2000 Hook-up Book

Draining Temperature Controlled Steam Equipment

conditions, a vacuum breaker and hydraulic pressure due to conden sate will prevent stall and allow the trap to drain the coil. Step 4. In many systems, the trap does not discharge freely to atmosphere and in our example, total back pressure on the trap is 15 psig, drawn as horizontal dot ted line (P 2 ). Coil pressure equals back pressure at the intersection of (P 2 ) with (P 1 /MT 1 ) which when dropped vertically downward to (R 1 ) occurs at 93% load. At less than this load, the required trap differential is eliminated, the sys tem “stalls,” and the coil begins to waterlog. In our air heating coil the air flows at a constant rate and extending the air tempera ture intersection horizontally to (R 2 ), stall occurs when the incom ing air is 6°F or more. The same procedure applies to a heat exchanger although the example temperature is not a common one. If the stall chart

example represented a heat exchanger where the liquid was to be heated through a constant tem perature rise from 0 to 80°F, but at a flow rate that varies, stall would still occur below 93% load. In this instance, if 100% load represents a 50 GPM exchanger, the system would stall when the demand was 46.5 GPM (50 x .93) or less. Draining Equipment Under “Stall” Conditions “System stall” is lack of positive differential across the steam trap and temperature controlled equipment will always be subject to this problem when the trap must operate against back pres sure. Under these conditions, a vacuum breaker is ineffective because “stall” always occurs above atmospheric pressure. Even when steam is supplied at a constant pressure or flow to “batch” type equipment, stall can occur for some period of time on startup when the steam condens es quickly and the pressure drops below the required differential. What happens when the sys tem stalls is that the effective coil area (“UA” in the formula) drops as the steam chamber floods and heat transfer is reduced until the control valve responds to deliver an excessive supply of steam to the coil. This results in a “hunting system” with fluctuating tempera tures and hammering coils as the relatively cooler condensate alter nately backs up, then at least some portion is forced through the trap. The solution to all system stall problems is to make condensate drain by gravity. Atmospheric sys tems tend to operate more predictably and are generally eas ier to control but major heating equipment is usually not drained into an atmospheric return because of the large amount of energy that is lost from the vent. In many process plants, venting vapors of any type is discouraged and a “closed loop” system is not only required but is less subject to oxygen corrosion problems.

An example plot is shown on Fig. 46 for a coil where air is heat ed to 80°F and the trap must discharge against back pressure. Step 1. The system is designed for 100% load when air enters at 0°F (T 1 ) and there is 0% load when air enters at 80°F (T 2 ). Draw line (T 1 /T2) connecting these points. Step 2. At maximum load, the arithmetic mean air temperature (MT) is 40°F. Locate (MT) on line (T 1 /T 2 ), extend horizontally to 0% load, and identify as (MT 1 ). Step 3. Allowing for pressure drop, the control valve has been sized to supply 25 psig steam to the coil at 100% load. This pres sure is (P 1 ) and has a steam temperature of 267°F. Mark (P 1 ) and draw line (P 1 /MT 1 ). Line (P 1 /MT 1 ) approximates the steam supply at any load condi tion and the coil pressure is below atmospheric when it drops below the heavy line at 212°F. In a grav ity system with sub-atmospheric

SYSTEM DESIGN

Figure 46: Air Make-up Coil 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

P 1

Inches Vacuum Pressure psig

P 2

3 0 5"

10" 15" 20" 25"

Temperature °F

Design MTD

80 60 40 20 0

Stall MTD

T 2

MT 1

MT

R 2

T 1

100 90 80 70 60 50 40 30 20 10 0

Percentage Load

R 1

34