Question 10.13: Suppose that at some point the vented dry returns feed into ......

Heating, Ventilating and Air Conditioning [987579]

Suppose that at some point the vented dry returns feed into a vented wet return, which drops 3 ft into a condensate return tank. The estimated total equivalent length (pipe plus fittings) is 120 ft. If the mass flow rate of the condensate is 9800 lbm/hr, what size pipe should be used?

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Table 10-7 applies to this situation. In this case the pressure drop is equal to the difference in elevation head between the condensate tank and the entrance to the wet return: 3 ft of head. Then the lost head per 100 ft is

∆P/L = 3(100/120) = 2.5 ft/100 ft

Referring to Table 10-7 at ∆P/L = 2.5 ft/100 ft, a 2 $\frac{1}{2}$ in. pipe can handle 9890 lbm/hr, which is close to the specified 9800 lbm/hr. Therefore, use 2 $\frac{1}{2}$ in. pipe.

Table 10-7a Vented Wet Condensate Return for Gravity Flow Based on Darcy–Weisbach Equation
Nominal Diameter, in. IPS	Condensate Flow, lb/hr, $^{a}$ at Condensate Head, ft per 100 ft
Nominal Diameter, in. IPS	0.5	1	1.5	2	2.5	3	3.5	4
$\frac{1}{2}$	105	154	192	224	252	278	302	324
$\frac{3}{4}$	225	328	408	476	536	590	640	687
1	432	628	779	908	1020	1120	1220	1310
1 $\frac{1}{4}$	901	1310	1620	1890	2120	2330	2530	2710
1 $\frac{1}{2}$	1360	1970	2440	2840	3190	3510	3800	4080
2	2650	3830	4740	5510	6180	6800	7360	7890
2 $\frac{1}{2}$	4260	6140	7580	8810	9890	10900	11,800	12,600
3	7570	10,900	13,500	15,600	17,500	19,300	20,900	22,300
4	15,500	22,300	27,600	32,000	35,900	39,400	42,600	45,600
5	28,200	40,500	49,900	57,900	64,900	71,300	77,100	82,600
6	45,800	65,600	80,900	93,800	105,000	115,000	125,000	134,000
$^{a}$ Flow is in lb/hr of 180 F water for schedule 40 steel pipes. Source: Reprinted by permission from ASHRAE Handbook, Fundamentals Volume, 1997.

Table 10-7b Vented Wet Condensate Return for Gravity Flow Based on Darcy–Weisbach Equation
Nominal Diameter, mm	Condensate Flow, g/s, $^a$ at Condensate Pressure, Pa/m
Nominal Diameter, mm	50	100	150	200	250	300	350	400
15	13	19	24	28	32	35	38	41
20	28	41	51	60	68	74	81	87
25	54	79	98	114	129	142	154	165
32	114	165	204	238	267	294	318	341
40	172	248	308	358	402	442	479	513
50	334	482	597	694	779	857	928	994
65	536	773	956	1110	1250	1370	1480	1590
80	954	1370	1700	1970	2210	2430	2630	2810
100	1960	2810	3470	4030	4520	4960	5379	5750
125	3560	5100	6290	7290	8180	8980	9720	10,400
150	5770	8270	10,200	11,800	13,200	14,500	15,700	16,800
$^{a}$ Flow is in g/s of 82 C water for Schedule 40 steel pipes.
Source: Reprinted by permission from ASHRAE Handbook. Fundamentals Volume, 1997.

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