Wet Process Heating and Cooling Equipment

A Layman's Guide to Immersion Heating

Authored by Steve Smith, Vice President of Process Technology

The following article appeared in the November/December 1998 issue of Finishers' Management magazine:

In today’s business environment, most companies have been forced to figure out how to operate more efficiently. Everyone is doing more with less. When it comes to immersion heating, often either the resident expert is no longer with the company, or they are too busy to spend time sizing heaters and coils. The result is that more companies than ever before have come rely on their suppliers for technical support. So what information does your heating equipment supplier need from you in order to recommend the proper heater?

As with any equipment, selecting the correct heating equipment requires data accumulation. With the proper information in hand, you can be confident that the equipment placed into service will provide long service life. Conversely, incorrectly specified equipment can result in improper operation, premature failure and substantial down time.

GENERAL SIZING CONSIDERATIONS

Some of the information needed to size heaters is the same as for other equipment that comes into contact with solution. For instance, when considering pump and filtration needs, one must consider the type of solution being used. Chemical compatibility is a major key to satisfactory heater life as well. Heater selection, however, often requires more information gathering than for some of your other equipment.

A sheath material needs to be chosen that will withstand the rigors of the chemistry. This is an especially important consideration for heating equipment, because the elevated temperature of the heater sheath or heat exchanger is a catalyst for increased chemical corrosion. Often several choices are available. All else being equal, pick the most durable heater material. If a quartz heater and a titanium heater will both work in your application, the titanium heater is the usually the better choice because it is not subject to damage as easily as quartz. Your heater supplier can assist in this selection. Another often overlooked resource for technical assistance is your chemical supplier. Refer to your chemical supplier’s MSDS, both for a breakdown of all the components of the chemistry, and also for heater sheath recommendation. Have this document available for your heater supplier should they require it.

In addition, the physical layout of the tank needs to be reviewed. What are the tank dimensions and what area in the tank is available for heater placement? What is the solution depth? These considerations will impact on the heater’s physical configuration. Ideally, for uniform heating, heaters would be spaced evenly around the tank. Usually, however, this luxury is impractical. Heaters should be placed in areas that promote even heating across the tank and prevent direct contact with work in process. This will minimize the chance of physical damage occurring to the heaters. Since heat rises, your tank’s heat source should be placed towards the bottom of the tank. Properly placed bottom heaters will give you more even heating than over the side heaters. Heating equipment that is positioned too high in the tank can result in heat stratification problems. This can adversely affect production quality.

The ambient temperature and operating temperature must also be noted. In addition to the temperature rise needed, you will need to determine how quickly you want to bring the tank up to temperature. To the uninitiated, the time frame required to bring a tank to temperature can be surprising. A 500 gallon tank being heated from 70° F to 160° F requires 110,000 watts (and 265 amps service at 240 volts!) to accomplish the task in one hour. If you allow for a six hour heat up, that same tank only requires a little less than 20kW and around 50 amps of power. As with many things, initial heat up time is invariably a compromise. Confirm that sufficient power is available and what voltage is accessible in the area.

If you run large parts or have high through put requirements for your production, the mass of the parts going into the tank often becomes a heat calculation factor. Load factors create a heat sink effect in the solution, and can dramatically draw down the tank temperature. If you have chosen a long initial heat up time, your heat recovery may be insufficient. With the temperature of the load going into the tank and the pounds per hour of production, your heater supplier can calculate the heat requirements needed for heat recovery and compare this to the tank heat up calculation. Heaters must be sized for the greater heat requirement of either the initial heat up or the heat recovery needed.

With the changes in ventilation requirements in recent years, surface heat losses have become a greater factor in heat calculations. Covering the tank during initial heat up will substantially reduce surface heat loss while the tank is covered. At 160° F, an open still tank will lose about 370 watts of heat (almost 1200 BTU) per square foot of solution surface per hour. With 120 FPM of ventilation or mild agitation that figure climbs to 650 watts/sq. ft. With higher ventilation rates and aggressive mechanical or air agitation, the heat loss factor can soar to four or five times that of a still tank. If you know the airflow rate of the ventilation system and volume/temperature of your air agitation, the amount of additional heat required to offset these losses can be determined. Automatic solution replenishment also must be reviewed to see how it impacts on heater sizing. The two important factors here are the incoming temperature and the flow rate. Sidewall losses are generally not critical unless the tank is outdoors or in an open environment subject to cold temperatures.

Heaters should be placed so that a minimum of two inches of solution covers the "hot zone" (the heated portion of the heater) at all times. Any portion of the hot zone exposed to air will cause an over-temperature condition to occur. You must also look to the bottom of the tank to ensure that the heater is situated above any sludge that may build up. In applications that generate high levels of solids, you will want to minimize any horizontal heating surfaces that may act as a "shelf". This contributes to solids accumulation on the heater. Sludge and scale encrusted on heating equipment acts as an insulation blanket. The heat being produced is unable to transfer into the solution. This causes inefficiencies with the equipment, and in the case of electric heaters, element burn out. Low solution level and the build up of solids on heaters are the two biggest causes for premature heater failure.

SAFETY CONSIDERATIONS

Electric heaters should never be installed without proper safeties. Thermal overload protection is an absolute necessity, especially in applications involving plastic tanks or other combustibles. Electric heaters exposed to air reach temperatures in excess of 1300° F. Many manufacturers now provide some type of thermal protection as a standard feature. Thermal protectors are factory installed devices located at the top of the heater hot zone. These devises minimize the potential for fire in the event of low solution level conditions by shutting off heater power. They are not designed to activate due to heater over-temperature because of scale/sludge buildup. Both single use and resettable devices are available. A resettable device is generally preferable because it alerts the operator to a tank solution level problem by way of an audible or visual alarm and requires intervention to restart the system.

A liquid level device should also be installed on the tank. Too many people rely on their thermal protection to tell them when they have a low level condition in a tank. Thermal overload switches are safety devices and should not be used as a substitute for level switches. Make sure that you understand the correct installation of thermal protector devices and inspect your equipment regularly to insure that none of the safeties have been bypassed. Call the manufacturer if you have any questions. Also, only use thermal devices that are specifically designed for the heaters in use. Never mix heater and protector manufacturers. If you opt for single use thermal protectors, always keep spare protectors on hand to minimize the temptation your workers might have to bypass the thermal safety on the heater.

Only use heaters that are certified by independent testing labs such as UL and CSA. These approvals mean that minimum construction standards are being met. These agencies also ensure that proper grounding procedures are being followed in the construction of the heater to help reduce any hazard potential. Proper grounding is also a function of proper installation. Always make sure that the heater ground is securely fastened to a suitable earth ground.

HEAT EXCHANGER SIZING

Many of the considerations for determining tank heating are the same regardless of the method of heating. However, there are a couple additional factors to consider when sizing immersion coils.

You need to know the steam pressure or, in the case of hot water systems, water temperature and pressure that is available at the tank. You cannot assume that a gauge at the boiler is giving you an accurate indication of what is available tank side. Line losses can be substantial and are affected by such factors as the distance to the tank, pipe size, insulation of the lines and the ambient temperature of the plant.

When sizing a coil, your supplier should provide you with information regarding the pounds of steam or water flow rate required for the heat transfer needed. Your plant likely has capacity for replacement equipment, but new applications may require a review of your central heating system.

SPECIAL CONSIDERATIONS

Many types of chemistry require special sizing and design considerations for heaters. For instance, electroless nickel tends to plate out more aggressively on horizontal heater surfaces. A heater design that maximizes vertical surfaces is preferable. Caustic solutions over 20% concentration and phosphate baths require derated heaters. A standard metal heater produces 35-40 watts per square inch of heater surface area. A derated heater is typically around half that. For the same overall heat output, you must increase the package size of the heater.

Solution guides published by immersion heater companies are for aqueous based solutions. Other types of solutions require a detailed look at the chemistry to determine a correct heater recommendation. One example is in the area of technical acids. These are acid applications with little or no water content. It is not unusual for this kind of chemistry to require a fluoropolymer heater derated to 3-5 watts per square inch and with a stretched open coil configuration to minimize hot spots.

CONCLUSION

Because of the tremendous variety of applications and their effect on heater choices, most companies today chose to contact their heater supplier for assistance. Some heater companies even have sophisticated computer-sizing programs to calculate the different variables. The next time you make that call to your heater supplier be armed with the all information you need to properly address your heating requirements right the first time.

2" of clearance required between the Minimum Liquid Level and the top of the Hot Zone. 2" of clearance required between the bottom of the Heater and the Maximum Sludge Depth.