment can be properly sized. However, refrigeration system design is much like modern thermal storage HVAC design in that the most important thing to know is the total btu's of cooling required over the entire 24 hour period of the design day.
Since 24 hour load totals are required, it is necessary to know the exact number of hours each load affects the refrigerated space. When products go in and leave, how long people work in the space, operating hours of lights and equipment, and defrost are all important time dependent loads. Keeping track of these time varying loads is difficult by hand, but easy with a computer program.
Although the 24 hour load total is the most important information for refrigeration design, it is still necessary to consider the hour by hour loads. Due to products moving in and out so often, a refrigeration application is often subject to an extreme hourly load fluctuation. The load variance can be so great that the momentary temperature in the refrigerated space could rise above the maximum allowable temperature for some products. In order to satisfactorily meet large sudden loads, it is sometimes necessary to size the refrigeration equipment larger than is required just for the 24 hour load total.
A special time dependent load in refrigeration design involving unitary equipment is the compressor run-time load. Although refrigeration loads are calculated on a 24 hour basis, most refrigeration compressors do not operate 24 hours per day because a certain amount of time (typically 2-4 hours) is needed for a defrost cycle. Since compressors are rated in catalogs based on their single hour btu capacity, an adjustment to the total refrigeration load must be made to properly select a compressor that runs less than 24 hours per day. This adjustment to the total load is the compressor run-time load. This is usually a simple calculation to perform, but with a refrigeration loads program it's automatically taken care of.
A final benefit of using refrigeration load calculation software is that nicely formatted reports are printed. While it's not that difficult for a designer to manually calculate accurate results, it is usually very time consuming to create neat and formal reports from hand calculations. Software not only produces neatly organized reports, it also prints more reports than what would normally be generated by hand.
Most refrigeration load programs provide both detailed and summary reports. A typical report shows the total 24 hour refrigeration load broken out by transmission gains, internal loads (people, lights, equipment), infiltration, product and container loads, compressor run-time load adjustment, and an additional safety factor load. The summary reports give quick bottom line information while the detailed reports show exactly how the final output was arrived at.
Software can also help the refrigeration designer beyond just calculating 24 hour load totals and printing reports. A major task faced by designers once loads have been calculated is the selection of an optimal coil and compressor combination. This can be a difficult chore because many factors such as the refrigerated space temperature, refrigerant type used, desired coil temperature difference, and the number of coils required for uniform air distribution must be considered. The typical selection process involves manually searching through numerous catalogs and comparing data. Software can make the selection task faster and easier by displaying only those coils and compressors that are good possible candidates. Rather than flipping through pages and pages of catalogs, designers can use software to quickly narrow down the possible alternatives.
Unlike HVAC designers, refrigeration designers have not had much in the way of software available to them. However, recent trends with refrigeration equipment manufacturers indicate that many of these manufacturers are starting to provide refrigeration load calculation software at no charge to qualified designers.
In addition, there is increased activity amongst independent software vendors towards developing software for refrigeration design. Refrigeration designers who would like to start using the computer more in their work will find that software specific to them is just now coming available.
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Elite Software Development, Inc., firstname.lastname@example.org