Food Engineering

Tech Update: Cooling & Freezing Equipment

September 1, 2010
Today’s economy creates hard choices for processors selecting cooling and freezing equipment systems. But solutions exist to decrease energy usage and convert the reject heat of cooling to useful energy to heat water.

The M10 spiral freezer from JBT FoodTech uses a compact design to save energy and minimize the use of hot water for cleaning. Source: JBT FoodTech.

We all want lower cooling bills. But food and beverage processors upgrading or purchasing cooling and freezing systems in hopes of reducing energy usage and costs face several concurrent challenges:

• EPA regulations are saying no to greenhouse refrigerants (R22 or Freon), but switching to new refrigerants can mean system modifications.
• OSHA safety regulations require processors to take precautions to protect workers from hazardous gases such as ammonia.
• FDA/USDA regulations stress food safety.
• Refrigeration is needed to provide the correct environment for several food processes. Newer two-stage, or cascade, systems may provide an effective solution, but cost is an issue.
• Replacing cryogenic gases in tunnel freezers with mechanical systems can be a solution, but the cost-of-entry may be too high.
• Processors would like to save energy over the long haul, but the cost of entry for a new refrigeration/cooling system/plant may be overwhelming.

Shown in the early build phase, J.T.M. Food Group’s AS/RS features small, automated doors at the input stage, reducing energy waste in the cooling incoming outside air, which is an issue with forklifts requiring large manually operated doors in conventional plants. Source: Dematic.

"First-offs" vs. energy savings

It might seem that processors would think long-term energy savings when planning cooling and freezing systems. But in today’s tight economy, it’s the “first-off” (the initial sticker shock) that seems to have a commanding grip over spending more to get long-term energy savings-at least for manufacturing plants (as opposed to warehouses).

In new designs for cooling systems, “it really comes down to the capital costs of the installation,” says Andrew Scott, a mechanical engineer working in refrigeration at The Dennis Group. Scott cites a typical example in designing a project with a two-stage ammonia system, which is fairly efficient. If the original, budgeted amount is held the same throughout designing, engineering and building the system, it’s an easy task to construct the system. But processors have been known to cut line-item amounts midstream, and engineers find themselves cutting wherever they can to meet the decreased budgetary amount.

“Energy and efficiency go hand-in-hand,” says Ekle Small, Stellar director of design, refrigeration services. “We’ll always try to do an energy-efficient system and that’s the primary factor, but first-offs will overshadow everything-especially with today’s budgets.” And while some systems may have been over-designed in the past-with more cooling than actually needed-cost constraints today preclude energy-wasting plans calling for more cooling plant equipment than is actually needed, says Small.

Then there is the opposite problem, says Tera Bristow, JBT FoodTech refrigeration systems engineer. No matter how large the cooling plant is or how well the reserve has been designed and built into it, it will never have enough capacity as the process staff keeps adding more equipment and environmental cooling.

There are some regions where energy rebates are very generous, says Scott. These include, for example, New Jersey, Massachusetts, Connecticut and California. In these states, where processors can show an ammonia system is far more efficient than an air-cooled chiller, energy companies will rebate some costs of a more expensive system to produce a one-year savings.

Small is especially happy with Title 24 in California, which forces manufacturers to be more energy conscious with new systems and creates a level-playing field for suppliers as well. He feels he doesn’t have to shortcut systems in California designs, where the use of VFDs (variable frequency drives) is required, and the rules demand energy-efficient cooling systems. Small says some of his non-California processors are actually using pieces of the California regulations in bid specifications to outline minimum cooling system requirements.

There are, of course, other factors in the decision-making process. “For most of our work, the factor driving the purchase, installation and retrofitting is usually a [food] product requirement-whether it’s for a new product or additional volume for a product,” says Mark Hoffman, SSOE Food Group project manager & business leader. Hoffman says that although cooling system plant upgrades are deemed vital to meet production and process requirements, sometimes efficiency improvements also justify their expense. Another consideration for upgrades involves the phase-out of CFCs (chlorofluorocarbons) and by 2020, HCFCs (hydro-chlorofluorocarbons) that affect the earth’s ozone layer. But changes like these can involve somewhat minor equipment changes, or in the case of converting to ammonia, major system-wide changes.

AS/RS storage spaces such as this at FAGE in Jonestown, NY make efficient use of space, allowing more products to be stored per cubic foot of warehouse space while minimizing energy requirements for cooling. Source: Wayne Labs.

Refrigerants: Ammonia rules for now

Because of the interest in non-polluting refrigerants, a research project involving NIST, Air Products and Toromont Process Systems actually ran a successful pilot application using closed-cycle air refrigeration (CCAR); ordinary air was compressed and used as the refrigerant. (See Food Engineering, November 2003, “Refrigeration, Naturally.”)

Unfortunately, according to a NIST status report (95-01-0150), CCAR technology was not commercially accepted because of a change in market demand. In addition, the cost of equipment construction was higher than anticipated. Nevertheless, Air Products developed two commercial applications based on the Advanced Technology Program-funded technology: a modified closed-cycle refrigeration system for use in manufacturing liquefied nitrogen and oxygen, and a liquid natural gas (LNG) reliquefaction system.

The ability to compress at higher pressures has made it practical to pressurize natural gas at 825psi, ready to enter a pipeline, says Sam Gladis, Vilter Manufacturing business director of heat pumps. This same technology can be applied to refrigeration to create high-pressure heat pumps. While LNG could actually be used as a refrigerant, it’s a scary thought, though Vilter has sold quite a few industrial compressors that actually use propane as a refrigerant.

Ammonia, however, has typically been the refrigerant of choice in food plant operations because it’s relatively inexpensive, efficient and friendly to the environment, should a leak occur. However, at concentrations beyond 200ppm, it’s not friendly to humans, says Gladis. Consequently ammonia plant systems are kept  far away from people and food, usually located in a separate structure or room off the main plant.

In addition, in some states such as New Jersey, if an ammonia plant is located near a school or hotel, a diffusion tank is necessary, says Scott. Should an emergency occur, this allows ammonia to be dissolved in water, preventing its escape into the atmosphere. Keeping ammonia in the machine room and using a secondary loop (or heat transfer fluid) to transfer cooling to the process plant seems to be the trend today, says Small. Popular secondary refrigerants are glycol and carbon dioxide (CO2).

Similarly, cascade systems keep ammonia in the engine room, and Scott has had clients looking at CO2 as a secondary refrigerant. According to the EPA, cascade systems consist of two independent refrigeration systems that share a common heat exchanger. Each system uses a different refrigerant suitable for the given temperature range. The high-temperature system (“high side”) located in the cooling plant/machine room uses high-boiling point refrigerants such as R-404A, R-507A, R-134A, propane, butane and ammonia, whereas low-temperature systems (“low side”) in the process area use low-boiling refrigerants such as R-744 (CO2) and R-508B.  The advantages of a cascade system include a reduction in the refrigerant charge and a reduced carbon footprint.

Secondary loop systems and cascade systems that use ammonia as their primary refrigerant keep the ammonia at low pressure (called a low-charge system). In the event of a power failure and a return to power, compressors start at low pressure. Small says one of his projects called for direct expansion, where only gas is brought back from the evaporator rather than a mixture of gas and liquid, creating a lower charge in the overall system.

To use ammonia at temperatures and pressures required for industrial heat pump applications, compressors must be able to handle a minimum of 750psia at 195

Reclaiming the heat of refrigeration

With ammonia refrigeration, the extent of pressures and temperatures range from 5psia/-60

Ways to save energy

On the food processing side, freezer and cooler makers are concentrating on saving energy. “We tend to tailor our equipment toward our customers’ needs, but we’ve also identified that our customers are getting smarter [about saving energy], and we’ve found that it’s been important to invest in reducing energy costs with our equipment,” says Andrew Knowles, JBT FoodTech applications engineer.

Knowles says JBT FoodTech spiral freezers use less power than typical low-tension freezers because they’re self-stacking and smaller than several low-tension systems. Therefore, less water is needed for cleaning.

The major load in a spiral freezer isn’t really cooling the food; it’s the heat gain of the fans because of the tremendous air turnover to remove heat from products, says Scott. The fans should be able to handle the heat load of the food and not be oversized.

The best way to conserve energy in a spiral freezer is to eliminate unnecessary fans and make the air flow more efficient, says Knowles. Instead of using horizontal air flow across the product, in theory, vertical air flow would be more efficient. In addition, a self-stacking design can accommodate more belts in the same footprint of a low-tension freezer. So while the trend right now might be toward the lowest price, as the economy picks up, Knowles thinks processors will realize the value of having a freezer with a smaller footprint so they don’t dump three times more water than needed to clean it.

In older plants where refrigerated spaces have been re-purposed, the coolers may not be right-sized for the new application, says Scott. A solution might be to install an evaporator that uses fractional-hp fan motors, rather than motors of 3hp or more.

The use of VFDs to control motor speeds was not very prevalent until recently, says Horn. “VFDs, in my way of thinking, are one of the single most energy-efficient things you can do for your refrigeration facility-both for condenser fans and evaporator fans, and certainly the compressor motors,” he states. VFDs allow fine-tuning of systems to make optimum use of energy, and screw compressor manufacturers have been including VFDs, which minimize the energy penalty associated with part-load performance.

The refrigerated space itself can be designed to save energy. Factors include doors, seals and penetrations into the room, says Hoffman. With older facilities, it’s not practical to redo walls, but sometimes a roof replacement can make sense, especially if it’s old. Since the roof is the major heat load of a refrigerated space, new reflective materials can be used, and insulation can be increased as well.

Forklift doors that tend to remain open and large roofs create energy waste. After building several conventional storage freezers, J.T.M. Food Group finally built a 20,000-sq.-ft. AS/RS freezer. Recognizing that forklifts let out a tremendous amount of cold air, Joe Maas, J.T.M. vice president of operations, wanted a freezer that was as efficient as possible, operated by a minimum number of people and designed with a minimal roof footprint. Designed and build by Dematic, the AS/RS freezer has small automatic doors and “uses one-third of the refrigeration we felt it was going to need based on everything we knew prior,” says Maas. “We originally built in two compressors, but only one of them ever comes on because the other is not needed.”

Cleaning, energy, food safety

Cleaning, conserving energy and maintaining food safety might seem at odds with each other, but nonetheless are critical considerations. “I would suggest that an overlooked factor [driving selection] is sanitation/cleanability of any food chilling or freezing equipment,” says Ron Idol, Air Liquide business development manager. With the escalating recalls of the last several years, the risk factor from equipment that either harbors a pathogen or used equipment that brings a pathogen into a plant is simply too great to ignore. “Food chillers and freezers designed to meet USDA Certified Cleanable standards or EHEDG cleanable standards will allow food plant managers and owners to sleep at night,” adds Idol.

“We have customers with spirals that have extended runs-three days before they go into a washdown and cleanup,” says Small. These rooms should be held to 40

Cryogenic vs. mechanical quick freezing

There’s no doubt that cryogenics has a role in IQF foods. As far as cryogenic refrigerants go, one of the greatest current issues is the seasonal shortages/surcharges for liquid carbon dioxide (LCO2) in some parts of the country, says Idol. Several LCO2 suppliers in the Midwest are experiencing outages and shortages that put their customers on allocation or can shut them down for days or weeks. For many cryogenic operations, it is now feasible to use either LCO2 or liquid nitrogen, and this is true for both freezing and chilling operations such as bottom injection for mixer/blenders. Consequently, operators should carefully evaluate their options and costs and select the cryogen that is going to have the most stable cost and be available year-round.

Today, a processor can opt to use conventional ammonia-based equipment for IQF foods, but this decision is akin to buying vs. renting software. There’s a simplicity associated with using LCO2 in freezing tunnels, says Gladis. It’s delivered in a tanker truck, and the processor’s tank is filled. The processor draws off the tank by feeding the LCO2 into the freezing tunnel. It vaporizes at -60