Achieving a Perfect Fit
Immersion heat exchangers provide a direct way to control heat in process tanks. Constructed from a range of industrial metals and plastics, this equipment offers a solution to hot and cold temperature control. Selecting the right heat exchanger demands careful attention to detail. Heat exchanger design requires a balance of engineering and economics. When done correctly, the right design disappears into the background of consistent operations.
Even before calculating the heat exchanger's required size, care must be taken in selecting the materials of construction. This step leads to designs that are long-lasting and economical. Processors should carefully review the contents of the tank or process flow and read material safety data sheets (MSDS), which are required of all chemical suppliers in the United States. These important documents list the chemical compositions of even proprietary chemicals. Then, the processor should ask the supplier which material of construction he suggests for your process heat exchanger.
Additional resources include corrosion charts and guide books, but take care to recognize the limits of these information sources. Such lists and summaries usually provide corrosion and operating data for simple chemistries. In typical operating environments, more complicated compositions contain two, three or more chemicals. More sophisticated media can have important impacts on material selection. For example, titanium is not a good choice for heating dilute sulfuric acid. But, by adding copper (common in printed circuit board manufacturing), it becomes a good material for process heating in dilute sulfuric acid.
The difference between a good design and the perfect fit in material selection often is found in "drag-ins" of trace chemicals. Even good material selection can turn sour if attention is not paid to the effect of unexpected chemical additions. Zirconium heat exchangers are a good choice for hot, sulfuric acid used in pickling. But if road salts are an unexpected drag-in, surprisingly quick corrosion failure can occur. Similarly, trace fluorine can find its way into a process flow from solder fluxes, causing unexpected failures in 316 stainless steel heat exchangers used in cleaning and polishing. Keeping track of all the possible process chemistries will lead to a material selection that contributes to the perfect fit.
Size MattersFour factors affect every heat exchanger design: the composition of materials being heated or cooled, heat load, temperature difference and time (table 1). Heat exchanger design also depends on the state of the material being heated or cooled and the type of material being used as a heat transfer medium.
The ability to efficiently conduct heat through a material is a critical design factor. The three phases of matter - gas, liquid and solid - react differently when used in heat exchange systems. Even the speeds at which these materials flow through an exchanger are important to good design. All material and flow rate information is captured in a single, experienced-based proportion called the U-factor. Some typical U-factors that should be figured into your design are summarized in table 2.
Heat load describes the amount of energy the exchanger is required to transfer. It is measured in BTUs or calories. A BTU is defined as the amount of heat required to heat 1 lb of water 1°F. Similarly, one calorie is defined as the amount of heat required to heat 1 g of water 1°C. To calculate static heat loads, multiply the weight of the material being heated or cooled by the required change in temperature. When calculating heat loads, keep in mind weather extremes. A startup heating project on a cold January day will require a larger heat exchanger than the same assignment in July.
Temperature difference is similar to the leverage used in a mechanical system. A larger temperature difference between the material being heated or cooled and the source of the heating or cooling medium will result in a smaller, more economical design. The most extreme example of this fact is sometimes seen in hot water rinses. Near boiling rinses (190°F [88°C] and hotter) heated with low pressure steam provide as little as 20°F of temperature difference and require surprisingly large heat exchangers. The engineering trade-off is between hotter, higher pressure boilers and larger heat exchangers.
The final factor affecting heat exchanger size is the time available for the heat transfer task. In an open top tank with no fluid flow (a static design), a longer heatup time translates into a smaller, more economical heat exchanger. With two hours available for heatup rather than one, the heat exchanger will be roughly 50% smaller. With four hours available at startup (after a long weekend for example), the exchanger can be half again as large (25% the size of the coil required for a 1 hr startup). The limit to the trade-off between time and size usually is reached around six to eight hours in tanks below 140°F (60°C) and around four hours in tanks approaching the boiling point of water. This is based on a balance between the heat load for the original heatup plus losses due to evaporation and losses in the work flowing through the tank.
Expert design, exact sizing and savings are available at your fingertips. Traditional attention to detail and online design tools can turn your requirements into a heat exchanger that will best fulfill your process needs. There is no longer any reason to settle for less than a heat exchanger that provides the perfect fit.
Heat Exchanger Designs: A Closer LookYour heating or cooling application may be best served using one of these heat exchanger designs.
Immersion Heat Exchangers. Immersion exchangers are available for a range of applications as small as a 15-gal tank to as large as a 100,000-gal behemoth used in strip processing of steel. Simple U-shaped coils providing surfaces up to 2 ft2 are capable of transferring approximately 30,000 BTU/hr. Larger, serpentine style, tube heat exchangers can be designed with up to 20 ft2 of surface.
Gridcoil and Plate Coil Heat Exchangers. Gridcoil and plate coil styles have been the workhorses in industry for decades. They are available in sizes up to 100 ft2 of surface in a range of materials.
Plastic Immersion Heat Exchangers. Available in bundle, loop and grid styles for corrosion applications that are too severe for metals, plastic immersion heat exchanger design is limited by plastic's low threshold for temperature and the lower U-factors associated with plastic's low thermal conductivity.
Electric Heaters. Available for immersion heating applications, these heat exchangers convert electric power into process heat by warming resistive elements, much like a toaster oven. They are rated in kilowatts and can be designed to provide hundreds of thousands of BTU per hour.
The connection from the heat exchanger to the heating or cooling source is the assignment of an HVAC contractor or system installer. Con-sidering this connection during the design phase can save you time and money later. By planning and specifying riser detail at the time of the heat exchanger design, substantial savings in connection costs can be realized during installation.