Question

The chilled water system for a 27-story building has a pump located at ground level. The lost head in a vertical riser from the pump to an equipment room on the twenty-seventhfloor is 40ftof water, and the pump produces 270ft of head. What is the pressure on the suction side of the pump for a pressure of 8 psig to exist in the riser on the twenty-fifth floor

Answer 1

This question is incomplete, the complete question is;

The chilled water system for a 27-story building has a pump located at ground level. The lost head in a vertical riser from the pump to an equipment room on the twenty-seventh floor is 40ft of water, and the pump produces 270ft of head. What is the pressure on the suction side of the pump for a pressure of 8 psig to exist in the riser on the twenty-fifth floor

Assume 12ft of elevation per floor

Answer: 48.68 psig

Explanation:

First  we calculate the elevation of the building

hb = 27 story * 12ft per floor/story

hb =  324 ft

given that the head lost in the vertical riser hL = 40 ft

now the delivery head required in the riser on he 27th floor;

hd = 8 psig *  (2.31 ft / 1 psig)

hd = 18.46 ft

Now calculate the suction head required by balancing the energy per unit weight of water, considering pump as the control volume

hp = (hb + hL + hd) - hs

hs = hb + hL + hd - hp

where hp is the head developed by the pump (270 ft)

hb is the elevation of the 27th floor of the building ( 324 ft)

hL is the head lost in the vertical riser ( 40 ft)

hd is the head required to exist in the riser on the 27th floor (18.46 ft)

so we substitute

hs = 324 ft + 40 ft + 18.46 ft - 270 ft

hs = 112.46

so 112.46ft * (1 psig / 2.31 ft)

= 48.68 psig