Electric IR Heating To Peel Tomatoes: A Water Saving, Cost-effective, and Eco-friendlier Alternative

For almost a decade now the development of non-chemical peeling technology has recently been identified as a top priority at the for the California Food Processing and Beverage Industry.

Lye peeling is the most industrially used method for processing tomatoes in the U.S. However, due to the pressure of cost and environmental regulations, some tomato processors were forced to use steam peeling to reduce chemical contamination of water. Unfortunately, steam peeling produces undesirable products with deteriorated peeling appearance, high loss in firmness and lowered yields.

Existing studies about the success of IR radiation heating for peeling of potatoes prompted interest in using the same technology for tomatoes. Initiatives for pilot plants and testing began.

A pilot-scale infrared tomato dry-peeling system was designed, built, and tested. It consisted of a section for feeding of the tomatoes, the infrared heating section, a peel eliminator, and the discharge section.

The IR heating unit is equipped a number of IR emitters and an automatically controlled variable speed conveyor system.  Surface temperature of tomatoes was measured immediately after IR heating using non-contact IR thermometers that gathered three different measurements on each tomato. The controlled temperatures were in the range of 103 deg. C to 110 deg. C. In this range, a yield of 70%- 85% fully peeled tomatoes could be obtained for all tomatoes sizes, depending on variety and maturity stage.

Since IR heating does not use water as a heating medium, the process can be referred to as “IR dry-peeling”. Early results showed infrared (IR) heating for peeling tomatoes as having "remarkable and promising" potential for commercialization by the food industry.

Because no water and salt are used in the new peeling process, IR dry-peeling could be the solution for long-term water supply and salinity problems caused by lye peeling. IR dry-peeling also reduced the tomato peeling loss significantly and resulted in similar or better firmness of the product with similar heating time compared to hot lye peeling. The reduced peeling loss and high product quality mean that more valuable and premium products can be produced. Because no salt is used in the peeling, the skins do not contain added salt and can be easily utilized as value-added food products.

Major Advantages to Infrared Dry Peeling:

• The average percentage of fully peeled tomatoes obtained from the IR peeling system was much higher than that from steam peeling. 
• The IR peeled tomatoes had a much better texture than the steam peeled ones.
• A commercial infrared peeling system is predicted to save about 22 percent and 28 percent of the energy when compared to energy used by steam and lye peelings.
• No water and chemicals are required for the infrared peeling system so there will be no need to treat any wastewater after the peeling process. 

Maintaining Close Temperature Control of a Fluid Process Flow

Temperature control is a common operation in the industrial arena. Its application can range across solids, liquids, and gases. The dynamics of a particular operation will influence the selection of instruments and equipment to meet the project requirements. In addition to general performance requirements, safety should always be a consideration in the design of a temperature control system involving enough energy to damage the system or create a hazardous condition.

Let's narrow the application range to non-flammable flowing fluids that require elevated temperatures. In the interest of clarity, this illustration is presented without any complicating factors that may be encountered in actual practice. Much of what is presented here, however, will apply universally to other scenarios.

What are the considerations for specifying the right equipment?

Know your flow. 

First and foremost, you must have complete understanding of certain characteristics of the fluid.

• Specific Heat - The amount of heat input required to increase the temperature of a mass unit of the media by one degree.
• Minimum Inlet Temperature - The lowest media temperature entering the process and requiring heating to a setpoint. Use the worst (coldest) case anticipated.
• Mass Flow Rate - An element in the calculation for total heat requirement. If the flow rate will vary, use the maximum anticipated flow.
• Maximum Required Outlet Temperature - Used with minimum inlet temperature in the calculation of the maximum heat input required.

Select system components with performance to match the project.

• Heat Source - If temperature control with little deviation from a setpoint is your goal, electric heat will likely be your heating source of choice. It responds quickly to changes in a control signal and the output can be adjusted in very small increments to achieve a close balance between process heat requirement and actual heat input. 
• Sensor - Sensor selection is critical to attaining close temperature control. There are many factors to consider, well beyond the scope of this article, but the ability of the sensor to rapidly detect small changes in media temperature is a key element of a successful project. Attention should be given to the sensor containment, or sheath, the mass of the materials surrounding the sensor that are part of the assembly, along with the accuracy of the sensor.
  
  The location of the temperature sensor will be a key factor in control system performance. The sensing element should be placed where it will be exposed to the genuine process condition, avoiding effects of recently heated fluid that may have not completely mixed with the balance of the media. Locate too close to the heater and there may be anomalies caused by the heater. A sensor installed too distant from the heater may respond too slowly. Remember that the heating assembly, in whatever form it may take, is a source of disturbance to the process. It is important to detect the impact of the disturbance as early and accurately as possible.
• Controller - The controller should provide an output that is compatible with the heater power controller and have the capability to provide a continuously varying signal or one that can be very rapidly cycled. There are many other features that can be incorporated into the controller for alarms, display, and other useful functions. These have little bearing on the actual control of the process, but can provide useful information to the opeartor. 
• Power Controller - A great advantage of electric heaters is their compatibility with very rapid cycling or other adjustments to their input power. A power controller that varies the total power to the heater in very small increments will allow for fine tuning the heat input to the process.
• Performance Monitoring - Depending upon the critical nature of the heating activity to overall process performance, it may be useful to monitor not only the media temperature, but aspects of heater or controller performance that indicate the devices are working. Knowing something is not working sooner, rather than later, is generally beneficial. Controllers usually have some sort of sensor failure notification built in. Heater operation can be monitored my measurement of the circuit current. 

Safety Considerations

Any industrial heater assembly is capable of producing surface temperatures hot enough to cause trouble. Monitoring process and heater performance and operation, providing backup safety controls, is necessary to reduce the probability of damage or catastrophe.

• High Fluid Temperature - An independent sensor can monitor process fluid temperature, with instrumentation providing an alert and limit controllers taking action if unexpected limits are reached.
• Heater Temperature - Monitoring the heater sheath temperature can provide warning of a number of failure conditions, such as low fluid flow, no fluid present, or power controller failure. A proper response activity should be automatically executed when unsafe or unanticipated conditions occur.
• Media Present - There are a number of ways to directly or indirectly determine whether media is present. The media, whether gaseous or liquid, is necessary to maintain an operational connection between the heater assembly and the sensor. 
• Flow Present - Whether gaseous or liquid media, flow is necessary to keep most industrial heaters from burning out. Understand the limitations and operating requirements of the heating assembly employed and make sure those conditions are maintained. 
• Heater Immersion - Heaters intended for immersion in liquid may have watt density ratings that will produce excessive or damaging element temperatures if operated in air. Strategic location of a temperature sensor may be sufficient to detect whether a portion of the heater assembly is operating in air. An automatic protective response should be provided in the control scheme for this condition.

Each of the items mentioned above is due careful consideration for an industrial fluid heating application. Your particular process will present its own set of specific challenges with respect to performance and safety. Share your requirements with process heat experts, combining your process knowledge with their expertise to develop safe and effective solutions.

Flanged and Screw Plug Electric Heating Assemblies for Industrial Applications

Flanged Tubular Electric Heater Assembly
Hotwatt

Electric heating, though not the most energy efficient means of delivering heat, provides some distinct advantages as a means of controlling the temperature or thermal component of fluids and solids throughout commercial and industrial settings.

Tubular elements are a common form of electric heater. Essentially a metal tube with resistance wire and electrical insulation inside, tubular elements can be configured into almost uncountable shapes and sizes. Manufacturers typically offer a range of standard sizes and ratings, but that should never deter you from making contact to discuss your ideas for a custom arrangement.

Two mounting schemes that are readily used on tanks or other vessels are the screw plug and flanged heater assemblies. In each case, tubular heaters are bent in a "U" shape and fitted into either a pipe flange or a threaded plug. A junction box encloses the electrical terminations for the heating elements, providing a single ended assembly that can be easily mounted to an industrial standard mechanical connection. These assemblies are useful for tank or vessel OEMs that wish to provide a fluid heating option to their customers.

Examples of Screw Plug Electric Heaters
Hotwatt

Electric heat enables a properly configured controller to proportion heat into a subject fluid across a wide range, from very small packets that could be fractional percentages of full capacity to the fully available output of the heater. The units are compact, rugged, and can be configured to accommodate a broad array of industrial environments and applications.

Selecting or specifying a unit is uncomplicated. Determine the amount of heating capacity needed, then select the assembly mounting type (flange, screw plug, or other). Select an element sheath material that is compatible with the process media and a termination enclosure that suits the surrounding environment. Application assistance is available from product specialists who are well versed in the available options and can help you specify an assembly that provides excellent performance and an extended service life.

Electric Heating Element Types and Selection Guide

The following, courtesy of Hotwatt,  is a good reference for selecting an electric heating element for both OEM and process heating applications.

Included are all types of industrial electric heaters - cartridge, band, tubular, immersion, duct, circulation and cable.

OEM & Industrial Electric Heating Element Catalog by Hotwatt from Belilove Company-Engineers

Handy Electric Heating Engineering Constants and OHMs Law

Here is a very handy "cheat-sheet" for calculating Ohm's Law and other engineering constants that come in very handy when calculating wattages, voltages and current draw of electric heating elements.

These equations are important when sizing any type of electric heating element including cartridge, band, immersion, or flexible heaters.

To calculate any wattage, voltage or current, you need to know two of the variables and refer to the diagram in the document to calculate the third.

Another important quick reference chart is Fahrenheit to Centigrade temperature conversion as shown below.

Industrial Electric Immersion Heaters

Screw plug and flanged
immersion heaters
(courtesy of Durex)

Electric immersion heaters are used in a myriad of industrial applications. From drying industrial gasses, to freeze protecting cooling tower sumps, to heating acids in plating applications, the versatility of electric heating element can save time, energy and space.

Industrial immersion heaters are used to directly heat a standing or moving fluid by using electric heating elements. There are three primary types of industrial electric immersion heaters; screw-plug heaters, flanged immersion heater, and over-the-side heaters.

At the heart of industrial immersion heaters are the individual heating elements, normally constructed from a stainless steel or Inconel tube containing a magnesium oxide filler and a nichrome resistance wire. Current is applied to the wire which produces the heat, while the compacted magnesium oxide powder provides the electrical insulation, and the metallic tube provides the physical protection.

Aluminum Nitride Ceramic Heaters Open Doors to Better Machine Design

Aluminum Ceramic Heaters
(Courtesy of Durex)

Aluminum Nitride (AlN) Ceramic heaters are a relatively new entry in the very high watt density heater market and are an attractive alternative to traditional metal sheathed heaters. Capable of achieving up to 2000 watts per square inch, and operating temperatures of up to 1000 deg. C, these heaters show great promise for semiconductor processing applications such as crucible heating, fluid and gas handling and chemical vapor deposition. 

The heaters are made by "tracing" a resistance material (Tungsten) on a the ceramic base at various thicknesses, corresponding to the performance requirements of the heater. The Tungsten and AIN expand and contract at very similar rates, which greatly reduces the mechanical concerns of delamination. Binders and trace additives are added to the ceramic and Tungsten for additional strength. The resulting construction allows for some pretty impressive thermal cycling - one example is an application with a 200 deg. C temperature swing every 30 seconds.

AIN ceramic heaters offer significant advantages over metal sheathed heaters and their inherent performance limitations. Material compatibility, fatigue, outgassing and thermal lag must be considered when applying metal sheath heaters. Ceramic heaters combine excellent thermal conductivity with outstanding chemical resistance, strength, inertness and design flexibility. Additionally, RTD sensors can be deposited right on the ceramic heater itself for optimum control. 

Capable of forming virtually any shape, along with their excellent mechanical, thermal, dielectric, chemical resistant and embedded sensors, Aluminum Nitride Ceramic heaters open the doors for engineers to design equipment to new levels of performance.

High Purity Fluid Electric Heaters

Transfer Line
Heater Assembly

In many biomedical, pharmaceutical, semiconductor, electronics or R&D laboratory applications, special purpose electric heaters are required for heating high purity fluids. These heaters typically must be ruggedly designed, made from materials immune to process contamination and be vacuum tight. They can be subject to high temperatures, harsh solvents, and corrosive gases. Many times they must maintain a seal for full vacuum, demonstrate a unique or even heating profile and be able to be closely controlled.

The misapplication of screw plug immersion heaters, screwed into a stainless steel welded vessel, offer more problems than solutions due to leaks, material compatibility, poor controllability, and bulky size.

Transfer line
heater assembly

The answer is in a custom high purity fluid heater designed with the process in mind.

Custom electric heating elements are available designed to handle high vacuum, high temperatures, utilize glass liners for ultra-pure gases, offer 316 stainless steel parts, provide internal RTDs for control and can be temperature profiled.

General Specs for these types of custom fluid heaters are:

• Variety of voltages.
• Wide range of watt densities.
• Temperatures up to 350°C.
• Heater length can be profiled to generate a liner temperature profile.
• Vacuum compatible up to 1.0 x 10-8  STD. CC/SEC Helium.
• Can be provided with internal sensors (RTD or thermocouples).
• Can be glass lined for ultra pure gas application.

Careful review of the application is important and the help of an experienced application engineer is required, but the outcome of the test, process or product will be infinitely improved.