Dispelling myths about catalytic gas heating.

Some thermoformers love its energy savings. Others doubt it'll do the job. Who's right? It's time to confront the misinformation that has fueled the controversy.

Catalytic gas heating has now been used in thermoforming for just over three years. More than 75 thermoforming companies have successfully converted over 230 machines to catalytic heating, and the number expands at a rate of one or two new converts a week. These processors are generally saving 60-90% in energy costs over electrical heating systems. As a result of customer demand, catalytic heating systems are now available on new equipment and not just as retrofits. More than a dozen new machines, from roll-fed to twin-sheet systems, have been supplied with catalytic gas heat.

Despite this success, a number of "myths" about this technology have impeded its growth. Misinformation about safety issues, control methods, and alleged declines in heater performance create doubt and confusion among thermoforming processors. Dispelling these myths requires first some basic knowledge of how catalytic heaters operate, what type of energy electrical and gas heaters emit, and the cost savings and energy efficiencies available with gas.

How Catalytic Heaters Work

In a catalytic heater, natural gas or propane enters the back of a gas-tight heater pan. The gas is evenly distributed via dispersion media to a pre-heated catalyst pad. Preheating is accomplished with a tubular electric heating element for 20 min. Once the catalyst pad has reached 300 F, safety devices are activated, allowing gas to enter the back of the heater. The gas contacts the hot platinum catalyst and reacts with oxygen in the air, raising the catalyst temperature to 575-775 F and emitting infrared energy. Since the reaction temperature with current catalysts reaches a maximum of 775 F, which is well below the auto-ignition point for gas (1300 F), the reaction is flameless. Five minutes after gas enters the heater, the catalytic reaction is sufficiently established that the preheater is turned off.

Surface temperature of today's catalytic heaters varies from a low of 600 F to a high of 800 F--well below electric heaters' standard operating temperatures of 1000-1200 F. These higher temperatures are designed to coincide with the most familiar infrared absorption band of plastics, lying between 3.4 and 3.7 microns. However, it is well documented that a second and much broader plastics i-r absorption band exists in the 5-10 micron range. These longer wavelengths correspond to the lower temperatures of catalytic heating.

Figure 2 shows i-r distribution curves for so-called "black-body" emitting sources (theoretical perfect emitters) at 750 F and 1200 F. Also shown is a curve for a catalytic heater operating at 750 F. This curve is much flatter than that of a black-body radiator and covers a larger area of absorption by the plastic. This effect is explained by the chemical reaction that takes place at the heater surface, producing moisture and C[O.sub.2] at 750 F. These hot gases emit i-r spectra of their own in addition to the spectrum of emissions caused by the catalytic reaction. Electric heaters behave much closer to black bodies. It is evident from the areas under the curves that catalytic heaters put out a greater percentage of their radiation in a combination of the wavelengths absorbed by common plastics.

Experience shows that energy emitted over both these absorption bands is more easily absorbed by plastics than is just the narrow-band i-r radiation emitted at a higher temperature by electric heaters. Although electric heaters emit some longer-wave i-r, they do so at much lower intensity than do catalytic heaters. What's more, the lower temperature of catalytic heaters produces more uniform heating of the plastic through its thickness with less temperature difference between the sheet surface and its core. Consequently, more uniform part thickness has been observed in deep-draw, roll-fed forming and in sheet-fed forming.

HOW EFFICIENT?

The efficiency of Vulcan catalytic heaters has been tested by Tokyo Gas Co. in Japan. Researchers measured the intensity of i-r radiation emitted, equated it to an energy output from the heater, and thereby calculated the efficiency of converting the input energy. Tokyo Gas determined that 72% of the input energy is converted into i-r; the remaining 28% is lost through conduction and convection of heat away from the heater surface.

True efficiency of an electrical heater will depend to a greater extent on how the heater is constructed and maintained. All electrical heaters have a resistance wire embedded in a ceramic, quartz, or other matrix. Only 50% of the i-r energy is emitted in the direction of the plastic sheet. Anyone who has observed the heat shimmering off the back side of an electric oven recognizes how much energy is being wasted. Although some types of electric heaters use reflectors to capture some of the energy that might otherwise be lost, the effectiveness of reflectors is often compromised by not being kept clean. In addition, the emitting effectiveness of electric heaters declines with use.

Comparisons of wavelengths and efficiencies may seem a bit abstract, but the real-world cost accounting of gas versus electricity is very simple. The national average for gas is 45[cents] per therm (100,000 Btu or 29.31 kwh), which is equivalent to 1.5[cents]/kwh. Electrical rates range from 4[cents]/kwh in Minnesota to 21[cents]/kwh in California. The national average is 6-8[cents]/kwh. The net effect of energy-cost and efficiency differences is apparent from recent production testing, summarized in Fig. 1.

Besides saving energy, users of catalytic heat need not bother with scheduling production when off-peak electrical charges apply. Gas-heat users also avoid the need to stagger the start-up times of multiple machines in order to hold down peak electrical demand charges.

Six Myths About Catalytic Heat

Myth #1

Catalytic heating offers slower cycle times than electric heat.

Many processors considering conversion to catalytic heat ask whether it can provide the same cycle times as electric heat. The answer lies in determining the watt density of the existing electric oven and comparing that figure to the watt density of the proposed catalytic heater.

The majority of electric heaters have a high connected load, sometimes as high as 30 w/[in..sup.2], although the actual operating watt density is usually much less. An electric oven with a watt density up to 2 kw/[ft.sup.2] (13.9 w/[in..sup.2]) for the total heater area can usually be converted to catalytic heat and retain the required cycle times, especially considering the added efficiency of catalytic heaters.

For the majority of thermoformers that process standard plastics--ABS, HIPS, OPS, PP, HDPE, LDPE, PET, and PVC--cycle time is limited by how long it takes to cool the formed part, not to heat the plastic sheet. Exceptions are certain engineering thermoplastic materials that require forming temperatures above 400 F. These resins require greater watt density than can be supplied by catalytic heaters.

Experience also shows that sheet-fed rotary machines with heating areas less than 12 [ft.sup.2] (3 x 4 ft) appear to require higher watt density than is offered by catalytic heaters. On such machines, the clamp frame and its support structure occupy a large projected area over the heaters. The clamp frame and supports absorb the infrared and prevent sufficient heat generation to reduce edge effects around the perimeter of the heater array.

Old rotaries are especially subject to this problem if the customer is used to running fast cycles with plastics such as ABS. Checking the kilowatt draw will help determine whether the watt density is too high for catalytic heaters to be an effective option.

Myth #2

Catalytic heat offers less control than electric heat.

Comparing the relationship between watt density and surface emitting temperature of electrical and catalytic heaters illuminates the control differences between the two systems. Figure 3 shows that the catalytic heater has a narrow operating-temperature range of 575-775 F and a corresponding watt-density range of 5.5-12.2 w/[in..sup.2]. The corresponding temperature range for an electrical heater having the same watt-density range is 625-1050 F.

The essence of the catalytic heater is that it delivers an equivalent watt density at much lower temperatures than an electric heater. The watt density of a catalytic heater is controlled by modulating the gas pressure and hence the volume of gas entering the heater.

Gas-pressure valves can be adjusted manually using an easy-to-read gauge. Gas valves can also be adjusted electrically, permitting remote setup from a PLC-based controller. Machine operators have easily mastered this new method of heater control and comment on how very little adjustment is required during operation. Gas-management hardware is carefully designed to provide uniform delivery of energy, unlike the problems associated with voltage drops and surges that affect electric heaters.

The optimum goal in thermoforming is to create an even blanket of heat across the entire sheet that penetrates to the core of the plastic. Catalytic heaters offer those desired optimum characteristics. They are typically larger than electric heaters and emit diffuse, uniform, low-intensity and penetrating radiation. By definition, uniform heating contradicts the idea of zoning. But in practice, the center of any heater array (electric or catalytic) is adjusted to emit less energy than at the periphery.

In the early days of converting thermoformers to catalytic heat, some machines were outfitted with only four zones and multiple gas heaters within each zone. Today, virtually every catalytic heater operates as an individual zone of 2 to 10 sq ft.

However, catalytic gas heaters generally require fewer zones than electric heaters. Using fewer heat zones may be a difficult concept to grasp for processors who are used to ceramic or quartz heaters. But seeing is believing. Reduced zoning with catalytic heaters has not compromised customers' part quality or limited their process-control capabilities on either sheet-fed or roll-fed machines.

In instances where close zone control is required within a narrow area--such as heating the edges of a sheet close to the chain rails--catalytic heaters can be supplemented by selective placement of a few electric heaters. This technique has proven to be necessary and effective on certain PS and HIPS roll-fed machines where the molding area is close to the cooled chain rails.

Myth #3

Catalytic heaters work better on thick sheet than thin.

At this early stage in the evolution of catalytic heaters for thermoforming, the majority of systems have been applied to sheet-fed machines for industrial parts. That is because these systems generally have large heating areas consuming electricity at rates of $10-20/hr. Switching to gas heat offers quick payback on such machines.

However, there has been a steady increase in roll-fed conversions to catalytic heat. The same processing and cost benefits apply to thin-gauge processors as to thick-sheet formers. Roll-fed users report energy savings, increased processing speeds, greater ease of heating control, and stiffer parts with more uniform wall thickness. An initial pilot program has been so successful at one large packaging firm that it is converting all its roll-fed machines worldwide.

Myth #4

Gas heating is unsafe

There have been two reports of plastic burning in catalytic systems, both ensuing from sheet having fallen onto the lower heaters. If these had been electric ovens, the fire would have started sooner and the machine would have been down longer. If a sheet drops onto a catalytic heater, there is very little chance of a fire. For one thing, the heater is at a lower temperature than an electric heater. And since the gas is completely oxidized during its passage through the catalyst, there is no available gas left over to ignite if the fallen plastic itself starts to burn. The fallen plastic may extinguish the catalyst directly under the sheet, but the gas will continue to be catalyzed around the perimeter of the sheet and slowly oxidize the sheet to ash. Thus, catalytic heaters tend to be self-cleaning.

Sheet can easily be prevented from failing on the heaters through the traditional placement of chicken wire 2-3 in. above the heaters across the oven.

Another safety advantage of catalytic heaters is that they reduce the amount of high, voltage, high-amperage power service at the thermoforming machine, thereby reducing hazards to maintenance personnel.

Myth #5

Catalytic heaters decline in performance over time.

A well-made catalyst pad has an indefinite life span. In fact, the first thermoforming machine converted to catalytic heat has been operating for three years with heaters running at the same gas pressures today as it did originally while producing similar cycle times.

True, there have been cases where catalytic heaters have shown diminishing efficiency after 8-12 months of operation due to a phenomenon known as "catalyst migration." Platinum catalyst molecules have affinity for themselves. If the catalyst is not securely adhered to the carrying substrate, migration takes place, creating "cold" areas without catalyst and consequent gas leakage.

More than 30 years' experience in manufacturing catalytic heaters has resulted in a product that exposes the maximum useful area of catalyst while ensuring that the catalyst-substrate bond is not broken during operation. The photo at left shows the fibrous substrate of a Vulcan catalyst system that has been used for more than six years in numerous paint-drying applications without any loss of efficiency. Vulcan offers a three-year catalyst warranty.

Myth #6

Higher-temperature catalysts would perform better.

Some makers of catalytic heaters have discussed development of higher-temperature catalysts that would emit shorter wavelengths similar to electric heaters. Such a move would appear unnecessary and perhaps dangerous.

By increasing the catalytic reaction temperature, more energy is lost to convection than is emitted as infrared. The i-r that is generated falls in the shorter-wavelength band of electrical heaters but is emitted at a lower watt density. This suggests electric heaters will outperform the higher-temperature catalysts. The higher-temperature catalytic reaction also approaches the auto-ignition point of gas, so such heaters are not expected to pass insurance standards.