2000 Hook-up Book - Spirax Sarco

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SYSTEM DESIGN 32 Makeup Air Heating Coils The steam trap must handle lots of air and drain condensate at saturated steam temperature continuously while the load and pressure are changing and thus a Float-Thermostatic type is recommended for all air heating coils. The trap is mounted below the condensate outlet from the coil with a vertical drop giving enough hydraulic head to enable a suitable size to be selected. A 14" head should be the minimum and represents about 1/2 psi, a 28" head about 1 psi, and to reduce possibility of freeze-up, a drop of 3 ft. to the trap is recommended. Preheat/Reheat Coils The preheat/reheat coil hookup shown in Fig. II-26 (page 96) may employ a direct-acting temperature control or with larger coils, a quicker responding pilot-operated type with a closer control band is recommended. This arrangement allows filtration and perhaps humidification of the air to be carried out at the controlled preheat temperature, and the reheat coil brings the dry bulb temperature of the conditioned air to the required value for distribution. The preheat coil is used to heat outside air up to the intermediate temperature but as outside temperature increases, the temperature control lowers the steam pressure in the preheat coil and condensate drainage tends to slow down. If the coil is being used where design loads occur at subzero temperatures, there can sometimes be only atmospheric pressure in the coil, although the air passing over it is still cold enough to lead to freeze-up problems. This difficulty is greatly reduced if the temperature sensor controlling the steam supply to the preheat coil is set to the needed distribution temperature. Part load conditions would then lead firstly to lowering the steam pressure in the reheat coil, where freezing will not occur, but pressure is maintained in the preheat coil until outside air temperatures are above the danger point. Such an arrangement reduces freeze-up problems in many instances on existing installations, at minimal cost. Corrosion And Waterhammer Problems Condensate mixed with air becomes corrosive and assuming the boiler water treatment is satis- Case in Action: Air Handling System Steam Coil Drainage Typical storage buildings are extremely large and difficult to heat. This example in specific has three floors with approximately 486,000 ft 2 of floor space and heated with 150 air handling units. These units are comprised of bay heaters, overhead door heaters and administrative office area heaters. The minimum steam supply pressure to all of them is 20 psig and are pneumatically controlled. In the preceding 12 month period, $201,000 was spent on labor and materials to repair damaged coils. The common problem was condensate standing in the coils, unable to drain, causing erosion due to presence of carbonic acid and bulging/splitting as a result of freezing. Solution Starting with a training session at the facility that addressed this problem and typical solutions, Spirax Sarco’s local sales office implemented a “Cooperative Research and Development Agreement” (CRDA). The purpose of the agreement was to test a proposed solution including Pressure Powered Pumps and Pump/Trap combinations to eliminate system stall, thereby assuring thorough condensate drainage, regardless of supply air temperature, control valve turn-down or over-sized heaters. A test was conducted on four air handling units. One unit was hooked up as usual, without Pressure Powered factory, coil corrosion problems are usually due to condensate regularly backing up or lying stagnant on the bottom of the tubes during shutdown. If the coil is trapped correctly, the most likely cause is an overhead return which prevents the coil from draining. One remedy for this is to fit a liquid expansion steam trap at the lowest piping level, as shown in Fig. II-26 (page 96), set to open when the temperature drops below 90°F. The coil then drains only cold condensate to a sewer. In high pressure systems where waterhammer on startup remains troublesome, a “safety drain” trap is sometimes used. This consists of a stock 15 psi rated inverted bucket trap fitted above the main trap which discharges to drain whenever coil pressure is low, but due to its design locks shut at higher pressure. While this is useful on pressurized mains, the safety trap may require a pressure considerably higher than its nominal rating to lock shut and on modulating service a considerable amount of condensate may be wasted. This makes the combination pump/trap a more viable solution to this problem. Pump drainage systems. The other three were drained by either open or closed loop PPP systems. Four days into the test and the unit without a PPP drainage system had three frozen coils. It was found that as outside supply air temperature dropped below 36˚F, it was necessary to close outside dampers and use 100% recirculated air, or the coils would freeze. The three units drained by PPP systems continued operating trouble-free. Benefits Employee Safety • Improved indoor air quality through the use of a higher percentage of outside air supply. • Reduced chance of injury by eliminating water leakage on the floor from broken coils and subsequent slippage. • Fewer burns because there are fewer steam leaks. • Greater employee awareness of hazards because of training. Cost Savings • Reduced steam and condensate losses resulting in energy savings. • Reduced cost for management support (paper-work). • Cost savings of up to 30% above the initial installation cost in a 12 month period.
Makeup air heating coils and other heat exchange equipment where the steam supply pressure is modulated to hold a desired outflow temperature must always be kept drained of condensate. Fitting a vacuum breaker and steam trap, no matter what the size, does not always result in trouble-free operation and problems with noisy, hammering, corroded and especially frozen coils are well documented. These problems are the result of coil flooding at some point when either: a. Incoming makeup air increases above minimum design temperature, or b. Flow rate through an exchanger decreases below the maximum equipment output. In a steam system, temperature regulation actually means controlling the pressure. Under partial load conditions, the steam controller, whether self-acting, pneumatic or any other type, reduces the pressure until the necessary trap differential is eliminated, the system “stalls,” and steam coils become waterfilled coils. Conditions Creating “System Stall” With the steam equipment and the operating pressure selected, the load at which any system stalls is a function of how close the equipment is sized to the actual load and any condensate elevation or other back pressure the trap is subject to. Other less obvious things can also seriously contribute to “system stall”; for instance, overly generous fouling factors and equipment oversizing. As an example, a fouling factor of “only” .001 can result in a coil surface area increase of 50% (See Table 10). Equipment oversizing causes the system to stall faster. This is particularly the case when the heating equipment is expected to run considerably below “design load.” Saturated steam temperature is directly related to its pressure Draining Temperature Controlled Steam Equipment and for any load requirement, the control valve output is determined by the basic heat transfer equation, Q = UA x ∆T. With “UA” for a steam-filled coil a constant, the amount of heat supplied, “Q”, is regulated by the “∆T,” the log mean temperature difference (LMTD) between the heated air or liquid and saturated steam temperature at the pressure delivered by the valve. Thus, the steam pressure available to operate the trap is not constant but varies with the demand for heat from almost line pressure down through subatmospheric, to complete shutdown when no heat is required. Actual differential across the trap is further reduced when the heating surface is oversized or the trap must discharge against a back pressure. Knowing these conditions, the system must be designed accordingly. Plotting A “Stall Chart” An easy way to determine the conditions at which drainage Temperature °F 400 380 360 340 320 300 280 260 240 220 200 180 160 140 120 100 80 60 40 20 Figure 45: Stall Chart Table 10: Percentage Fouling Allowance Velocity Fouling Factor in Ft./Sec. .0005 .001 1 1.14 (14%) 1.27 (27%) 2 1.19 (19%) 1.38 (38%) 3 1.24 (24%) 1.45 (45%) 4 1.27 (27%) 1.51 (51%) 5 1.29 (29%) 1.55 (55%) 6 1.30 (30%) 1.60 (60%) 7 1.31 (31%) 1.63 (63%) problems will occur, and prevent them at the design stage is to use the “stall chart” shown in Fig. 45. The steam supply pressure is shown on the vertical axis, with corresponding temperatures on the opposite side, and the plot will indicate graphically what will occur for any percentage of the design load. This method provides a fairly accurate prediction of stall conditions even though the chart uses “arithmetic” rather than “log mean” temperature difference. 0 100 90 80 70 60 50 40 Percentage Load 30 20 10 0 235 180 140 105 psig 75 55 34 Pressure 20 10 3 0 5" 10" 15" Vacuum 20" 25" Inches 33 SYSTEM DESIGN
Page 1 and 2: DESIGN OF FLUID SYSTEMS HOOK-UPS
Page 3 and 4: Spirax Sarco Spirax Sarco is the re
Page 5: Section 1: System Design Informatio
Page 8 and 9: SYSTEM DESIGN 2 The Working Pressur
Page 10 and 11: SYSTEM DESIGN 4 Sizing Steam Lines
Page 12 and 13: SYSTEM DESIGN 6 Sizing Superheated
Page 14 and 15: SYSTEM DESIGN 8 Draining Steam Main
Page 16 and 17: SYSTEM DESIGN 10 Draining Steam Mai
Page 18 and 19: SYSTEM DESIGN 12 Steam Tracing The
Page 20 and 21: SYSTEM DESIGN 14 Steam Tracing Sizi
Page 22 and 23: SYSTEM DESIGN 16 Steam Tracing Trac
Page 24 and 25: SYSTEM DESIGN 18 Steam Tracing A si
Page 26 and 27: SYSTEM DESIGN 20 Pressure Reducing
Page 28 and 29: SYSTEM DESIGN 22 Parallel and Serie
Page 30 and 31: SYSTEM DESIGN 24 How to Size Temper
Page 32 and 33: SYSTEM DESIGN 26 Temperature Contro
Page 40 and 41: SYSTEM DESIGN 34 Draining Temperatu
Page 42 and 43: SYSTEM DESIGN 36 Draining Temperatu
Page 44 and 45: SYSTEM DESIGN 38 Steam Trap Selecti
Page 46 and 47: SYSTEM DESIGN 40 Steam Trap Selecti
Page 48 and 49: SYSTEM DESIGN 42 Flash Steam Flash
Page 50 and 51: SYSTEM DESIGN 44 Flash Steam Flash
Page 52 and 53: SYSTEM DESIGN 46 Condensate Recover
Page 54 and 55: SYSTEM DESIGN 48 Condensate Pumping
Page 56 and 57: SYSTEM DESIGN 50 Clean Steam Printi
Page 58 and 59: SYSTEM DESIGN 52 Clean Steam Overal
Page 60 and 61: SYSTEM DESIGN 54 Clean Steam Figure
Page 62 and 63: SYSTEM DESIGN 56 Testing Steam Trap
Page 64 and 65: SYSTEM DESIGN 58 Spira-tec Trap Lea
Page 66 and 67: SYSTEM DESIGN 60 Steam Meters Densi
Page 68 and 69: SYSTEM DESIGN 62 Compressed Air Sys
Page 70 and 71: SYSTEM DESIGN 64 Compressed Air Sys
Page 72 and 73: SYSTEM DESIGN 66 Compressed Air Lin
Page 74 and 75: SYSTEM DESIGN 68 Typical Steam Cons
Page 76 and 77: SYSTEM DESIGN 70 Specific Heats and
Page 78 and 79: SYSTEM DESIGN 72 Specific Heats and
Page 80 and 81: SYSTEM DESIGN 74 Conversions To Con
Page 82 and 83: SYSTEM DESIGN 76 Flow of Water thro
Page 84 and 85: SYSTEM DESIGN 78 Moisture Content o
Page 86 and 87: SYSTEM DESIGN 80 ANSI Flange Standa
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HOOK-UP APPLICATION DIAGRAMS Sectio
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Figure II-3 Draining and Air Ventin
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Figure II-8 Draining Steam Main whe
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Figure II-12 Typical Pressure Reduc
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Figure II-16 Installation of Pressu
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Figure II-20 Typical Pneumatic Sing
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Condensate Return Steam Supply Stea
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Steam Supply Steam Supply Figure II
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Figure II-31 Temperature Control of
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Elevated or Pressurized Return P 2
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Figure II-39 Controlling Temperatur
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Page 113 and 114:
Page 115 and 116:
Figure II-50 Trapping Small Utensil
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Figure II-55 Draining and Air Venti
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Figure II-59 Air Venting and Conden
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Evaporator Pressure Figure II-64 Dr
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Figure II-68 Installation of Pump/T
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Figure II-73 Draining Small Condens
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Figure II-78 Flash Steam Recovery a
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Figure II-80 Heating Water using Re
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Figure II-82 Recovery of Flash Stea
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Figure II-84 Flash Steam Condensing
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Figure II-87 Tank Sterilization Cle
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Steam Supply Figure II-93 Typical S
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Figure II-97 Hand Operated Rotary F
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Figure II-102 Freeze Proof Safety S
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Figure II-106 Continuous Boiler Blo
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Figure II-111 Controlling Temperatu
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141
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PRODUCT INFORMATION Section 3
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Steam Traps • Balanced Pressure T
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147
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Subject Index Safety...............
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Subject Index Radiation—Baseboard
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Argentina Spirax Sarco S.A. Ruta Pa
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2000 Hook-up Book - Spirax Sarco

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