Old Ammonia Technology Made New
The contract assigned by Anthony Marano Co. in 1999 to AMS Mechanical Systems Inc., Burr Ridge, Ill., called for a process cooling refrigeration system in a new 220,000 ft2 facility with 35 individual coolers and ripping rooms consuming more than 120,000 ft2. Room temperatures ranged from 33 to 60oF (0.56 to 15.6oC) depending on produce and humidity requirements. Requesting a hands-on system, the owners needed simple equipment without elaborate controls or large ammonia inventories.
During the design stage, three refrigeration systems were considered: Freon rooftop split systems, direct ammonia and indirect ammonia/glycol.
The rooftop systems were eliminated due to the number required, energy consumption, maintenance, life expectancy and inability to provide tight enough temperature control. Direct ammonia didn't make the cut because of the large operating charge required that would have forced compliance with OSHA and EPA programs. Another concern was the adverse effect that even a small concentration of ammonia, inadvertently released, would have on the produce.
Indirect ammonia/glycol was chosen for its low ammonia charge, low maintenance, lower power consumption, longevity and ability to offer excellent temperature and humidity control.
Hands-On ControlIn keeping with the owner's keep-it-simple approach, individual control panels were installed at each cooler. Using a PID temperature controller, the system monitors room temperature, controlling it within +/-0.5oF. Because the greater concern is produce quality over actual room temperature, having individual and localized control offered adjustability.
A microprocessor-based system controls the ammonia engine room; monitors and controls the compressors, chillers, condenser fans and pump exhaust fans; and provides ammonia leak detection.
The Ammonia Side. The refrigeration system consists of three twin-screw compressors, operating at 10oF (-12.2oC) suction with thermosyphon oil-cooling providing 1,125 tons of capacity, and two plate-and-frame heat exchangers, each rated at 500 tons. The compressors discharge to two 500-ton evaporative condensers. The returning condensate drains into a combination thermosyphon/high pressure receiver that supplies liquid ammonia to the compressors for oil-cooling and makeup liquid for a flash cooler. The flash cooler drops the liquid temperature to 40oF (4.44oC), then feeds the surge drums for the heat exchangers, which cool the glycol down to 15oF (-9.44oC). The system provides 1,000 tons of cooling with fewer than 6,000 lb of ammonia.
The Glycol Side. A solution of 33 percent propylene glycol is pumped through 16" mains to the plate-and-frame heat exchangers by two 125 hp variable-speed pumps, each rated at 2,500 gal/min. The system pressure is monitored with a differential pressure transmitter that controls the variable frequency drives. Consistent flow is maintained as the system loads change by monitoring and controlling the differential pressure between the glycol supply and return headers. The glycol solution is pumped throughout the facility to the control stations located at each cooler. In addition, a 100-ton secondary loop provides comfort cooling for the corporate offices.
Two types of control stations were used, depending on the cooler requirements and product to be stored.
The first was for a low temperature cooler capable of maintaining consistent and tight temperature control. In this design, a three-position motorized butterfly valve regulates the glycol flow through the cooling coil. Using a PID temperature controller with relay outputs, the butterfly valve is positioned at 0 percent, 50 percent or 100 percent open, depending on room load. The valve is cycled on/off utilizing time-proportional control. The capacities of the evaporators were selected based on a 5oF temperature difference to minimize product shrinkage. The control scheme maintains temperature within +/-0.2oF of setpoint. Tight control allows the produce company to reduce temperatures near freezing without damaging the fruits or vegetables, extending shelf life and providing higher quality product.
The second control scheme is for the ripping coolers, which often run higher temperatures for produce-ripping and lower temperatures for storage. The stations incorporate a circulation pump and secondary glycol loop. With a secondary loop, higher glycol temperatures are obtainable and the room temperatures actually can be increased using electric heating elements in the evaporators. A PID temperature controller outputs a proportional 4 to 20 mA signal that modulates a motorized mixing valve between 0 percent and 100 percent, depending on room load and temperature requirements. The rooms typically maintain temperature within +/-0.5oF.
No matter if it is old technology or new, knowing the products' requirements is what's important. It's been 80 years since South Water Market first opened with its ammonia-brine refrigeration system. And now there are powerful screw compressors, variable frequency drives, large plate-and-frame heat exchangers and high-tech microprocessors controlling the system, but it still relies on ammonia and a secondary coolant to meet the need for healthy and appealing produce. PCE
Sidebar: Quick MaintenanceRefrigeration systems work hard and live long with little attention. Perform regular system-wide maintenance, including comprehensive weekly or biweekly inspections, depending on the season.
Check for proper compressor operation, verify all fluid levels and operating temperatures. Perform regular oil draining. Check all wearing items such as V-belts. Check room temperatures and calibrate temperature probes if necessary.
Don't forget about the glycol. Glycol systems should be tested every six months for pH and inhibitors. The inhibitors are considered consumable and need to be replenished in order to protect the piping and components from corrosion.
Sidebar: 8 Design TipsWhen designing a refrigeration system for produce, know your products literally inside and out. Temperature and humidity tolerance levels for each type of fruit and vegetable differ significantly. Some produce gives off heat while ripening. And that's just the beginning. Here are eight suggestions to help you develop a system with the tight control you need to provide good-eating produce for your customers.
1. Know your produce. Different produce has different temperature and humidity requirements. Tomatoes like it warm and berries like it cold.
2. Design for high traffic loads. During peak hours, the cooler doors never close.
3. Don't forget "heat of respiration" in your load calculations. Many fruits generate heat as they ripen.
4. Clean and flush the glycol system with an approved cleanser prior to filling the system.
5. Use propylene glycol with food-grade inhibitors to protect the piping and components from corrosion.
6. Use deionized or softened water with food-grade inhibitors. Check water hardness for compatibility with the inhibitors.
7. Use 100 percent duty-cycle actuators on control valves.
Install a side-stream filter to remove any particulates in the glycol along with a chemical feed pot, which can be used to replenish inhibitors.