Page 222 - Handbook of Energy Engineering Calculations

Locate the surge tank so that it is the highest point in the cooling system.
Some engineers recommend that the bottom of the surge tank be at least 10 ft
(3 m) above the pump centerline and connected as close as possible to the
pump intake. A 1½- or 2-in (38.1- or 50.8-mm) pipe is usually large enough
for connecting the surge tank to the system. The line should be sized so that

the head loss of the vented fluid flowing back to the pump suction will be
negligible.

6. Determine the pump net positive suction head
The pump characteristic curve, Fig. 9, shows the net positive suction head

(NSPH) required by this pump. As the pump discharge rate increases, so does
the NPSH. This is typical of a centrifugal pump.
The greatest flow, 330 gal/min (20.8 L/s), occurs in this system when all
the coolant is diverted through the bypass circuit, Figs. 4 and 5. At a 330-

gal/min (20.8-L/s) flow rate through the system, the required NPSH for this
pump is 8 ft (2.4 m), Fig. 9. This value is found at the intersection of the 330-
gal/min (20.8-L/s) ordinate and the NPSH curve.
Compute the existing NPSH, ft (m), from NPSH = H − H + 2.31(P −
s
s
f
P )/s, where H , = height of minimum surge-tank liquid level above the pump
v
s
centerline, ft (m); H = friction loss in the suction line from the surge-tank
f
connection to the pump inlet flange, ft (m) of liquid; P = pressure in surge
s
2
tank, or atmospheric pressure at the elevation of the installation, lb/in (abs)
(kPa); P = vapor pressure of the coolant at the pumping temperature, lb/in 2
v
(abs) (kPa); s = specific gravity of the coolant at the pumping temperature.

7. Determine the operating temperature with a closed surge tank

A pressure cap on the surge tank, or a radiator, will permit operation at
temperatures above the atmospheric boiling point of the coolant. At a 500-ft
(152.4-m) elevation, water boils at 210°F (98.9°C). Thus, without a closed
surge tank fitted with a pressure cap, the maximum operating temperature of
a water-cooled system would be about 200°F (93.3°C).

2
If a 7-lb/in (gage) (48.3-kPa) pressure cap were used at the 500-ft (152.4-
m) elevation, then the pressure in the vapor space of the surge tank could rise
2
to P = 14.4 + 7.0 = 21.4 lb/in (abs) (147.5 kPa). The steam tables show that
s
water at this pressure boils at 232°F (111.1°C). Checking the NPSH at this