Introduction

This paper will focus on Food Processing Oven design as it relates to air circulation and heat sources. A general description of main heat sources and heat consumption are presented. The concept of air changes, importance of supply air uniformity, and fresh air intake and exhaust systems are briefly discussed. Typical utility consumption and processing costs are depicted in graphical form.

Oven Design Overview

Features commonly found in ALKAR ovens:

• Stainless steel internal structures for corrosion resistance.

• Inflatable gasket doors for a smoke tight seal.

• Floor plate with raised 4-way tread pattern for slip resistance.

• Modulating temperature control outputs for process accuracy.

• Microprocessor based controls for recipe storage and repeatable processes.

• Air handling units fabricated from stainless steel for corrosion resistance.

• Oven roof decks with raised 4-way tread or anti-slip tape.

• Clean-in-place piping for limited access ductwork areas.

• Polyurethane foam insulation for insulating value and panel rigidity.

• Single main air blower for total mixing of recirculated air.

• Fresh air intake and exhaust systems with modulating dampers for wet bulb (relative humidity) control.

• Rotating dampers for enhanced air distribution.

Recirculated Air Stream

Most common equipment locations are on top of the oven cabinet or on the floor behind the oven. The equipment is sized and configured to match oven size, space limitations, heat source and process requirements. Rotating dampers in the supply air ductwork are used to vary the supply air velocities.

• Figure 1A - Damper controlling air to the right side of the oven is open and the left side damper is closed.
   
• Figure 1B - Illustrates air flow into the oven when the dampers are at 45° or when there isn't any damper control.
   
• Figure 1C - Illustrates the change in air flow as the dampers reverse.

Horizontal Airflow

Utilizes two fundamental air handling features, single main blower for total air mixing and rotating dampers for alternating air flow in the product zone. Horizontal air flow ovens are most beneficial when processing products that are loaded on screens or trays. The product zone has limitations of approximately 12 feet wide and 12 feet long.

Main Heat Sources

Gas, steam and electricity are the most common heat sources for ovens.

• Gas System Gas systems are either direct heat or indirect heat. Direct heated systems fire the gas burner flame directly into the return air stream and impart heat to the air by mixing with it. An indirect heated system fires the gas burner flame into a heat exchange device and the heat is imparted to the return air by heat transfer from the heat exchanger. Natural gas and liquid propane are the two most commonly used fuels for the gas burner.
   
• Steam System Steam systems are indirect heat systems and supply heat to the return air by passing the return air stream through steam coils. These coils are usually constructed of copper, cupro-nickel or stainless tubes, and copper or stainless steel fins.
  ALKAR steam coils normally have stainless steel tubes and copper fins. Coils constructed of all stainless steel are available but have less heat transfer capacity. When using all stainless steel coils, additional coils are added to compensate for the reduced heat transfer capacity.
   
• Electric System Electric systems are direct heat systems and supply heat to the return air stream by passing the air through a bank of tubular heater elements. Fins can be added to heater elements to increase heat transfer. Fins allow the heater elements to run at a lower temperature which would extend their life, or fins would allow the designer to select a smaller heater element with higher watt density.

Comparison of Heat Sources

Heat Transfer Efficiency and Effectiveness

• Gas-Direct: Highly efficient and effective. The combustion process is 97±% efficient and all heat produced is absorbed by the recirculating air stream.
   
• Steam: Less efficient than a direct gas system. Coil construction is a major factor in heat transfer rate. Copper gives the best heat transfer (high thermal conductivity) and stainless steel provides less heat transfer (low thermal conductivity). The number of fins per inch and the fin material will also affect heat transfer capability. Tight fin spacing makes cleaning difficult and can result in coils being plugged with smoke tars. Too few fins will require a deeper coil (more rows) to provide an adequate amount of surface area for heat transfer. ALKAR normally provides coils with 6 fins per inch. A steam coil with 6 fins per inch provides 6.5 times more heat transfer than a bare tube coil of equal size. An all stainless steel coil is approximately 70% as effective as a copper fin and tube coil.
   
• Gas-Indirect: Less efficient than a direct gas system because an air-to-air heat exchanger is required. Depending upon the type of heat exchanger design and construction, heat transfer can be reduced significantly. Air is a low heat transfer medium and stainless steel has poor thermal conductivity. In some systems, up to 50% of the heat could be wasted.
   
• Electric: Highly efficient. All heat generated by the heater elements is picked up by the recirculating air as it passes through the bank of heater elements.

Air Temperature Rise

• Gas-direct: Fast air temperature rise because it is an efficient system and has a high heat source temperature. Also, gas burners are available in certain capacities and, as a rule, the burner used will have a capacity larger than what the design actually requires.
   
• Steam: Reduced air temperature rise because of heat exchanger Btu requirements and lower heat source temperature.
   
• Gas-indirect: Reduced air temperature rise because of heat exchanger design and Btu requirements. With good designs, temperature rise can approach equivalent rates of steam coils.
   
• Electric: Designed with reduced air temperature rise to maintain a balance between equipment cost and operating cost.

High Temperature Processing

• Gas-direct: Can process at high temperatures (over 225ºF) because of the heat source temperature.
   
• Gas-indirect: Can process at high temperatures, but longer process times are required because of heat exchanger heat transfer capacity.
   
• Electric: Can process at high temperatures, but will have longer process times than direct gas because of reduced heat input. Heater element temperature can be as high as 1500ºF.
   
• Steam: Typically cannot be used for high temperature processing. Nominally, the maximum process temperature is limited to two-thirds of the steam temperature (e.g. if 300ºF steam at 72 psig, maximum of 200ºF process temperature).

Note: Maximum operating temperature for standard design ALKAR High Temperature Ovens is 400ºF.

Effects on Product

• Gas-direct: Cooking of non-cured meat products such as roast beef or turkey breast in a direct-gas fired oven will cause discoloration around the outer edges of the product cross-section. This ring is usually 1/8" to 3" thick and can be pink, red or purple, based on product type. Studies have shown that the ring is caused primarily by nitrogen dioxide (NO2) which is a combustion by-product or natural gas.
   
• Gas-indirect: No affect.
   
• Steam: No affect.
   
• Electric: No affect.

Equipment Costs

• Gas-direct: Heating equipment costs are less for direct gas than for other systems. An exception to this would be small ovens, 3-truck or smaller, where electrically heated ovens have the lowest equipment cost.
   
• Steam: Equipment costs are more than direct gas because of the steam coils and equipment design (larger fan cabinet).
   
• Gas-indirect: Equipment costs are more than direct gas and steam, primarily because of the heat exchanger and equipment design.
   
• Electric: Equipment costs are more than direct gas because of the electrical heater elements and control components. An exception to this would be the smaller size ovens, 3 truck or smaller, where an electric system would be less expensive than a gas or steam system.

Air Flow Design

Characteristics of low air change ovens (3-9 air changes/minute)

• Slower drying and slower internal temperature rise rates.
• Longer process times.
• Typically this type of oven is used for fermented sausage products (either dry or semi-dry types).

Characteristics of medium air change ovens (9-15 air changes/minute)

• Most common air flow rate in process ovens.
• Best suited for the general processing rates of most products.
• Oven lends itself to acceptable process times while allowing the processor to work with the product.

Characteristics of high air change ovens (15-20 air changes/minute)

• For products that can tolerate fast surface drying and/or rapid internal temperature rise.
• Most common in poultry processing and other products that are considered very stable (example: 95% fat free hams).

Air flow calculations

As defined by ALKAR, an air change is the recirculated air flow rate (CFM) divided by the empty oven volume (cu.ft.).
 

• Recirculated air volume = area of duct outlet x velocity of air at the outlet.
• Oven volume = inside width of oven x inside length of oven x oven height. Oven height is the distance from the floor to the bottom of the overhead ductwork.
• Air flow = recirculated air volume ) oven volume = air exchange rate. Commonly expressed as air changes per minute.

Note: When doing air flow calculations it is required that all measurements be of compatible units e.g. area of duct outlet in ft2, air velocity in feet per minute and oven volume in ft3. A detailed set of calculations is shown in attached figures.

Importance of uniform supply air

• Air is the media by which drying, smoking and heating is accomplished.
• Proper high to low air velocity ratios are required to maintain a uniform but turbulent air flow pattern in the product zone.
• Non-uniform air flow will lead to color, shrink, and internal temperature variations.

Fresh air intake and exhaust systems

• Fresh air can be taken from inside or outside of the process room.
• Air condition, moisture content, and temperature may determine best source.
• Drafts in process room doorways usually indicate a shortage of make-up air.
• Make-up air units should be sized for smokehouse requirements PLUS any other room or adjacent room exhaust fans.
• Energy cost comparisons for make-up air systems need to account for three variables: Energy cost, air condition (may be a seasonal factor), and process requirements.

Effects on the process

• High moisture content in make-up air can limit how dry you can run a process. Relative humidity or wet bulb control at process set point requires that the moisture removal capacity of the fresh air and exhaust system be equal to the amount of moisture being given up by the product.
• Low dry bulb temperature in make-up air can increase the time required to achieve dry bulb set point. Low relative humidity or wet bulb settings will keep the make-up air and exhaust system at maximum capacity, which in turn consumes much energy.

Heat Consumption

How much heat is used to cook the product and how much is used elsewhere? Graph 1 shows the different percentages of heat consumption in the oven. These are calculated values based upon a general boneless ham cycle. Other product cycles would yield different percentages for these values.

From the illustration, you can see that 20% of the heat is required to heat the product from its entering temperature (45°F internal) to its final internal temperature (152°F). Moisture evaporation accounts for 22% based upon a 10% shrink rate. In order to remove the moisture from the product, exhaust must be provided, and this will result in fresh air being drawn into the oven. For processes using a low wet-bulb setting, or for high moisture loss product (such as beef jerky), heating the fresh air can account for a larger percentage since the exhaust volume is increased. Heating the equipment (i.e. fan cabinet, ducts, trucks/trees/cages; sheet steel), accounts for 19% of the heat consumption, and 4% is lost through the fan cabinet.

Utility consumption

Having seen where the heat is used in the process, how much heat (or utility) was required to process this product? Graph 2 shows the amount of each utility consumed during the boneless ham cycle based upon 100 pounds of product. They all require the same amount of heat (Btu's).

Heating costs

Knowing how much utility was consumed per 100 pounds of product, what was the heating cost? Graph 3 shows the cost of processing the 100 pounds of boneless hams based on the utility rates as shown on the graph. Electric heat has the highest processing cost, followed by steam and gas heat systems. The gas processing cost was based on a direct heated system.

Summary