Question 10.12: Referring to Example 10-11, a water heater at the end of the......
Referring to Example 10-11, a water heater at the end of the longest run has a capacity of 50,000 Btu/hr. Condensate flows into a vented dry return that slopes \frac{1}{8} in./ft. Size the steam and condensate line.
The enthalpy of vaporization for the steam is about 960 Btu/lbm from Table A-1a. Then
\dot{m}_{\text{v}} = \dot{q}/i_{fg} = 50,000/960 = 52 lbv/hr = \dot{m}_{c}
Using Fig. 10-46 with a system steam pressure of 5 psig, ∆P/L of 0.2 psi/100 ft, and mass flow rate of 52 lb/hr, the pipe size is between 1 and 1\frac{1}{4} in. and the velocity based on 0 psig is 2400 ft/min. The velocity at 0 psig may be converted to the velocity at 5 psig using Fig. 10-47 to obtain 2200 ft/min. This velocity is a little high; therefore, use the 1 \frac{1}{4} in. pipe, which will have a lower velocity and pressure loss. The dry condensate return for this case may be sized by referring to Table 10-6a. With a line slope of \frac{1}{8} in./ft and flow rate of 52 lb/hr, select the nominal \frac{1}{2} in. pipe, which is rated at 54 lb/hr.
| Table 10-6a Vented Dry Condensate Return for Gravity Flow Based on Manning Equation | ||||
|
Nominal Pipe Diameter, in. |
Condensate Flow, lbm/hr,^{a} at Condensate Line Slope, in./ft |
|||
| \frac{1}{16} | \frac{1}{8} | \frac{1}{4} | \frac{1}{2} | |
| \frac{1}{2} | 38 | 54 | 76 | 107 |
| \frac{3}{4} | 80 | 114 | 161 | 227 |
| 1 | 153 | 216 | 306 | 432 |
| 1 \frac{1}{4} | 318 | 449 | 635 | 898 |
| 1 \frac{1}{2} | 479 | 677 | 958 | 1360 |
| 2 | 932 | 1320 | 1860 | 2640 |
| 2 \frac{1}{2} | 1500 | 2120 | 3000 | 4240 |
| 3 | 2670 | 3780 | 5350 | 7560 |
| 4 | 5520 | 7800 | 11,00 | 15,600 |
| 5 | 10,100 | 14,300 | 20,200 | 28,500 |
| 6 | 16,500 | 23,300 | 32,900 | 46,500 |
| ^{a} Flow is in lb/hr of 180 F water for schedule 40 steel pipes.
Source: Reprinted by permission from ASHRAE Handbook, Fundamentals Volume, 1997. |
||||
