by Paul Abelson, technical editor
When I started to think about this issue’s assignment, I figured it would be a snap. After all, there are only two makers of refrigeration units, Carrier Transicold and Thermo King. They both do pretty much the same thing. They’ve both been around for years, so their products are well proven. All I’d need to do (I thought) was call around and find out what each company’s unique selling point was, what differentiated it from its competition.
How simple minded of me! Little did I suspect that reefers are one of the most misused, abused and over used products on a truck, and for no other reason than reefer owners don’t take time to learn. For example, does this seem familiar? You go to shop for a trailer. You sit with the salesman and either review what’s in stock or you spec what you want: suspension, tires, insulation, doors and a reefer. You rely on the trailer salesman to provide what you need because you, as I did, assume “reefers is reefers.”
The salesman, anxious to close the sale (or at least not lose it on price), quotes you the lowest priced reefer on the lot or in the spec book. You spend the next 10 years with the wrong unit for your needs, wasting whatever dollars you saved on purchase price, many times over. Unfortunately, this is how some owner-operators buy a reefer. It makes as much sense to go to your Carrier or Thermo King dealer first, and ask them to spec your trailer. Both are specialty items, and each should be purchased as such.
Before we examine how to specify and maintain refrigeration units, let’s examine a few basic principles. Then we’ll look at how reefers function and how the properties of your loads alter the requirements placed on your equipment. We’ll also look at a few essential accessories and a few that, while not strictly essential, can make your work, and your unit’s, far less demanding.
First of all, in scientific terms, there is no such thing as cold, unless you define it as the absence of heat. In order to make something cold, you remove heat from it. In order to warm it up, you add heat. In trucking, we measure heat the old-fashioned way, using British Thermal Units (BTUs). One BTU is the energy required to raise the temperature of 1 pound of water 1 degree Fahrenheit. That heat can be added to raise temperature, or be removed to lower temperature. Heat energy can enter a closed container, like a trailer, in many ways. It can be conducted through trailer walls (that’s why we insulate trailers). It can flow around gaps in door openings or through cracked door seals. Heat can be conducted in through the trailer floor or out through the ceiling. A structural member or a steel bolt will act as a channel for heat if it passes through from outside to inside. Better reefer trailers have internal structures for rub-rails and E-tracks, separated and insulated from the outside walls. If you ever have to make repairs or add accessories, keep this thermal integrity in mind and never drill bolt holes through the walls.
Other sources of heat are often the loads themselves. Chocolate ice cream is one of the worst heat generators, as any experienced dairy hauler will tell you. The sugars in foods react and give off heat, and the chocolate has a biological reaction. Other notorious exothermic (giving off heat) foods are broccoli and beans.
Mobile refrigeration units are designed to maintain, not change temperature. Although it may happen given enough time, reefers are not designed to chill down field loads from 100 degrees ambient temperature to 40 degrees storage temperature. They are designed to remove any increased heat and to keep product temperature stable. Reefers also control humidity by condensing moisture from the air.
To do these things, refrigeration units require four things: thermal integrity of the trailer to prevent the inflow of additional heat; sufficient BTU capacity to remove the expected amount of heat; sufficient airflow, measured in cubic feet per minute (cfm), to exchange the air inside a trailer about once every minute; and sufficient air velocity to move the air through the trailer and over and through the load. Compromise any of these, and you may have compromised your cargo and opened yourself to freight claims.
Recent research done by the University of Florida, in cooperation with the Florida and U.S. departments of Agriculture, indicates that 46 percent of the drivers surveyed had claims, averaging one claim in a little over two years. Claims from high temperature, gases and humidity averaged more than $7,500. Yet, according to researchers, more than a third of drivers rarely or never check cargo temperatures before accepting a load, and more than half rarely or never use any recording thermometer to monitor the load in transit.
In theory, refrigerated loads come from temperature-controlled storage. In theory, they spend no time on docks or exposed to higher temperatures. In theory, the interior of a pre-cooled trailer maintains its temperature even when its doors are opened in the yard and the trailer is backed into its dock. In the real world, theory doesn’t always apply.
In the real world, loads are often too warm when they are loaded. In the real world, drivers don’t shut their reefers off before opening doors. In the real world, debris blocks airflow. When a pre-cooled reefer operates with the doors open, warm, humid outside air is sucked in. The moisture hits the evaporator, which ices up. That cuts off cold airflow, while warm air fills the previously pre-cooled trailer. The same effect results when debris gets into return air passages, blocking the flow of cold air.
Most of you who have read this far are familiar with proper loading procedures, but allow me to explain to those new to “temperature-controlled” operations why the procedures are important. It takes a significant difference in temperature to cause heat to flow from a warm object to a colder one, or from one with more heat to one with less. Once the heat of two objects is equal, heat energy will cease to flow. Remember, the function of your refrigeration unit is to maintain the temperature of a load, which means removing heat from any sources, both from inside the load and outside. When cooling, the load is the warmer object, and the air surrounding the load is, or should be, cooler. That cooler air must be in contact with the load in order for it to remove heat energy. If the air does not flow around the load, either because the load is stacked so as to block flow or because there are restrictions, the air will quickly reach the same temperature as the load. As the load warms up, so will the air. Then no cooling, not even temperature maintenance, will occur.
Airflow can be restricted if the load is stacked too high. Pressure builds towards the rear. The air from the evaporator, seeking the path of least resistance, flows down through the front and middle of the trailer. Circulation to the rear ceases, and loads are ruined. That’s why airflow and velocity are important. BTU capacity is the ability to remove the heat absorbed by the air, so it can again remove heat from the load. It is also the measure of how much heat can be added to the air to keep the product from freezing.
Getting ready to load
Here are the procedures you should follow when preparing to load your trailer. Before you start, make sure the trailer is clean and free of debris. Any foreign matter, like pieces of Styrofoam or parts of plastic or paper sheets can block both the air return and the floor channels. Then:
- Pre-cool the trailer to the desired temperature.
- Shut the reefer off before opening doors.
- Open doors, then back the rest of the way to the dock.
- Follow good loading practices (more on that later).
- Close the doors.
- Start the reefer. It should be set to the desired temperature, since it had been pre-cooled.
- Switch the unit to “defrost” to de-ice the coils (with today’s units, nothing will happen if the evaporator is not iced). The unit will automatically return to its cooling (or heating) mode.
Tips when loading
- Always stack boxes or cartons so air can flow around and through the load. There should be space around the edges. Stacking with space inside the center is called “chimney stacking.” “Staggered stacking” is when boxes are stacked with space between them horizontally, and with a stagger when looked at vertically. When stagger stacking, use load locks to make sure loads don’t shift. If they do, they can block airflow, preventing circulation around the cartons.
- Pallets should always be placed with their vertical structural boards running fore and aft. Pin-wheeling pallets or placing them with the boards running side-to-side will obstruct return airflow.
- Use a bulkhead in the nose of the trailer. Bulkheads keep the load away from the trailer wall to allow air to return to the evaporator. They have openings for airflow at floor level.
- Do not load to the rear doors. Leave space for ... you guessed it ... air to circulate.
- Load to a uniform height so flow to the rear of the trailer is not obstructed. Some operators paint a red line on the walls about 18 inches from the roof, as a reminder. Some stencil the words “Load Height” or “Do Not Load Above This Line.” Some warehouse personnel even pay attention.
These tips should help you get the most from your equipment. Next month, we’ll look at what makes a reefer work, and how you can best work with your reefer dealer to assure getting the unit best for you. We’ll also look at the features offered by Carrier and Thermo King, and the accessories that will make them perform even better. Thanks again to Dave and Bob at Illinois Auto Central and Bruce and Brian at Chicago Carrier Transicold for their help. n
These guys are the experts
I would be remiss if I didn’t give proper credit to the experts who gave of their time to help me understand this complex subject. Bruce Borowicz and Brian Hufnagl from Chicago Carrier Transicold, and Dave Smith and Robert Stearns from Illinois Auto Central Thermo King were all articulate teachers, and patient with me as I worked to comprehend this (for me) new technology. They all stressed that you should seek out your local refrigeration dealer, and get on his mailing list. It’s not so they can send you sales promotions, but so they can get you service bulletins and invite you to attend the free Risk Management Seminars that both refrigeration manufacturers host at dealerships around the country. If you attend, you’ll get an in-depth education on topics I can only touch on in Land Line. They’ll even buy lunch.
Paul Abelson is Land Line’s technical editor and freelances from his office in Lisle, IL.