2000 Hook-up Book

Draining Temperature Controlled Steam Equipment

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 prob lems 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 increas es above minimum design temperature, or b. Flow rate through an exchang er decreases below the maximum equipment output. In a steam system, tempera ture regulation actually means controlling the pressure. Under par tial load conditions, the steam controller, whether self-acting, pneumatic or any other type, reduces the pressure until the nec essary trap differential is eliminated, the system “stalls,” and steam coils 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 “sys tem 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 caus es 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 become waterfilled coils. Conditions Creating “System Stall”

and for any load requirement, the control valve output is determined by the basic heat transfer equa tion, 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 tem perature 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 sub atmospheric, 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

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 condi tions even though the chart uses “arithmetic” rather than “log mean” temperature difference. 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%)

SYSTEM DESIGN

Figure 45: Stall Chart

235 180 140 105

400 380 360 340 320 300 280 260 240 220 200 180 160 140 120 100

75 55 34 20 10

Inches Vacuum Pressure psig

3 0 5"

10" 15" 20"

Temperature °F

25"

80 60 40 20 0

100 90 80 70 60 50 40 30 20 10 0

Percentage Load

33