SPECIAL REPORT: Energy To Burn

You’ve heard the old adage, “If you can’t stand the heat…” Because kitchens are the soul of every foodservice operation and the source of your livelihood, no one’s about to get out when things get a little toasty. Instead, many of you are taking the heat out of the kitchen these days and using it to warm water, temper makeup air or run warewashing equipment more efficiently.

No doubt by now you’ve also heard that foodservice facilities are among the most energy-intensive buildings per square foot on the planet. And the by-product of practically every piece of equipment you plug in to an electrical socket or hook up to a gas (or steam) line is heat. Doesn’t it make sense to capture some of that wasted heat and reuse it? Fortunately, some equipment manufacturers agree and are designing products to do just that.

While waste-heat recovery technology has been around for a long time, foodservice operators took little interest in it until energy costs began spiraling upward a decade ago. The trend began in Europe where energy costs, particularly for electricity, have been much higher than in the U.S. Now, despite a glut of natural gas on the market, waste-heat recovery can make sense for many operators.

To date, manufacturers have developed ways to recover waste heat from three equipment categories: warewashers, refrigeration systems and the cookline.

Waste-Not Warewashing

For a number of reasons, the most highly developed equipment category is warewashing. Advances in technology and design have made it possible for manufacturers to build warewashing equipment that uses much less water, decreasing operators’ incoming/outgoing water costs as well as the required heating energy—an attractive proposition. Now, manufacturers can build waste-heat recovery right into the equipment, making it possible to achieve additional energy savings.

So, how does it work? Warewashers with waste-heat recovery incorporate a heat exchanger at either the load end (on one maker’s version) or the unload end. Cold water pipes through the heat exchanger, and heat generated by the machine’s operation warms it to at least 110°F. The 110°F water then flows to the machine’s booster heater where it’s heated another 70° to the NSF-required 180°F for the final rinse. Savings can be found in the reduced load on the facility’s building hot water supply. If the operation relies on an electric water heater, the savings will be higher than if a gas water heater is used; in most parts of the U.S., electricity costs more than gas.

At least three warewasher manufacturers offer waste-heat recovery on some machines, either as standard or optional equipment. Models include door-type, rack-conveyor and flight-type machines. Depending on the manufacturer, each has its own idiosyncrasies, but in general they all work on the same principle.

Door-style machines can be natural candidates because it’s easy to capture heat in a closed chamber. Because they often run continuously throughout the day, flight-type machines offer the greatest potential for energy savings simply because of the volume of heat they generate and hot water they use. Rack-conveyor machines tend to be much shorter in length, so they don’t generate as much waste heat, meaning there’s somewhat less to capture compared with flight-type machines. But depending on the operation and local utility costs, rack machines with heat recovery still can deliver savings, according to the manufacturers.

Flight and rack-conveyor warewasher installers will hook up the washtanks to a hot water supply and connect the waste-heat recovery system (and indirectly, the booster heater) to a cold water supply. However, because the booster heater supplies final rinse water (which, again, must be at least 180°F at the final rinse manifold), manufacturers use different tricks to make sure it produces an adequate amount.

All warewashers use hot-water fill at start-up for both the washtanks and booster heater. During start-up mode, washtanks and booster heaters heat incoming hot water to the appropriate temperatures, whether 150°F for the wash or 180°F for the final rinse. Because machines with waste-heat recovery use a cold-water hook-up to supply the booster heater (through the heat exchanger), the heat exchanger won’t do much good until the machine gets going and warming up—that’s when waste-heat recovery begins to subsidize the booster.

Likewise, if a machine has been idling for a while, the booster heater will have to kick in to achieve proper water temps until the waste-heat recovery has enough heat to recover. Warewasher manufacturers have developed a couple of ways to ensure that the booster kicks in as needed whenever the temperature of water coming from the waste-heat recovery falls below 110°F.

One manufacturer’s machines monitor the temperature of the heat exchanger coil. If the coil temperature falls below 110°F, the sensor signals the booster heater to kick on. Another maker uses a slightly different technique. Its machines monitor water temperature in the waste-heat recovery unit. If the water temperature falls below 105°F, the sensor signals a solenoid switch that diverts hot water from the washtanks’ hot-water line to the booster heater until the warewasher gets running. Whichever option is presented, be sure to ask the manufacturer how long the machine needs to run before waste-heat recovery begins.

The most ideal scenario for waste-heat recovery is one in which the warewasher runs for long periods, not intermittently.

Sample Savings

One manufacturer cites a customer whose facilities are equipped with electric water heaters rather than gas. The customer put dedicated electric meters on rack-conveyor machines with waste-heat recovery in a few of its stores as a test. In one store, the customer saved an average of 148 kW per day. A lower-volume store saved an average of about 69 kW per day. The manufacturer’s warewasher also includes a feature that allowed the customer to remotely monitor how much cold water the machine was using (vs. hot water shunted from the wash-tank’s hot-water line). The high-volume warewasher used about 92% cold water on average.

In simple terms, if you figure an operation typically is open six days per week and estimate electricity costs about $0.09/kW, the savings for the first store would be more than $4,150 per year. The lower-volume store would save around $1,900 per year. At $0.12/kW, the savings would be higher—more than $5,500 and nearly $2,600 per year respectively.

Based on these numbers, payback on the waste-heat recovery premium for this particular operator would be approximately a year; however, the operator relied on electric water heaters to supply the facilities. The savings would not be as dramatic, and the payback would take longer, if gas hot water heaters supplied the building’s hot water. Manufacturers say the payback on warewashers with waste-heat recovery is generally anywhere from one to four years depending on the size and operation of the machines, the cost of utilities and whether the building hot water supply is gas or electric. Any warewasher manufacturer can run an energy audit and show you how much you’ll save in energy costs each year; in fact, you should insist on it.

Where Does The Air Go?

Another big advantage to waste-heat recovery systems is that they typically reduce the temperature of the air being vented from the machine from about 140°F to around 95°F. This air is still pretty moist, so you need to vent it out of the dishroom, but at a dramatically reduced cfm—typically about a third less than that needed for conventional flight and rack-conveyor warewashers.

Depending on the size of the machine, one manufacturer says that you’ll need to vent anywhere from 500 cfm to 1,500 cfm on its warewashers equipped with waste-heat recovery (you may need up to an additional 600 cfm if your machine has a blower dryer). Other manufacturers say you can go as low as 250 cfm to 275 cfm on their smaller rack-conveyors (although typical rack-conveyors only need about 400 cfm anyway, so the lower cfm doesn’t save much). The bottom line is that your HVAC system won’t work as hard, saving you even more energy, and the dishroom will be more comfortable for employees.

Door-type machines available with waste-heat recovery systems use the same basic principle, but work slightly differently. Because they wash dishes in batch cycles, rather than continuously, the heat exchanger needs more time to convert heat generated by the wash and rinse cycle into hot water.

Because door-type machines are closed during operation, the heat exchanger is designed to act as a condenser, using incoming cold water to condense and cool the hot water vapor generated during the wash/rinse cycle. Once that’s accomplished—usually adding about 30 seconds to each cycle—the air can be vented directly into the room at about 80°F instead of using an exhaust vent.

As a result, these are called “ventless” door-type machines. Although the extra cycle time they require may put some operators off, the ventless feature, rather than energy savings, makes them a good choice in certain applications. Because these warewashers don’t require a Type II hood or venting, a lot of operations from schools to QSRs are installing these high-hood door models in place of three-compartment sinks to wash pots, pans and utensils.

One disadvantage is that unless the incoming water is relatively cold—about 50°F to 60°F—the condenser isn’t very effective. Places such as Arizona where incoming water temperature is around 70°F or so aren’t conducive to ventless warewashers. The warmer water can’t pull enough heat or moisture out of the air into the machine.

Costs And Considerations

Manufacturers estimate the premium for waste-heat recovery systems on warewashers adds about $2,500 to $3,000 to the cost of a door-type machine, around $3,000 to $5,000 to a rack-conveyor and anywhere from $5,000 to $7,000 to a flight-type. While that sounds like a lot of money, it can turn out to be a bargain when you consider operating costs over the life of the equipment.

There are, however, a number of other costs and factors to consider:

Utility costs. A manufacturers’ audit should be able to tell you if you’re better off in your area and facility to heat water going to the machine with your building hot water heater than to heat water at the machine with waste-heat recovery.

Cold water line. Does your dishroom currently have a cold water supply? (It’s likely if it has a three-compartment sink and/or hand sink.) Installation costs may be higher for a machine with waste-heat recovery if you have to add (or move) cold water lines.

Water treatment. If you have hard water, you might already be using a water softener for your hot water supply to the dishroom. But you’ll have to treat your cold water as well if you aren’t already. (Most operations with hard water typically treat the supply for the entire facility, but make sure.) Hard water and waste-heat recovery do not play well together.

Maintenance. Make sure you ask the manufacturer if the waste-heat recovery unit requires maintenance; if so, what is entailed and who will do it? It can be an added cost, and it’s critical.

Energy rating. Compare the rated kW of the machine you want to purchase with the one you’re replacing. A new warewasher with a waste-heat recovery unit could use up to 85 total rated kW. It may be more efficient than your present warewasher with the same rated kW, but another new machine without waste-heat recovery at 60 kW might get you a big rebate from your local utility. Check out both scenarios and do the math.

Cold Walk-In, Hot Water

Refrigeration is another area from which waste heat can be recovered and reused. Systems that use hot gas from refrigeration systems to heat water have been available for nearly a century. A Milwaukee brewery reclaimed heat from its refrigeration system back in 1916.

Two types exist today, both of which require some advanced planning and consideration before installation. Tube-in-tube essentially runs the refrigeration line inside a water line, transferring heat directly to the water. They’re fairly flexible in terms of installation, but are modestly efficient, and maintenance costs can be high.

The other type is a passive tank system. Refrigeration lines are routed into this water storage tank that’s wrapped with an insulated waffle-shaped heat exchanger. The exchanger acts as a “super de-heater,” and while it doesn’t replace the condenser in your refrigeration, it does pull a lot of the heat from the refrigerant gas. This heats the water in the tank, and as the warmer water rises to the top, it pipes from there to your hot water heater. Temperature rise ranges from 10°F to 20°F up to 70°F to 80°F depending on the size of the refrigeration system and volume of hot water usage.

With no moving parts, these tank-type units cost virtually nothing to maintain, and with no heating elements there’s little build-up of scale inside. They’re also very efficient.

Major drawbacks of the system are that they require a fair amount of floor space (the tanks are much larger than a typical water heater), and they must be located fairly close to refrigeration compressors. To be truly cost-effective, these units are designed for facilities with large refrigeration systems and high-volume hot water requirements. Makers typically recommend you have a minimum of a 3-ton refrigeration system before you consider installing a heat-recovery tank. QSRs probably don’t use enough hot water to warrant such a system, for example, but a full-service restaurant might, and large facilities such as hotels, casinos and stadiums definitely can benefit.

Installation costs of tank-type systems for restaurants typically run $2,500 for the system, $300 for copper pipe and labor costs for two people, two days.

The question then becomes how much energy you can reclaim from your refrigeration and/or air-conditioning system. A 3-hp walk-in freezer and 2-hp walk-in cooler generate about 50,000 BTUs of waste heat per hour. At a $1.00-per-therm cost of gas or $0.12 per kW cost of electricity, that’s about $0.84 to $1.96 an hour, depending on which energy source you use.

An average restaurant uses 2.4 gals. of heated water per day per customer, according to ASHRAE. But for the sake of an example, let’s say you heat 100 gals. of water per day from 55°F to 140°F. You have a gas water heater that’s 60% efficient. Multiply 100 gals. by 8.33 lbs./gal. (1 gal. weighs 8.33 lbs.) times that 85º temperature rise to get 70,806 BTUs. Divide that by 60% and you get 118,010 BTUs to heat 100 gals. of water. If you’re paying $1.00 a therm for gas, your energy cost is $1.18 to heat 100 gals. of water. An electric heater at 90% efficiency would use 78,672 BTUs to heat 100 gals. of water. At 3,413 BTUs per kWh and $0.12 per kWh, your energy cost would be $2.77.

Compare those numbers with the amount you’d save if you apply 50,000 BTUs of captured waste heat per hour in the example shown above, and you’ll get an idea of what kind of payback you can expect. An added bonus is that in general every 1°F drop in condensing temperature increases the efficiency of your refrigeration system by 1%. If you cool your condenser down by 10°F using waste-heat recovery, you’ll improve the efficiency of your refrigeration system 10%.

Now You’re Cooking

Relatively new to the market are systems designed to reclaim heat from cooking equipment. There are a few different types available, giving you options depending on your operation.

The first type is a back-shelf proximity hood for gas cooking equipment such as fryers or griddles. The hood contains a heat exchanger that transfers clean combustion heat from gas appliance flues to water lines. When you’re talking exhaust flue temperatures of anywhere from 400ºF to 1,100°F, there’s a lot of heat that can be reclaimed.

The heat reclaim hood preheats water and sends it to the hot water heater. Depending on what type of equipment you place under the hood and exhaust flue temperatures, however, the manufacturer says that the hood can provide enough 140ºF water to supply a typical QSR when the cooking equipment is working at full capacity.

Another type of heat-reclamation from cooking equipment comes from the same company in the form of a makeup air unit (MAU). Situated on the roof over the hood exhaust fan, the unit captures exhaust air at about 110ºF. The air passes through a closed glycol-loop heat exchanger that in turn warms incoming makeup air before it’s introduced into the kitchen.

The unit can be equipped with supplemental heating for facilities in cold northern winter climates, and in the summer the unit is capable of heating water instead of makeup air. The unit basically eliminates the need for a fan and standard MAU.

Maintenance is nominal, essentially consisting of quarterly cleaning. An indicator on the control panel alerts you to dirt on the coil, and if the air pressure drops enough, the system automatically shuts down.

Obviously more effective in northern-tier states than in the south, the unit can save you about $1.00/cfm of makeup air per year in Minneapolis, about $0.80/cfm per year in Chicago and $0.76/cfm per year in Boston based on average winter temperatures, operating 14 hrs./day, 6 days/wk. Payback for most operators using the units is about three years.

A third type of heat-recovery unit is a specially designed grease filter which contains water lines. Quick-disconnect stainless steel flex hose makes the filters relatively easy to install and remove for cleaning in a dishmachine. The water lines are encased in a Teflon-coated copper heat exchanger. Hot air from the cooking surface heats the encased water lines and the heated water pumps from the filters to your hot water tank, but only when the cooking equipment is on.

Though less efficient than the other two forms of heat recovery designed for cooking equipment, according to tests conducted at PG&E’s Food Service Technology Center in San Ramon, Calif., the filters may offer a less costly alternative for some operators.

Whichever alternative you consider, recovering waste heat in your operation ultimately can save you thousands of dollars in energy costs over the life of the equipment.

Two More Ideas

A couple of energy-saving ideas you may want to look into depending on your operation and where your stores are located are heat-pump water heaters and a heat-recovery unit designed for warewasher drains.

A number of manufacturers make heat-pump water heaters for residential use, but they can just as easily be used in foodservice facilities. In warmer climates, they operate like refrigeration in reverse. Instead of removing heat from a box and venting it into the air, heat-pump water heaters use a low-pressure refrigerant to remove heat from the outside air and transfer it to water in the tank. For more information, go to the EPA’s ENERGY STAR website, energystar.gov, and look up water heaters under “Find Energy Star Products.”

The heat-recovery drain unit actually was originally designed in Canada for residential shower drains, but its potential in foodservice is tremendous. The FSTC tested the unit’s effectiveness at recovering wasted heat from warewasher drain  water and found that under certain circumstances, it resulted in as much as 36.8% water-heater savings. For a copy of the test results, go to fishnick.com/publications/appliancereports/dishmachines/.”””