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Immersion heater design
IMMERSION HEATER DESIGN by Tom Richards Process Technology, Mentor, Ohio The immersion heater represents a sound, economical method of heating process solutions in the finishing industry. Classical heater installations consisted of hanging a steam coil on one tank wall, sized to heat up water to a "rule-of-thumb" temperature in two hours. While this method has proved adequate in providing heat and covering a multitude of oversights, it has also proved unsatisfactory with regard to energy costs and control. As the cost of energy rose, the finisher increased heat-up times in an attempt to conserve energy. Soon, heat losses prevented achieving desired temperature levels so tank insulation, covers, and other methods of loss conservation were added. Again, adequate solutions to most of the challenges were found, but the hanging steam coil remained unchanged. Today, we have the knowledge that allows us to adequately plan, design, install, and operate economical, efficient heating systems. Molecular activity, chemical solubility, and surface activity are enhanced through temperature elevation. The reduced solution surface tension, low vapor pressure of some organic addition agents, and heat-sensitive decomposition or crystallization of other additives are major considerations that modify the benefits gained as solution temperature rises. To achieve a proper balance of all these factors, while providing economical installation and operation, it is necessary to analyze the individual heating requirements of each process. Your best source of process information is your process chemical supplier, which can tell you: 1. 2. 3. 4. 5. 6.
Recommended materials of construction. Maximum (minimum) solution temperature. Maximum heater surface temperature. Specific heat of the process solution. Specific gravity of the process solution. Recommended heel (sludge) allowance.
To size the heater, first determine the tank size: space required for the part, parts rack or barrel, space required for busing (anodes), in-tank pumps and filtration, sumps, overflow dams, level controls, air or solution agitation pipes, and any other accessories. From this data, a tank size and configuration can be determined. Calculate the weight of solution to be heated. For rectangular tanks: Weight = L X W × D x S.G. X 62.4 lb/ft3 where L, W, and D are length, width, and depth in feet (substitute 0.036 lb/in) for dimensions in inches). S.G. is the specific gravity of the solution (water is 1.0). For cylindrical tanks: Weight = R2D × S.G. X 62.4 lb/ft3 where R is the radius of the tank. Calculate the temperature rise required by subtracting the average (or lowest) ambient temperature from the desired operating temperature (if the shop temperature is kept very cool during winter months, it might be wise to use this temperature as the average ambient temperature). Temperature rise = T operating minus T ambient [To - Ta = T rise] Determine an adequate heat-up time to suit your production requirements. The traditional 2-hour heatup may prove costly and unnecessary since using this value usually provides a heater more than twice the size necessary for heat maintenance. A 4- to 6-hour heatup more 755
Table I. Heat Losses from Liquid Surfaces Solution Temperature (°F)
Nonventilated Losses (BTU/hr/fte)
Ventilated Losses (BTUIhrlfi e)
100 120 140 160 180
170 340 615 990 1,590
290 560 995 1,600 2,750
closely approximates the heat maintenance value but may impose production constraints deemed impractical Long heat-up times can be overcome through the use of 24-hr timers; however, unattended heat-starts carry the responsibility of tank liquid level monitoring and approved overtemperature safety shutoffs. With this data, the initial tank heating requirements can be determined. A BTU is the amount of heat required to raise one pound of water one degree Fahrenheit. A BTUH is that amount per hour. Initial BTUH(Q) = Weight × T rise X s.h./Heat-up time where s.h. is specific heat. This should be the actual value from the process supplier (water is 1.0). Calculate the approximate heat loss from the tank surface and tank walls. (Use the data shown in Tables I and II.) The losses from the tank surface can represent the most significant loss affecting heater sizing. The addition of even a partial or loose-fitting cover will reduce these losses. The tank surface area is simply the width in feet times the length in feet. You can use inches instead of feet, but then must divide the results by 144 to obtain square feet. If you install partial covers, such as removable covers extending from the tank edge to the anode busing, use the remaining "open" dimensions. The covered area uses the reduced loss values shown in Table III. The use of partial covers reduces exhaust volume requirements and associated energy demands as well. Air agitation can be said to primarily affect losses from the tank surface. Breaking bubbles increase the surface area and expose a thin film of solution to accelerate evaporative losses. Air agitation spargers sized at One cfm per foot of length affect a 6 in. (1/2 ft) wide path along their length. Thus, a three foot by four foot tank surface with two lanes of air agitation running, on the four foot dimension has:
3 x 4 = 12 fi2 surface plus 2 x 1/2 x 4 = 4 fts agitation increase, a total 16
ft 2
effective
Multiply the effective area by the values shown in Table I. Be sure to deduct any cover area (if used) and use the reduced loss values shown in Table III.
Table II. Heat Losses from Tank Walls and Bottoms
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Solution Temperature (°F)
Metal Tank or Thin Plastic (BTU/hrlft 2)
Insulated Tank or Heavy Plastic (BTUlhr/ftz)
100 120 140 160 180
30 90 140 190 240
10 30 45 45 80
Table IH. Cover Loss Values (BTUfnr/ft2) Cover Style
Still Air
Ventilated (150 fpra )
[Looseor partials Insulated Floating balls
Metal tank values shown in Table II Insulated tank values shown in Table II 0.25 times the value obtained from Table I
Twice that for still air Same as still air Twice that for still air
The tank wall area equals the tank length in feet, times the depth of solution in feet, times two plus the tank width in feet, times the depth of solution in feet, times two plus the tank length in feet, times the tank width in feet. L X D × 2 + W X D × 2 + L X W = wall area. (You can use inches instead of feet but you must divide the result by 144 to convert into square feet.) Multiply the tank wall area times the values shown in Table II. Calculate the heat loss through parts being immersed. Racks per hour, times the weight of the loaded racks, times the specific heat of the parts (use 0.1 for most metals, 0.2 for aluminum), times the temperature rise (use the same value used in calculating the tank temperature rise). racks/hr × weight/rack X s.h. X T rise A plastic or metal plating barrel must be included with the parts weight. A metal barrel has a specific heat value close to the average parts (0.1), and can be included in the parts weight, but a plastic barrel has a specific heat of 0.46 and will require an independent calculation. Weight of barrel, times barrel loads per hour, times the specific heat of the barrel, times the temperature rise. barrels/br X weight/barrel × 0.46 X T rise Add to this the parts per barrel barrels/hr X weight of parts/barrel X s.h. × T rise The heat loading and the actual heat-up time for immersed parts are distinct values. The heated solution can lose temperature to the immersed parts in ~rmatter of seconds. This heat loss is replaced by the heater. To determine the temperature drop of the process solution, divide the heat loss through parts (barrels) being immersed by the weight, times the specific heat of the solution. Heat loss (parts)/[Weight (solution) × s.h. (solution)] = Temperature drop Calculate the heat loss through solution additions such as drag-in and make-up water when working on small process tanks with high operating temperatures. In some operations, it is customary to replenish evaporative losses by rinsing parts over the tank. This practice increases the heat loading. Gallons of water each hour (drag-in or add), times 8.33 (lb/gal), times the temperature rise (water temperature to tank operating temperature). gallons per hour x 8.33 × T rise Now determine total heating requirement by comparing initial heat-up requirements with the sum of the various losses. Assuming no additions or operating losses during the initial heatup, we can equate our heater size based on the initial heat-up requirement, plus the tank surface losses, plus the tank wall losses. This value must be compared with the operating requirements--tank surface losses, plus the tank wall losses, plus the rack (barrel) losses, plus the drag-in (make-up) losses. The larger value becomes the design basis for heater sizing. Heater sizing can proceed based on the heating method employed. Electric ~mmersion heaters are sized based on 3.412 BT.UH per watt-hour (3,412 BTUH per kilowatt-hour). Divide the design heating requirement by 3,412 to findkilowatts of electric heat required. design heating requirements (BTUH)/3,412
758
The immersion heater sheath temperature will be higher than the solution temperature. Consult your immersion heater supplier for its recommendations where solutions have high temperature limits. Electric heaters have the potential of achieving sheath temperatures, particularly in air, and are capable of igniting flammable materials; therefore, it is essential that liquid level switches and high sheath temperature cutoffs be employed. Look for (or ask about) Underwriters Laboratory or other independent agency listing labels on electric heaters for assurance that the product meets a recognized standard. Verify and install the sheath ground to minimize personnel shock hazard and, as with all heaters, use a quality temperature controller for economical operation. Steam immersion heaters are sized based on steam pressure, overall transfer coefficients, area, and log mean temperature difference. The overall transfer coefficient is a value determined by several basic values: the ability of the heater material to conduct heat, the ability of the two fluid films that form on the inside and outside of the heater to conduct heat, and the resistance to the flow of heat caused by fouling or buildup. You can significantly alter the performance of irmnersion heaters by the choice of materials and the supply or the lack of supply of tank agitation. By selecting proper materials the fouling caused by corrosion is either reduced or eliminated. Clean quality steam will reduce internal fouling while properly placed agitation can enhance overall thermal performance. The precise calculation of the overall transfer coefficient is detailed and will not be covered here, but is available from your heater supplier. The following rule-of-thumb values can be used for estimating steam heater size. For metal coils, the range of values for the overall heat transfer coefficient is 100-200 BTU/hr/fte/°F. For plastic coils, the overall heat transfer coefficient ranges from 20-50. Use 150 for metal and 40 for Teflon. Now calculate the log mean temperature difference (LMTD) because the driving force for the heat exchange is a varying quantity that is expressed as this value. LMTD = (AT 1 -- AT2)/[ln(AT1/AT2)] where In = Naperian-(natural) logarithms. Steam pressure produces specific temperatures that will be used in the calculation of the LMTD. Typical values are given in Table IV. As an example, assume 10 psig steam is to be used to heat a solution from 65°F (ambient shop temperature) to 140°F (solution operating temperature). Steam temperature (from Table IV): 240°F AT1 = 240 - 65 = 175°F AT2 = 240 - 140 = 100°F LMTD = (175 - 200)/[ln (175/100)] = 75/0.55 = 134°F The heater area required to steam heat a process solution equals the design heating requirement, divided by the overall heat transfer coefficient, times the log mean temperature. Design heating requirement (BTUH)/Overall heating requirement X LMTD As with any immersion heater, the heater surface temperature will be higher than the solution temperature. Obviously, it cannot exceed the steam temperature. If the solution has a high temperature limit below available steam temperatures, you may require a custom electric immersion heater or a hot water (or thermal fluid) heater with a lower heating temperature. Although the heater temperature is limited to the steam temperature, damage to process tanks and accessories can result from overtemperature or low liquid levels..It is wise to equip your process tank with overtemperature and low liquid level cutoffs. Once a coil size is selected, piping size should be investigated. The quantity of steam used for a specific coil size varies with the steam pressure (see Table V) and the heat released
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Table IV. S t e a m Table Steam pressure (psig) Steam temperature (*F) Heat of evaporation (BTU/Ib)
5 226 960
10 240 950
15 250 945
20 260 940
25 266 935
30 274 930
is the heat of evaporation (latent heat) only. The values in the tabie are in BTUs per pound of steam. So the quantity of steam required equals the design heating requirement, divided by the heat of evaporation of the steam. Design heating requirement (BTUH)/Heat of evaporation (from Table IV) The result, in pounds of steam per hour, can be equated to pipe size as shown in Table V. The condensate generated (condensed steam) must be "trapped," that is, equipped with a steam trap. Steam traps are sized based on pounds per hour times a safety factor. Since the amount of condensate varies with the temperature of the solution, it is wise to use a safety factor of four or better. Trap capacity equals the steam required times four. The condensate piping is smaller than the steam pipe since the condensate is liquid. Some of the condensate will convert back to steam because of condensate temperature and pressure. The use of piping smaller than 1/2 in. nominal is not recommended since scale and buildup inside the pipe is a factor in all steam lines. We recommend using 3/4 in. nominal pipe for condensate lines. This size will handle up to 1,920 Ib/hr with a modest pressure drop. Steam coil valve sizing is usually smaller than the pipe size since a pressure drop across the valve is required for proper operation. Some typical sizes for diaphragm solenoid valves are shown in Table VI. Since the performance of the valve and trap can be affected by foreign matter in the steam, it is wise to place a 100-mesh strainer of the same pipe size as the steam pipe ahead of the valve. Metal steam heaters, when suspended in electrified tanks, may conduct current through the steam lines to ground so it is a good practice to install nonconductive couplings between the heater and the pipe lines. This can be accomplished using a proprietary insulating coupling, dielectric union, or section of steam hose. Finally, because some steam heaters may be buoyant (tend to float) when in service, it is necessary to secure these heaters through the use of ballasts or proprietary hold-down fixtures. Hot water (thermal fluid) heating is similar to steam heating in the methods used for sizing. The basic differences involve the usually lower heating solution temperatures and the lower performance, overall heat transfer coefficient of the heater. As in the case of steam heating, the overall transfer coefficient is subject to varying performance and its precise computation is beyond the scope of this presentation. The following rule-of-thumb values can be used for estimating hot water heater sizes. For metal, the overall heat transfer coefficient is 70-100 BTU/hr/ft2/°F. For plastic, the range is 20-50. Use 9! for metal and 40 for Teflon. The calculation of the LMTD uses the same equation but now the heating fluid temperature must change since it is yielding the fluid heat and not the evaporative heat Table V. N o m i n a l Pipe Size for Various S t e a m R e q u i r e m e n t s
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Nominal Pipe Size (in.)
Steam Required (lb/hr)
1 I 1/2 2 3
Up to tO0 100-300 300-500 300-1,000
Table VI. R e c o m m e n d e d Valve Sizes CV Factor
Diaphragm Valve Pipe Size (in.)
Steam Required (lb/hr )
4 5 13.5 15 22,5
I/2 3/4 1 I 1/4 I 1/2
120 150 400 450 675
available in steam. It is wise to limit the heat drop of the heating fluid to 10°F since greater -drops may be impossible to achieve in a field-installed condition. Also, it is wise to design the exiting heating fluid temperature to be 15°F higher than the final solution temperature to ensure field reproduction of design performance. Consult your heater supplier for assistance if you experience any difficulty in sizing a heater. As an example, heat a solution from 65°F (ambient shop temperatul:e) to 140°F (operating temperature) using 195°F hot water. Limit the hot water temperature drop to 10°F or 185°F outlet. This temperature is more than 15°F above the final bath temperature. AT1 = 195 - 65 = 130°F AT2 = 185 - 140 = 45°F LMTD = (130 - 45)/ln(130/45)] = 95/1.0607 = 80.56°F The heater area required to heat a process solution equals the design heating requirement divided by the overall heat transfer coefficient times the LMTD. Design heating requirement/[Overall transfer coefficient × LMTD] With hot water heaters, it is a wise precaution to install high liquid level cutoffs that will shut off hot fluid flow in the event of a heater leak. If a high temperature heating fluid is used, solution temperature sensitivity must be evaluated and high temperature, low liquid level cutoffs may be in order. Once the coil area has been selected, the hot water (thermal fluid) flow must be calculated. The flow is equal to the desigr~ heating requirement, divided by the temperature drop of the heating fluid, times the specific heat of the heating fluid, times the specific gravity of the heating fluid. Design heating requirement/[Temperature drop X s.h. X s.g. (all of the heating fluid)] This 'results in the pounds per hour of heating fluid. To convert this into gallons per minute, divide the pounds per hour by the weight of fluid per gallon times 60 (water weighs 8.33 lh/gal). This value is used to evaluate pipe size (both inlet and outlet). Table VII gives a reasonable flow for water through various pipe sizes. The control valve may be smaller than the pipe size. Some typical sizes for diaphragm valves with a water pressure drop of 5 psig are given in Table VIII. Table VII. W a t e r F l o w Rates for Various N o m i n a l Pipe Sizes
764
Nominal Pipe Size (in.)
Flow Rate (gal/min)
I/2 3/4 1 1114 11/2
6 10 20 30 45
Table VIII. 'l~pical Valve Sizes a n d Flow Rates for a Pressure D r o p o f 5 psig CV Factor
Diaphragm Valve Size (in.)
Flow Rate (gal/min)
4.0 6.5 13.5 22.5
U2 3/4 1 1I/2
9 14 30 50
As with steam heaters, it is a good practice to install a strainer to minimize foreign particles that may affect valve performance. A 60-mesh strainer is usually fine enough for hot fluid systems. Metal heaters, when suspended in electrified tanks, may conduct Current through supply lines to ground so it is a good practice to install nonconductive couplings between the heater and the pipe lines. A proprietary insulating coupling or dielectric union can be used. Plastic heaters and some empty metal heaters may be buoyant, so be sure to provide adequate anchoring if floating is suspected. Thermal stratification is a fact of life in heated process tanks. To minimize this effect good agitation (mixing) is required. Classic air agitation is sized at one cfm per foot of length. When placed beneath a cathode (or anode) it provides sufficient agitation to that surface to enhance deposition rates. It does not, in this form, eliminate thermal stratification. Top-down mixing can be provided through recirculation pumping. Pumps sized for 10 turnovers or more per hour provide good mixing and uniform temperatures. Skimming style pump inlets with sparger bottom discharges are best since higher temperature solutions are forced to the cooler areas. In tanks three feet deep and more, a vertical sump pump can be mounted on the tank flange with a length of discharge pipe anchored to the tank bottom. These can often be coupled to in-tank filters for removal of particulates while providing mixing. Air agitation, when properly placed, can "average" temperature in their zone of influence (usually 6-12 in.) and can be used to enhance response time for temperature controller sensors. As the air agitation is increased, heat losses also increase, making air agitation a less desirable means of dealing with thermal stratification. Heat sensitive solutions can be addressed by either electric or hot water (thermal fluid) heaters. Electric is the easiest to control since the heater surface temperature can be varied by varying the input voltage. A heater surface temperature controller can limit surface temperatures while still providing sufficient heat for the solution. Similarly, hot water systems can be sized for maximum hot water temperatures (and thus heater temperatures) but control and response are usually inferior to electric systems.
Gold Plating Technology by F.H. Reid and W.. Go/die
630 pages
$200.00
This js a critical review of the literature with over 900 references cited. The 42 chapters written by 23 authorities cover everything from historical background to applications, and is a worthwhile investment since it takes only a little pointer from the wealth of information presented to save thousands of dollars. Send Orders to: METAL FINISHING, 660 White Plains Rd., Tarrytown, NY 10591-5153 For faster service, call (914) 333-2578 or FAX your order to (914) 333-2570 All book orders must be prepaid. Please include $5.00 shippingand handling for delivery of each book via UPS in the U.S., $10.00 for each book shipped expressto Canada; and $20.00 for each book shippedexpress to all other countries.
766
Recommended materials of construction. Maximum (minimum) solution temperature. Maximum heater surface temperature. Specific heat of the process solution. Specific gravity of the process solution. Recommended heel (sludge) allowance.
To size the heater, first determine the tank size: space required for the part, parts rack or barrel, space required for busing (anodes), in-tank pumps and filtration, sumps, overflow dams, level controls, air or solution agitation pipes, and any other accessories. From this data, a tank size and configuration can be determined. Calculate the weight of solution to be heated. For rectangular tanks: Weight = L X W × D x S.G. X 62.4 lb/ft3 where L, W, and D are length, width, and depth in feet (substitute 0.036 lb/in) for dimensions in inches). S.G. is the specific gravity of the solution (water is 1.0). For cylindrical tanks: Weight = R2D × S.G. X 62.4 lb/ft3 where R is the radius of the tank. Calculate the temperature rise required by subtracting the average (or lowest) ambient temperature from the desired operating temperature (if the shop temperature is kept very cool during winter months, it might be wise to use this temperature as the average ambient temperature). Temperature rise = T operating minus T ambient [To - Ta = T rise] Determine an adequate heat-up time to suit your production requirements. The traditional 2-hour heatup may prove costly and unnecessary since using this value usually provides a heater more than twice the size necessary for heat maintenance. A 4- to 6-hour heatup more 755
Table I. Heat Losses from Liquid Surfaces Solution Temperature (°F)
Nonventilated Losses (BTU/hr/fte)
Ventilated Losses (BTUIhrlfi e)
100 120 140 160 180
170 340 615 990 1,590
290 560 995 1,600 2,750
closely approximates the heat maintenance value but may impose production constraints deemed impractical Long heat-up times can be overcome through the use of 24-hr timers; however, unattended heat-starts carry the responsibility of tank liquid level monitoring and approved overtemperature safety shutoffs. With this data, the initial tank heating requirements can be determined. A BTU is the amount of heat required to raise one pound of water one degree Fahrenheit. A BTUH is that amount per hour. Initial BTUH(Q) = Weight × T rise X s.h./Heat-up time where s.h. is specific heat. This should be the actual value from the process supplier (water is 1.0). Calculate the approximate heat loss from the tank surface and tank walls. (Use the data shown in Tables I and II.) The losses from the tank surface can represent the most significant loss affecting heater sizing. The addition of even a partial or loose-fitting cover will reduce these losses. The tank surface area is simply the width in feet times the length in feet. You can use inches instead of feet, but then must divide the results by 144 to obtain square feet. If you install partial covers, such as removable covers extending from the tank edge to the anode busing, use the remaining "open" dimensions. The covered area uses the reduced loss values shown in Table III. The use of partial covers reduces exhaust volume requirements and associated energy demands as well. Air agitation can be said to primarily affect losses from the tank surface. Breaking bubbles increase the surface area and expose a thin film of solution to accelerate evaporative losses. Air agitation spargers sized at One cfm per foot of length affect a 6 in. (1/2 ft) wide path along their length. Thus, a three foot by four foot tank surface with two lanes of air agitation running, on the four foot dimension has:
3 x 4 = 12 fi2 surface plus 2 x 1/2 x 4 = 4 fts agitation increase, a total 16
ft 2
effective
Multiply the effective area by the values shown in Table I. Be sure to deduct any cover area (if used) and use the reduced loss values shown in Table III.
Table II. Heat Losses from Tank Walls and Bottoms
756
Solution Temperature (°F)
Metal Tank or Thin Plastic (BTU/hrlft 2)
Insulated Tank or Heavy Plastic (BTUlhr/ftz)
100 120 140 160 180
30 90 140 190 240
10 30 45 45 80
Table IH. Cover Loss Values (BTUfnr/ft2) Cover Style
Still Air
Ventilated (150 fpra )
[Looseor partials Insulated Floating balls
Metal tank values shown in Table II Insulated tank values shown in Table II 0.25 times the value obtained from Table I
Twice that for still air Same as still air Twice that for still air
The tank wall area equals the tank length in feet, times the depth of solution in feet, times two plus the tank width in feet, times the depth of solution in feet, times two plus the tank length in feet, times the tank width in feet. L X D × 2 + W X D × 2 + L X W = wall area. (You can use inches instead of feet but you must divide the result by 144 to convert into square feet.) Multiply the tank wall area times the values shown in Table II. Calculate the heat loss through parts being immersed. Racks per hour, times the weight of the loaded racks, times the specific heat of the parts (use 0.1 for most metals, 0.2 for aluminum), times the temperature rise (use the same value used in calculating the tank temperature rise). racks/hr × weight/rack X s.h. X T rise A plastic or metal plating barrel must be included with the parts weight. A metal barrel has a specific heat value close to the average parts (0.1), and can be included in the parts weight, but a plastic barrel has a specific heat of 0.46 and will require an independent calculation. Weight of barrel, times barrel loads per hour, times the specific heat of the barrel, times the temperature rise. barrels/br X weight/barrel × 0.46 X T rise Add to this the parts per barrel barrels/hr X weight of parts/barrel X s.h. × T rise The heat loading and the actual heat-up time for immersed parts are distinct values. The heated solution can lose temperature to the immersed parts in ~rmatter of seconds. This heat loss is replaced by the heater. To determine the temperature drop of the process solution, divide the heat loss through parts (barrels) being immersed by the weight, times the specific heat of the solution. Heat loss (parts)/[Weight (solution) × s.h. (solution)] = Temperature drop Calculate the heat loss through solution additions such as drag-in and make-up water when working on small process tanks with high operating temperatures. In some operations, it is customary to replenish evaporative losses by rinsing parts over the tank. This practice increases the heat loading. Gallons of water each hour (drag-in or add), times 8.33 (lb/gal), times the temperature rise (water temperature to tank operating temperature). gallons per hour x 8.33 × T rise Now determine total heating requirement by comparing initial heat-up requirements with the sum of the various losses. Assuming no additions or operating losses during the initial heatup, we can equate our heater size based on the initial heat-up requirement, plus the tank surface losses, plus the tank wall losses. This value must be compared with the operating requirements--tank surface losses, plus the tank wall losses, plus the rack (barrel) losses, plus the drag-in (make-up) losses. The larger value becomes the design basis for heater sizing. Heater sizing can proceed based on the heating method employed. Electric ~mmersion heaters are sized based on 3.412 BT.UH per watt-hour (3,412 BTUH per kilowatt-hour). Divide the design heating requirement by 3,412 to findkilowatts of electric heat required. design heating requirements (BTUH)/3,412
758
The immersion heater sheath temperature will be higher than the solution temperature. Consult your immersion heater supplier for its recommendations where solutions have high temperature limits. Electric heaters have the potential of achieving sheath temperatures, particularly in air, and are capable of igniting flammable materials; therefore, it is essential that liquid level switches and high sheath temperature cutoffs be employed. Look for (or ask about) Underwriters Laboratory or other independent agency listing labels on electric heaters for assurance that the product meets a recognized standard. Verify and install the sheath ground to minimize personnel shock hazard and, as with all heaters, use a quality temperature controller for economical operation. Steam immersion heaters are sized based on steam pressure, overall transfer coefficients, area, and log mean temperature difference. The overall transfer coefficient is a value determined by several basic values: the ability of the heater material to conduct heat, the ability of the two fluid films that form on the inside and outside of the heater to conduct heat, and the resistance to the flow of heat caused by fouling or buildup. You can significantly alter the performance of irmnersion heaters by the choice of materials and the supply or the lack of supply of tank agitation. By selecting proper materials the fouling caused by corrosion is either reduced or eliminated. Clean quality steam will reduce internal fouling while properly placed agitation can enhance overall thermal performance. The precise calculation of the overall transfer coefficient is detailed and will not be covered here, but is available from your heater supplier. The following rule-of-thumb values can be used for estimating steam heater size. For metal coils, the range of values for the overall heat transfer coefficient is 100-200 BTU/hr/fte/°F. For plastic coils, the overall heat transfer coefficient ranges from 20-50. Use 150 for metal and 40 for Teflon. Now calculate the log mean temperature difference (LMTD) because the driving force for the heat exchange is a varying quantity that is expressed as this value. LMTD = (AT 1 -- AT2)/[ln(AT1/AT2)] where In = Naperian-(natural) logarithms. Steam pressure produces specific temperatures that will be used in the calculation of the LMTD. Typical values are given in Table IV. As an example, assume 10 psig steam is to be used to heat a solution from 65°F (ambient shop temperature) to 140°F (solution operating temperature). Steam temperature (from Table IV): 240°F AT1 = 240 - 65 = 175°F AT2 = 240 - 140 = 100°F LMTD = (175 - 200)/[ln (175/100)] = 75/0.55 = 134°F The heater area required to steam heat a process solution equals the design heating requirement, divided by the overall heat transfer coefficient, times the log mean temperature. Design heating requirement (BTUH)/Overall heating requirement X LMTD As with any immersion heater, the heater surface temperature will be higher than the solution temperature. Obviously, it cannot exceed the steam temperature. If the solution has a high temperature limit below available steam temperatures, you may require a custom electric immersion heater or a hot water (or thermal fluid) heater with a lower heating temperature. Although the heater temperature is limited to the steam temperature, damage to process tanks and accessories can result from overtemperature or low liquid levels..It is wise to equip your process tank with overtemperature and low liquid level cutoffs. Once a coil size is selected, piping size should be investigated. The quantity of steam used for a specific coil size varies with the steam pressure (see Table V) and the heat released
760
Table IV. S t e a m Table Steam pressure (psig) Steam temperature (*F) Heat of evaporation (BTU/Ib)
5 226 960
10 240 950
15 250 945
20 260 940
25 266 935
30 274 930
is the heat of evaporation (latent heat) only. The values in the tabie are in BTUs per pound of steam. So the quantity of steam required equals the design heating requirement, divided by the heat of evaporation of the steam. Design heating requirement (BTUH)/Heat of evaporation (from Table IV) The result, in pounds of steam per hour, can be equated to pipe size as shown in Table V. The condensate generated (condensed steam) must be "trapped," that is, equipped with a steam trap. Steam traps are sized based on pounds per hour times a safety factor. Since the amount of condensate varies with the temperature of the solution, it is wise to use a safety factor of four or better. Trap capacity equals the steam required times four. The condensate piping is smaller than the steam pipe since the condensate is liquid. Some of the condensate will convert back to steam because of condensate temperature and pressure. The use of piping smaller than 1/2 in. nominal is not recommended since scale and buildup inside the pipe is a factor in all steam lines. We recommend using 3/4 in. nominal pipe for condensate lines. This size will handle up to 1,920 Ib/hr with a modest pressure drop. Steam coil valve sizing is usually smaller than the pipe size since a pressure drop across the valve is required for proper operation. Some typical sizes for diaphragm solenoid valves are shown in Table VI. Since the performance of the valve and trap can be affected by foreign matter in the steam, it is wise to place a 100-mesh strainer of the same pipe size as the steam pipe ahead of the valve. Metal steam heaters, when suspended in electrified tanks, may conduct current through the steam lines to ground so it is a good practice to install nonconductive couplings between the heater and the pipe lines. This can be accomplished using a proprietary insulating coupling, dielectric union, or section of steam hose. Finally, because some steam heaters may be buoyant (tend to float) when in service, it is necessary to secure these heaters through the use of ballasts or proprietary hold-down fixtures. Hot water (thermal fluid) heating is similar to steam heating in the methods used for sizing. The basic differences involve the usually lower heating solution temperatures and the lower performance, overall heat transfer coefficient of the heater. As in the case of steam heating, the overall transfer coefficient is subject to varying performance and its precise computation is beyond the scope of this presentation. The following rule-of-thumb values can be used for estimating hot water heater sizes. For metal, the overall heat transfer coefficient is 70-100 BTU/hr/ft2/°F. For plastic, the range is 20-50. Use 9! for metal and 40 for Teflon. The calculation of the LMTD uses the same equation but now the heating fluid temperature must change since it is yielding the fluid heat and not the evaporative heat Table V. N o m i n a l Pipe Size for Various S t e a m R e q u i r e m e n t s
762
Nominal Pipe Size (in.)
Steam Required (lb/hr)
1 I 1/2 2 3
Up to tO0 100-300 300-500 300-1,000
Table VI. R e c o m m e n d e d Valve Sizes CV Factor
Diaphragm Valve Pipe Size (in.)
Steam Required (lb/hr )
4 5 13.5 15 22,5
I/2 3/4 1 I 1/4 I 1/2
120 150 400 450 675
available in steam. It is wise to limit the heat drop of the heating fluid to 10°F since greater -drops may be impossible to achieve in a field-installed condition. Also, it is wise to design the exiting heating fluid temperature to be 15°F higher than the final solution temperature to ensure field reproduction of design performance. Consult your heater supplier for assistance if you experience any difficulty in sizing a heater. As an example, heat a solution from 65°F (ambient shop temperatul:e) to 140°F (operating temperature) using 195°F hot water. Limit the hot water temperature drop to 10°F or 185°F outlet. This temperature is more than 15°F above the final bath temperature. AT1 = 195 - 65 = 130°F AT2 = 185 - 140 = 45°F LMTD = (130 - 45)/ln(130/45)] = 95/1.0607 = 80.56°F The heater area required to heat a process solution equals the design heating requirement divided by the overall heat transfer coefficient times the LMTD. Design heating requirement/[Overall transfer coefficient × LMTD] With hot water heaters, it is a wise precaution to install high liquid level cutoffs that will shut off hot fluid flow in the event of a heater leak. If a high temperature heating fluid is used, solution temperature sensitivity must be evaluated and high temperature, low liquid level cutoffs may be in order. Once the coil area has been selected, the hot water (thermal fluid) flow must be calculated. The flow is equal to the desigr~ heating requirement, divided by the temperature drop of the heating fluid, times the specific heat of the heating fluid, times the specific gravity of the heating fluid. Design heating requirement/[Temperature drop X s.h. X s.g. (all of the heating fluid)] This 'results in the pounds per hour of heating fluid. To convert this into gallons per minute, divide the pounds per hour by the weight of fluid per gallon times 60 (water weighs 8.33 lh/gal). This value is used to evaluate pipe size (both inlet and outlet). Table VII gives a reasonable flow for water through various pipe sizes. The control valve may be smaller than the pipe size. Some typical sizes for diaphragm valves with a water pressure drop of 5 psig are given in Table VIII. Table VII. W a t e r F l o w Rates for Various N o m i n a l Pipe Sizes
764
Nominal Pipe Size (in.)
Flow Rate (gal/min)
I/2 3/4 1 1114 11/2
6 10 20 30 45
Table VIII. 'l~pical Valve Sizes a n d Flow Rates for a Pressure D r o p o f 5 psig CV Factor
Diaphragm Valve Size (in.)
Flow Rate (gal/min)
4.0 6.5 13.5 22.5
U2 3/4 1 1I/2
9 14 30 50
As with steam heaters, it is a good practice to install a strainer to minimize foreign particles that may affect valve performance. A 60-mesh strainer is usually fine enough for hot fluid systems. Metal heaters, when suspended in electrified tanks, may conduct Current through supply lines to ground so it is a good practice to install nonconductive couplings between the heater and the pipe lines. A proprietary insulating coupling or dielectric union can be used. Plastic heaters and some empty metal heaters may be buoyant, so be sure to provide adequate anchoring if floating is suspected. Thermal stratification is a fact of life in heated process tanks. To minimize this effect good agitation (mixing) is required. Classic air agitation is sized at one cfm per foot of length. When placed beneath a cathode (or anode) it provides sufficient agitation to that surface to enhance deposition rates. It does not, in this form, eliminate thermal stratification. Top-down mixing can be provided through recirculation pumping. Pumps sized for 10 turnovers or more per hour provide good mixing and uniform temperatures. Skimming style pump inlets with sparger bottom discharges are best since higher temperature solutions are forced to the cooler areas. In tanks three feet deep and more, a vertical sump pump can be mounted on the tank flange with a length of discharge pipe anchored to the tank bottom. These can often be coupled to in-tank filters for removal of particulates while providing mixing. Air agitation, when properly placed, can "average" temperature in their zone of influence (usually 6-12 in.) and can be used to enhance response time for temperature controller sensors. As the air agitation is increased, heat losses also increase, making air agitation a less desirable means of dealing with thermal stratification. Heat sensitive solutions can be addressed by either electric or hot water (thermal fluid) heaters. Electric is the easiest to control since the heater surface temperature can be varied by varying the input voltage. A heater surface temperature controller can limit surface temperatures while still providing sufficient heat for the solution. Similarly, hot water systems can be sized for maximum hot water temperatures (and thus heater temperatures) but control and response are usually inferior to electric systems.
Gold Plating Technology by F.H. Reid and W.. Go/die
630 pages
$200.00
This js a critical review of the literature with over 900 references cited. The 42 chapters written by 23 authorities cover everything from historical background to applications, and is a worthwhile investment since it takes only a little pointer from the wealth of information presented to save thousands of dollars. Send Orders to: METAL FINISHING, 660 White Plains Rd., Tarrytown, NY 10591-5153 For faster service, call (914) 333-2578 or FAX your order to (914) 333-2570 All book orders must be prepaid. Please include $5.00 shippingand handling for delivery of each book via UPS in the U.S., $10.00 for each book shipped expressto Canada; and $20.00 for each book shippedexpress to all other countries.
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