by Paul Abelson, technical editor
In the previous issue, we looked at some functional issues concerning the operation of refrigeration (reefer) units and the handling of temperature sensitive loads. The objective is to eliminate spoilage and, therefore, claims against you. If you’re not familiar with all the fundamentals, if you’re just thinking of investing in a reefer trailer, or if you just want to brush up in case you forgot something, you might want to read Part 1 in the March/April 2001 Land Line.
The principles of physics that allow refrigeration to work
- Heat is a form of energy.
- Heat energy travels from warmth to cold.
- Refrigerants are gases at room temperature and atmospheric pressure.
- When compressed, refrigerant gases liquefy.
- When pressure is released, the gases vaporize.
- The gases are contained in a closed system, going through the complete cycle.
- When a liquid expands to a gas, it absorbs heat, getting warmer itself but cooling the immediate surroundings.
- When a gas is compressed to liquid state, it releases heat, thus getting cooler itself, but warming its immediate surroundings.
Let’s start this month by looking at how refrigeration works. We’ll examine two comparable, popular units: one each from the two reefer manufacturers, Carrier-Transicold and Thermo King. We’ll review a few maintenance tips to help keep your reefers running well for years to come. And we’ll take a brief look at the future, examining some new technology.
Last issue, we described heat as a form of energy, and cold as the absence of heat. The idea of refrigeration is to remove heat, to take it from one place, transport it, and release it to another place. Since reefers can collect and pump heat from A to B or from B to A (although not with equal efficiency) they can be used to heat cargoes as well as cool them. But since most applications involve cooling, we’ll concentrate on that.
For those of you who operate reefers, please bear with me. This review of basics is for those who may be considering changing vocations or operating reefers for the first time.
Since refrigeration systems are closed, we can start our examination of the refrigeration cycle at any point. For no reason at all, I’ll start at the compressor.
In a truck’s reefer unit, the compressor is driven by a small diesel engine. The compressor draws gaseous refrigerant in and compresses it. The pressure liquefies the gas. The now-liquid refrigerant gives off heat to the body of the compressor, and ultimately, to the air. It is still relatively warm, so it is pumped to the condenser. The condenser is a heat exchanger. Warmth flows from the liquid to the walls of the tubing, to fins on the tubing. The fins present more surface area to cooling outside air drawn through the condenser by a fan. This is similar to the way a radiator cools an engine.
The refrigerant, having given up much of its heat in the condenser, is now a cool liquid under pressure. It now flows through a metering valve into the evaporator. The evaporator is located in the trailer or compartment to be cooled. The metering valve controls the amount of refrigerant released into the evaporator, acting like a throttle to control the amount of cooling. It also helps maintain backpressure in the high-pressure part of the system, which runs from the compressor to the evaporator.
In the evaporator, the refrigerant rapidly expands, once again becoming a gas. As it does, it absorbs a great amount of heat from its surroundings. Those surroundings are finned coils, which help transfer heat from air flowing over the fins to the refrigerant. Air from inside the trailer is blown over the evaporator. The refrigerant gas, now under low pressure, is drawn back to the compressor where the cycle starts again. The trailer air, now cooled by giving up some of its heat to the evaporator, circulates back into the trailer to keep the cargo cool.
By the way, when you want heat in your trailer to keep a load from freezing in winter, you operate the reefer in reverse. The evaporator becomes the condenser, giving off heat instead of absorbing it, while the condenser becomes the evaporator, absorbing heat from the outside air. Remember, at anything above absolute zero (459.69 F below zero, the temperature at which all molecular motion ceases) some amount of heat exists. It is, therefore, possible to extract heat from what we would normally consider extremely cold air. That’s how a reefer warms during a cold winter night.
All reefer units operate this way, with one exception we’ll look at later. When you get down to it, there are more similarities between Carrier and Thermo King units than there are differences, but there are enough differences to affect your choice one way or another. In order to make a fair comparison, we’ll look at two comparable models, the Carrier Ultra and Thermo King SB-300. Both flow about 3,300 cubic feet of air per minute (cfm) around the interior volume of a 48-foot trailer. Both units give the desired complete change of air every minute.
The Carrier Ultra develops as much as 54,000 BTU/hr. cooling with 35 F return air at the evaporator. At lower temperatures, less cooling capacity is needed. At zero degrees, capacity is 35,000 BTU/hr., and at 20 below, it’s 20,500 BTU/hr. The Thermo King SB-300 puts out (or actually takes in) a bit more, with 64,000, 37,000 and 25,000 BTU/hr., respectively. The Carrier figures are determined at 1,900 engine rpm, significantly lower than the Thermo King’s 2,600 rpm. Measurements for both units are done at an outside ambient temperature of 100 F.
The Carrier’s claimed advantages are higher airflow, at 3,350 cfm, and a reconfigured evaporator opening to enhance airflow throughout the trailer, especially along the sidewalls. The Ultra is slightly more compact allowing for a 60-inch trailer turning radius, two and three-quarter inches less than their competitor’s. Its microprocessor controls are claimed to be among the most advanced in transportation, with automatic start-and-set operation, tactile feedback in the keypad, backlit digital displays, with the Advance controller, and a message center that communicates instructions and faults in plain English. You can download diagnostics and operating history onto a DataShare Download Card, or onto a Data Link unit that can store up to four years of trip data. DataManager lets you analyze information on a PC or laptop computer.
While the Thermo King SB-300 provides fault codes rather than plain language, the codes are posted on the inside of the reefer access door, so there’s no fear of losing them. The SB-300 features OptiSet, which pre-programs temperature management cycles for up to 10 pre-determined conditions, for cargoes you regularly carry. The unit manages return air temperature, operating mode, temperature range, run time and optimized fuel economy.
Both manufacturers claim the ability to maintain set point two degrees above or below the preset temperature.
There are several options, sold under different names by each manufacturer, to make your reefer operate more efficiently. A front bulkhead provides space for return air to flow from the floor back up to the evaporator. Duct work, either fabric tubing or rigid polymer, ensures that air leaving the evaporator will reach all the way to the end of the trailer, resulting in even cooling of the entire load. Without ducting, you run the danger of short cycling. That occurs when air does not travel the full length of the trailer. Instead, it travels part way, a short distance, and then returns to the reefer. The air completes a “short cycle” due to obstructions to the flow caused by improper loading practices.
Noise is getting to be a critical factor in trucking. Besides environmental standards, drivers with sleeper cabs find their beds only a few feet from their reefers’ engines. Engine and compressor noise will interfere with sleep. Carrier provides “Stealth,” a comprehensive package of sound-deadening devices that include sound-reducing baffles, intake silencers, acoustic barriers and special door seals. A similar package, “Whisper Technology,” is available for Thermo King units. Noise reduction is on the order of 10 decibels, or about a 90 percent sound reduction.
Some operators use curtains at the rear of their trailers. These transparent, flexible plastic strips are easily brushed aside by loading equipment, but when in use, they help maintain temperature inside. Unfortunately, they are prone to damage by dockhands and forklift operators. Those operators may be more careful when the trailer’s owner is standing by supervising, but the curtains should be considered high-maintenance items.
Although reefers use both diesel engines and compressors, maintenance is relatively simple. Pre-trip inspections rely heavily on the built-in electronic self-diagnostic systems, but units should be checked regularly for oil leakage at both engine and compressor. Belts and hoses should be checked regularly, too. Standard units call for oil and filter changes at 1,500 hours, or about once every few months, depending on how much the reefer operates when the truck is idle.
Extended maintenance packages may increase engine oil capacity, add bypass filtration and/or use synthetic oil. These lengthen oil drain intervals as much as two or three times, to 3,000 to 4,000 hours. While mechanical components go a long time between service intervals, regular maintenance is necessary before taking on each new load. Make sure all air passages, including the channels on your floor, are clean. Any debris can interfere with airflow. Periodically, remove your forward bulkhead and make sure airways are clean. Check the evaporator for any paper or plastic scraps that may affect cooling. With sufficient care, any reefer will give long life and the performance it was designed for. To make sure that the unit matches your needs, check with a refrigeration specialist – not a trailer salesman – before you buy.
Last month, we acknowledged the help given by Bruce Borowicz and Brian Hufnagel at Chicago Carrier Transicold and by Dave Smith and Robert Stearns of Illinois Auto Central Thermo King. Without them, I never would have realized how much there was to cover, and how little I knew, about refrigeration. Without their help, you wouldn’t have this information. That help is very much appreciated.
The future of refrigeration
Thermo King is now selling an easy-to-operate refrigeration unit with no engine and no compressor. The SB-III CR cryogenic unit is engineered for 53-foot trailers, providing 61,000 BTU/hr. at 20 F below zero and 58,000 BTU/hr. at 35 F. It operates on liquid carbon dioxide. The expanding gas operates a vapor motor, which, in turn, powers the evaporator blower and a brushless alternator. A propane burner superheats the carbon dioxide for defrosting the evaporator and heating the cargo. With 926 pounds of carbon dioxide, the unit weighs 2,200 pounds. Propane and its tank add 140 pounds. At 85 F ambient, the SB-III CR will operate 14 hours on a charge of carbon dioxide with no door openings, and eight hours with 10 15-minute door openings. Using Air Curtains, operating time increases to 10 hours.
Refueling must be done at a special site with high-pressure equipment, which limits the market to fleets with dedicated runs or multiple terminals with refueling facilities. Work is proceeding on getting carbon dioxide refueling stations in truckstops, so it may be feasible for irregular route refrigerated carriers to use these cryogenic reefer units.
Full of fuel, an SB-III CR weighs about the same as a regular reefer. When down to a 100-pound reserve, it weighs less than 1,400 pounds. Cryogenic reefers are clean, quiet, efficient, durable, economical and environmentally friendly. Once infrastructure problems are resolved, they may be the reefers of the future.