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 judgment 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.
