When the Power is Cooking the Food Stays Cold

Before processed food even reaches the grocer's freezer cases or is served up at a restaurant, it experiences an extended storage life. And to adequately refrigerate product throughout its storage calls for lots of power—reliable power. Taking a close look at a food-processing or storage facility's total critical power system—from utility entrance to shipping dock—offers up ma...

06/01/2003


Before processed food even reaches the grocer's freezer cases or is served up at a restaurant, it experiences an extended storage life. And to adequately refrigerate product throughout its storage calls for lots of power—reliable power.

Taking a close look at a food-processing or storage facility's total critical power system—from utility entrance to shipping dock—offers up many options and opportunities to improve a plant's reliability.

It's a compelling issue. The value of food—and pharmaceuticals—dictates that facilities maintain adequate environmental temperatures during loss of normal power. Not only that, but the U.S. Department of Agriculture and the Food and Drug Administration govern both storage and process environments. Contaminated product is a danger to the public, and product recalls can be expensive to a producer's reputation and bottom line.

Step One

The first step in a power-quality strategic plan is to have primary power from two reliable sources. Utilities can provide the owner with a report of historical outages to help determine their reliability. Primary power should be provided from two independent substations to reduce the chances of an outage and allow maintenance of the primary distribution. Each service should have capacity to serve 100% of the plant if the other service fails or is down for maintenance. A selective primary loop distribution such as the one shown in Figure 1, p. 14, allows for balancing the load on each substation and also for isolating a failed loop cable.

Underground distribution in ductbanks will provide physical protection of the cables and ease replacement of any faulted cable, and a spare conduit allows for future expansion. Overhead distribution systems are less reliable due to weather and lightning strikes. Consequently, lightning arrestors on the primary and surge-protection devices at the secondary services can reduce the risk of damage due to transient voltages. Proper grounding will also reduce power quality problems in the plant. In addition, providing redundant primary distribution to many locations reduces voltage drop by placing the transformer closer to the load served. Secondary unit substations can be designed with tie breakers to provide full or partial redundancy on the secondary distribution as shown in Figure 2, p. 14.

Designing to standard sizes and stocking a spare of the most critical transformer, circuit breaker and starter will reduce plant downtime. When a food processing plant is designed using the standard equipment provided by the local electrical cooperative, this allows the owner to contract with the utility for maintenance and emergency stock of the 18 primary pad-mounted switches and transformers. Power monitoring meters will also help diagnose power quality or distribution problems and provide information for future expansion or changes to the system. Monitoring systems, as well, can be networked to provide centralized information and control.

The Clock is Ticking

How much time does one have to get the power back on before any damage is done? The first step is to identify what loads are critical to maintaining the product or process. A well-insulated storage warehouse is not going to rapidly rise in temperature if the refrigeration goes off. But losing power to a sub-zero blast freezer will stop production. Thus, providing connection points to the electrical distribution for temporary rental generators may suffice if hours of lead-time to install equipment after a power outage are available.

Refrigeration for freezers, cold storage and processes is a large percentage of many food processing loads. For example, a 600 hog per hour pork processing plant has 4,300 hp of refrigeration compressors, which is 30% of the plant total connected load. The two medium-voltage motor control centers are fed from two independent 4,160-volt distribution transformers to provide half of the refrigeration capacity if a single transformer fails. Figure 3, p. 16, illustrates this system.

Specifying the large refrigeration compressor motors at medium voltage insulates the plant distribution from electrical disturbances due to motor starting. In such a case, motors over 250 hp should also be considered for medium-voltage service to reduce transformer, secondary distribution and starter equipment costs. In general, medium-voltage motor controllers with vacuum contactors have reduced the physical size of the starters. If the motors are to be on standby power, the generator must be medium voltage or feed into step-up transformers.

Segregating critical loads and feeding them from separate power paths is another method of improving power reliability. Additionally, separate electrical distribution for critical loads can be more easily fed from a temporary power source. And separate electrical rooms for critical equipment can prevent a fault in the normal system from affecting the critical loads. An example would be to have the refrigeration compressors and evaporators required to maintain the cooler storage on a separate transformer from the remainder of the process refrigeration. With the refrigeration circuits on a common header or valved to allow switchover, the remaining the remaining refrigeration equipment would maintain the temperature in the cooler box if one transformer failed or lost power refrigeration equipment would maintain the temperature in the cooler box if one transformer failed or lost power.

Switchgear should not be overlooked in the critical power path. Also, designing with drawout gear reduces downtime.

Standby for Power

Optional standby systems are typically specified to provide an alternate source of electric power for food processing and storage facilities' refrigeration systems to prevent damage to the products during outages. This system provides power during extended power outages. The transfer of selected loads may either be automatically or manually initiated. These optional standby systems should be designed to comply with the National Electric Code (NEC 702). Normal and standby system wiring is permitted to occupy the same raceways, but segregating them will reduce changes of a fault affecting both systems.

Standby power systems typically consist of an engine/generator set, transfer switch, controls and an emergency power distribution system (see Figure 4, p. 16). Multiple generator sets are operated in parallel to obtain capacity in excess of 2 MW. Most storage facilities have enough mass—thermal inertia—to ride through a short power outage and could use a manually initiated start and transfer of the critical loads. It's best to keep the manually initiated steps simple and well documented to prevent errors. Processes with a small temperature tolerance may need an automatic start and transfer system that can restore power in 30 seconds or less. Monitoring equipment and critical controls should be designed with battery backup or on an uninterruptible power supply (UPS). The UPS can provide energy automatically during the transfer time to a standby source.

Standby generator sets are available in diesel, natural gas or dual fuel. On-site storage of liquefied petroleum can be a backup fuel to a natural gas/LP engine if there is a probability that the natural gas utility could be interrupted at the same time when normal power is lost. On-site diesel fuel storage can also be sized to provide extended operation during power outages. Additionally, generators can be located indoors or outdoors with weather enclosures. Sound attenuation enclosures are available for sites with noise ordinances and air quality restrictions must also be considered when siting a standby generator system.

Engine-driven refrigeration compressors are also available. A natural-gas spark-ignition engine-driven compressor could be designated as a standby to the electric-driven compressors to reduce the size and cost of a standby generator set. Engine-driven compressors may also be operated during times of high electrical demand to reduce the plant monthly demand charges. An analysis should predict when it is economical to operate this kind of compressor, because these units may have a higher maintenance cost than electrical motors and are only available in limited capacities.

For loads or processes that cannot tolerate any outage, voltage sag or transients, a UPS is ideal, even though most of the storage and cooling loads do not require this level of power quality. These units are typically specified with ten minutes of battery backup and then connected to generators for extended outages. Flywheel-type UPS units can provide enough ride-through time for the generator to start, and they eliminate the need for backup batteries and maintenance. Note that most UPS are in the 30- to 500-kVA range, with modules paralleled to provide more capacity.

Another option now available, medium-voltage static transfer switches, can transfer a load from one independent source to another within four milliseconds. This is fast enough to prevent most critical loads from seeing an outage.

An optional standby system could also be used to peak shave or produce power in parallel with the utility service. Peak shaving consists of a transfer scheme to disconnect a load from the normal power and connect to the standby system when the plant is nearing its peak demand limit. Peak shaving can be initiated manually or by an energy management system connected to the utility meter. The transfer scheme must have protection to avoid transfers to loads out of phase with the source.

Closed-transition transfer switches can prevent an outage on the retransfers from standby to normal sources and programmed transfer from normal to standby. This is accomplished by making the source contact before breaking the alternate contact. The closed transition can pay for itself by reducing costly restarts of critical processes. Transfer switches with bypass isolation allow for manual transfer to perform maintenance and testing. Therefore, bypass isolation transfer switches may be a wise investment for high value processing or storage.

If an owner wishes to parallel with the utility, this will require more expensive controls and switchgear that must meet the utility's protection standards. This means that there is no possibility that a utility circuit can be energized if power is lost. Fortunately, manufacturers of generator sets are now packaging paralleling switchgear, which has reduced the installed cost and shortened the lead-time for an installation. However, utility rate structures should be investigated to see if an interruptible rate or other options may be available.

In the Final Evaluation

In short, an economic evaluation of the power options should be prepared so the owner can make an informed decision. The return on investment will vary widely based on the options chosen and the value of the product saved from spoilage. Typical economic life of the generator sets is 15 to 25 years for standby usage, so a life cycle analysis should be conducted.

In sum, there are many options to provide critical power to food processing and storage facilities. In fact, there are almost as many possibilities as there are different food processing plants and storage facilities. Consequently, it takes planning and good design to protect a facility from power events which could potentially spoil its products.



Choosing Cogen

Interested in cutting down the fuel bill? Plants that have a concurrent usage for the thermal energy from the generating source may consider cogeneration power systems. A cogeneration gas turbine/generator with a waste heat recovery boiler could replace a gas-fired boiler and provide economical electrical power to the plant.

Cogeneration units have combined thermal efficiencies of 70% to 80% vs. 30% to 40% for combustion engine generators without heat recovery. This essentially means cutting fuel costs in half. For example, a 4.6-MW gas turbine installed in a confectionary plant boiler house produced 24,000 lbs. per hour of steam at 160 psig with 11,630 Btu/kW input. The electricity generated is 29% and the steam is 53% of the input for a total efficiency of 82%. This particular project had a projected payback of 5.8 years and 17.9 rate of return. The payback can be calculated to be even shorter if credit is taken for avoided loss of product.

The balance of thermal and electrical energy production to a plant's loads is very important. A study of the electrical and thermal load profiles needs to be prepared to pick the most economical sized unit. The need for thermal energy should coincide with the electrical power requirements to avoid dumping waste heat into the atmosphere. Gas turbine exhaust may also be used for many drying processes.

Excess waste heat can be fed to an absorption chiller to produce cooling. Excess steam can also be fed to a steam turbine/generator to produce more electricity. Recovering heat from gas and diesel engine coolant and exhaust produces lower temperature energy, which can be used for water heating processes. Most utilities will purchase any excess electricity that may be generated, but the rate may not be economically viable.

Independent energy companies can also provide and operate an energy plant on one's site as well as sell steam, electricity, chilled water or refrigeration.



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