Ovens in Commercial Food Service Operations

END-USE BRIEFING Summary A Quick Tutorial on an Important Energy End Use An oven can be simply described as a fully enclosed, insulated chamber used to heat food. There are many variations of this basic concept in the commercial Ovens in kitchen, but each type of commercial oven cooks by controlling the temperature and/or humidity of the oven cavity. Commercial The oven’s versatility makes it useful in Food Service many types of food service operation. Efficiencies vary between the different Operations types of ovens and fuel sources. Stan- dard-efficiency gas ovens are typically 30 to 40 percent energy efficient; high- efficiency gas ovens between 40 and 50 percent. Electric efficiencies run between 50 and 80 percent.

This end-use briefing offers a quick tutorial on the common types of ovens in commercial food service operations. It covers the following topics:

· Definitions of key terms, such as standard and convection. Summary ...... 1 · Description of common oven Definitions of Key Terms ...... 2 types and their operational characteristics. Conceptual Overview ...... 3 Common Oven Types and · Description of new oven tech- Categorization...... 3 nologies and their operational characteristics. New Oven Technologies ...... 7 · Typical applications of each type Typical Customer Applications...... 8 of oven. What to Look for in the Field...... 8 · Key field observations to help as- Tips for Efficient Operation And sess operational efficiency and energy Maintenance...... 8 conservation opportunities. References to More Information...... 9

Copyright © May 1997, Pacific Gas and Electric Company, all rights reserved. Revised 12/10/1999 · References to other documents that induced in the oven cavity by fans or provide detailed information on the top- blowers. ics in this briefing. · Indirect Fired: Describes gas ovens which route the combustion products Definitions of Key Terms around the bottom, sides and top of the oven without entering the cooking cavity. The oven cavity walls of indirect · Air Impingement: Jets of air are di- fired ovens become hot and heat is rected onto the food product using a transferred to the food by either con- ported manifold. vection or forced convection.

· Atmospheric Burner: A burner · Idle Energy Rate: The rate at which which jets gas and air through a hole an empty oven uses energy to maintain (“burner port”) at which point it mixes its cavity temperature at the thermostat and is ignited. This “Bunsen” type set point (e.g., 350ºF). burner shows a blue flame when prop- erly adjusted. · Infrared Burner: A burner made of porous ceramic plates or metal screens. · Conduction: A cooking process in Combustion of premixed air and gas which heat is transferred to the food via takes place on the burner surface, direct contact with a heated medium which can reach 1800ºF. The high sur- (e.g., ceramic hearthstone, firebrick or face temperatures cause the material to composite hearth.) emit radiant heat. Up to 50 percent of the energy in the gas can be converted · Convection: A cooking process in to radiant heat. which currents of hot air transfer heat to the surface of the food. Convection ov- · Natural Convection: Convection ens use fan(s) to circulate hot air in the created in the oven cavity due to the oven cavity. natural movement of hot air heated by burners, elements or heated cavity walls · Cooking Energy Efficiency: The (i.e., without the use of fans or blowers). ratio of the quantity of energy absorbed by the specified food to the quantity of · Oven Capacity: Maximum amount energy input to the oven during a cook- of food that can be cooked at one time ing energy efficiency test, expressed as (e.g., 6 12”-diameter pizzas, 40 pies, a percent. 20 to 40 sheet pans).

· Direct Fired: Describes gas ovens · Oven Cavity: The cooking zone, which route the combustion products chamber or compartment in an oven. from the burners through the cooking cavity, transferring heat directly to the · Radiant heat: Hot surfaces emit ra- food from the hot gases. diant heat. In ovens, the heat radiated by burners, elements or the heated cav- · Forced Convection: Convection ity walls moves towards the food.

PG&E Energy Efficiency Information© “Ovens in Food Service Facilities” Page 2 · Rated Input: (Also “Nameplate In- convection ovens are divided into sub put”) The maximum rate of energy con- categories (rack, combination, con- sumption for an appliance. veyor, rotisserie, etc.). The matrix in Table 1 presents the characteristics of each sub category, along with common Conceptual Overview use and customer application. Standard Commercial ovens are available for use with gas, electricity or liquid propane Standard ovens use natural convection and come in many sizes. (hot air currents) and radiant heat to cook food products. These ovens can An oven heats food by surrounding it be used for nearly all types of food with hot air. Heat is transferred to the preparation. Because they do not use food via convection, conduction, radia- blowers or fans to move air inside the tion and steam, either singularly or in oven cavity, standard ovens are used combination. In standard ovens the for precision-baking sensitive pastry burners or elements heat the air inside products such as meringues, cream the cavity causing natural convection puffs and pastry shells. Standard ovens currents which transfer heat to the sur- are the least expensive to purchase, but face of the food. What the industry calls they are not as fast-cooking or flexible a “convection” oven employs motorized as forced convection ovens. fans or blowers to create forced convection currents in the cavity; this cooks Standard ovens usually have simple the food faster while maintaining uni- controls, limited to a thermostat and a form heating. selector that allows the oven to bake or broil. Modulating thermostats, which Heat may be transferred to a oven ei- adjust the burner incrementally, are ther indirectly or directly. In indirect-fired most common. ovens, the hot combustion products of the gas heat the bottom, sides and top Range Oven of the oven without entering the cooking cavity. In direct gas-fired ovens the hot The most common standard oven is the combustion products are directed range oven. Range ovens are part of through the cooking cavity rather than the base of a rangetop, and are the fa- around the cavity. Heat is transferred miliar type of oven seen in most resi- directly from the hot gases to the food. dential applications.

Gas range ovens are heated with at- Common Oven Types and mospheric gas burners located directly Categorization below the oven cavity. The flue gases are routed around and/or through the cavity. In electric ovens the elements Ovens fall into two common categories: are placed in the top and bottom of the standard (or natural convection) and oven cavity, where they add both radi- forced convection. Standard and forced

PG&E Energy Efficiency Information© “Ovens in Food Service Facilities” Page 3 ant and convective heat; they also may fired. Manufacturers differ in how they be placed underneath the oven cavity. route the flue gases and how they mix them with cavity air. Gas burners may Deck Oven be protected from air currents by an ar- rangement of baffles, and the flue gases These ovens have a flat, wide cavity. directed around or through the cavity. The floor of the cavity is referred to as a Alternatively, the flames and flue gases deck, and it is usually possible to place may be directed into tubes that act as pans or the food product itself directly heat exchangers and vent into the flue. on the deck surface. These ovens are usually freestanding, and may consist of Forced convection ovens come in full- one to four stacked compartments. size or half-size capacities, depending on whether they are dimensioned to ac- Compartment size and construction cept standard full-size 18 x 26 x 1” or vary. Manufacturers list deck ovens by half-size 18 x 13 x 1” sheet pans. Full- intended use. Baking and roasting com- size ovens have large interior cavities partments may be combined into one capable of handling up to six full-size oven with multiple cavities; the baking pans. Half-size models accommodate compartments are about half the height up to five half-size pans. Countertop of roasting compartments (7” vs. 15”). and range type convection ovens are Specialized deck ovens for baking pizza also available, as are high-capacity roll- may have modified decks and/or damp- in or rack ovens. The convection princi- ers to adjust temperature. ple has also been applied to conveyor and rotisserie ovens. Deck ovens also can be used to cook a wide variety of other foods; the limiting In general, convection ovens offer more factor is the height or thickness of the control over cooking than standard ov- food product. ens. Convection ovens generally use accurate electronic sensors and ther- Convection mostats. Many gas models feature electronic ignition and controls. Also, Convection ovens force air through a most of the newer gas and electric motorized fan (or blower) which blows models have programmable cooking heated air throughout the oven’s cavity. computers. Some ovens allow the user The speed of the fan affects cook time to control cooking by regulating fan and uniformity, as does the pattern of speed as well as temperature, humidity airflow through the interior. Gas con- and cooking time. vection ovens are available with single or multiple burners. Burners are usually Rack Oven located at the bottom of the oven cavity, or between the cavity and the insulated Rack ovens are tall stainless steel oven wall. Until recently, most gas con- boxes. The rack oven is capable of pro- vection ovens have used atmospheric ducing uniformly cooked products in rather than infrared burners. high volume. They typically have a capacity of 20 to 40 standard sheet pans. Most gas convection ovens are indirect-

PG&E Energy Efficiency Information© “Ovens in Food Service Facilities” Page 4 Oven Type Standard Convection Rack Combination Conveyor Rotisserie Range Deck/Pizza

Fuel Source Gas or Gas and Gas and Typically Gas and Gas and Gas and Electric Electric Electric Gas Electric Electric Electric

Typical Input Rates: Gas (kBtu/h) 35 - 45 20 - 120 15 - 100 125 - 375 68 - 212 120 -150 40 - 60 Electric (kW) 7 - 9 6 - 12 2 - 40 10 - 63 35 - 45 4 - 12

Typical 1 Cavity 1-3 Cavities Full-Size: 6 1 Cavity 1 Cavity 1-3 Units 1 Cavity Capacity/ 36” wide x 7” - 15” high sheet pans 20 - 40 Varied sizes Varied Varied sizes Dimensions 30” deep x Half-Size: 6 sheet pans sizes 30” high sheet pans Heat Transfer Natural Natural Forced Forced Forced Forced Forced Convection Convection Convection convection Convection Convection Convection Method

Radiant Heat Radiant Heat Radiant Heat Radiant Radiant Heat Radiant Radiant and Con- Heat Heat ductive Conduction Steam Heat Natural Convection Steam with Con- duction Common Baking and Baking and Baking and Baking Multi-purpose Primarily Cook and Use roasting a roasting a roasting a and for a variety used for market a variety of variety of variety of roasting or of food: pizza. variety of food. Ideal for food. Ideal food. reheating Baking foods. Pri- precision for baking a variety Roasting marily used baking. and pizza. of food. Broiling for chicken. Steaming Customer Normally Pizza opera- Bakeries, High- Institutional, High- Chain res- Application specified for tions, baker- chain restau- volume restaurants, volume, taurants smaller ies, hotels, rants, hotels, operations kiosks and limited and Super- operations hospitals, hospitals, Supermar- menu op- markets cafeteria cafeteria kets erations kitchens kitchens

Table 1: Oven Types, Common Uses and Typical Customer Applications

PG&E Energy Efficiency Information© “Ovens in Commercial Food Service Operations” Page 5 Pans are loaded into a metal rack which and speed of the conveyor. Newer con- is rolled into the oven. Inside the oven, veyor oven designs may incorporate a motorized lift revolves the rack for multiple cooking zones within the cook- even cooking. Rack ovens use forced ing chamber, with three cooking zones convection and many have the ability to being typical. Some conveyor ovens inject steam into the cavity to enhance have a hinged glass door along side of shine and crust on baked goods. the chamber to allow loading and un- loading of food. Combination Oven Conveyor ovens are available using Combination ovens are convection ov- four different heating processes: infra- ens that include a steam generator. The red; natural convection with a ceramic oven can be operated as a convection baking hearth; forced convection; or a oven, as a pressureless steamer, or in combination of infrared and forced con- “combination” mode. vection. The ovens are available in many different sizes and configurations. Combination ovens can hold either tra- Most ovens can be stacked up to three ditional sized half- and full-sized sheet units high, significantly increasing pro- pans or steam pans. They start with duction capacity without requiring in- smaller countertop or half-size models, creased floor space. Gas leads in this move towards full-size combination ov- market. ens and extend to large, floor-mounted, full-sized units that accept up to 20 Rotisserie Oven standard full-size sheet pans. Large capacity roll-in rack models are also avail- Rotisserie ovens are designed for batch able. cooking, with individual spits arranged on a rotating wheel or drum within an Electric combination ovens dominate enclosed cooking cavity. The heat the market place, however recently sev- source may be a gas burner or electric eral manufacturers have introduced gas elements, and some rotisseries incorpo- alternatives. Many gas models are now rate high-wattage quartz lamps for dis- available. play and/or browning.

Conveyor Oven For gas rotisserie ovens a number of gas-fired burner systems are available. Essentially, conveyor ovens are a rec- Single heat-source systems include at- tangular housing containing a baking mospheric flame type, radiant and infra- cavity or chamber which is open on the red. There also are dual burner systems two opposite sides. A conveyor system that combine infrared with an open carries the product through the baking flame and radiant heat. Most gas mod- chamber on a wire rack. Some conveyor els feature electronic ignition systems. ovens can be outfitted with multiple conveyors so that products may be sent Rotisserie ovens range in size from through the oven at different speeds. high-volume floor models to space- saving countertop models. Most models Oven controls adjust both the heat input are equipped with basic time and tem-

PG&E Energy Efficiency Information© “Ovens in Commercial Food Service Operations” Page 6 perature controls, optional cook-and- Conduction hold controls, or more sophisticated control packages with programmable A recent entry into the market place is channels. an electric cook-and-hold conduction oven which circulates heat transfer flu- ids through the oven’s heat transfer State-Of-The-Art plates. The heat is conducted directly Oven Technologies through the pans to the food. This method of heat transfer, according to the manufacturer, allows food to be Infrared Burners brought evenly to a cooked state without burning or drying. More efficient infrared burners are re- placing the traditional atmospheric Combination Convection burners in gas ovens. An infrared/forced Microwave convection oven combines the pene- trating heat of infrared radiation with A few oven manufacturers are using a convection to sharply reduce baking modified impingement system that pro- time compared to natural convection pels hot air directly down onto the food, ovens. then pulls it around and underneath the product. This oven is frequently sup- Air Impingement plied with a built-in microwave to further speed the cooking process. Air impingement is a relatively new technology applied to conveyor and Some manufacturers are combining some rotisserie ovens. Air impingement “smarter controls” with the ovens that typically uses a ported manifold to direct use air impingement and microwave jets of air, or “fingers,” onto the prod- cooking technologies. Among the inno- uct’s surfaces. The “fingers” of air blow vations are computer-controlled focus- away the layer of air and moisture that ing of the hot-air impingement above insulates the food, thus increasing the and below; a non-rotating cooking speed of the cooking process service, which means that the whole oven can be used to prepare food; and Quartz Halogen Lamps an innovative microwave technology which will permit the use of metal trays Quartz halogen lamp ovens use a com- and pans. bination of infrared energy and visible light to cook food. The ovens use radi- A large microwave oven manufacturer is ant heat to brown and crisp the exterior being touted as producing the first as would a conventional oven. The commercially approved compact oven lamps start heating instantly, thus have for food service use. It packs a “double no preheat time, and remain off when whammy”, offering 1000 watts of micro- the oven is in idle mode. wave energy and 2,200 watts of con- vected air. With this combination comes a host of usage options: browning,

PG&E Energy Efficiency Information© “Ovens in Commercial Food Service Operations” Page 7 baking, steaming, sautéing, cooking and determine if the steam mode is used roasting. and if so, for which food products?

· Nameplate rating: Energy input Typical ratings on the nameplate of range ovens typically list the combined in- Customer Applications put of the range oven with the burners or elements comprising the Operators specify ovens based on a va- range top. riety of factors including its function and versatility, production capacity, tem- · Oven sized to match production perature uniformity, and first cost. Table volume: Oversized oven capacity 1 shows the typical applications of ov- can cost the facility money by wast- ens in food service operations. ing both energy and space.

What to Look for Tips for Efficient Operation in the Field · Turn it off: Because it can take up · Preheat Time: Most ovens require to half an hour to preheat, the oven preheating for half an hour or less. can’t be turned off when it is used spo- Ask the operator how long he/she radically. But if you can identify a con- usually allows the oven to preheat to tinuous two-hour period when there is determine if there is an opportunity no use on the oven, the operator can to minimize the oven on time. save up to $400 each year by turning it off every day during that period. · First use: Is there a regular time each day when the oven is first used · Start-up and shut-down schedule. to prepare food? Note this time as Sometimes employees simply walk well as when the oven is usually down the line of appliances in the turned on. morning, turning on every one, hours before they may be needed. Identify · Last Use: Is there a regular time when the appliance is first used in the when the last load of product is pre- morning, and turn it on just long enough pared? Note this time as well as beforehand to allow for an appropriate when the oven is usually turned off. preheat.

· Long Idle Periods: Note any long Likewise, turn the oven off after its periods (e.g. between last lunch and last use of the day, not some time later first dinner prep) when the oven is as part of cleanup. Turning an oven off idle for two or more hours. for an extra hour each day can mean savings of $40-$200 a year, depending · Food Product: What foods are on the oven. cooked in each oven, and at what · Don’t use steam if you don’t need settings? For combination ovens, it: Often combination ovens are used in

PG&E Energy Efficiency Information© “Ovens in Commercial Food Service Operations” Page 8 combination mode (i.e., forced convec- Convection Ovens” prepared for the tion plus steam) when convection mode Research and Development would be just as effective. Vaporizing Department, Report No. 1994. water for steam may increase the energy use of these ovens by as much as 100%.

References to More Information

1. “Cooking Appliance Performance Report: PG&E Production-Test Kitchen.” PG&E Research and De- velopment Report No. 008.1-90.8, May, 1990.

2. "Montague Model SE70AH Gas Full- Size Convection Oven: Appliance Performance in Production." PG&E Products and Services Report No.5011.95.21, 1995.

3. "Montague Model SEK15AH Electric Full-Size Convection Oven: Appliance Performance in Production." PG&E Products and Services Report No.5011.95.22, 1995.

4. "Appliance Performance in Production Blodgett Model DFG-60 Gas Half-Size Convection Oven." PG&E Research and Development Report No.008.1-9.11, December, 1992.

5. "Blodgett Model CTB-1 Electric Half- Size Convection Oven: Appliance Performance in Production." PG&E Products and Services Report No.5011.95.17, 1995.

6. “Development and Application of a Uniform Testing Procedure for