Temperature Rise and Minimum Flow

Temperature rise in average pump during operation 

ATr = temperature rise, °F/min Pso = brake horsepower at shutoff or no flow W, = weight of liquid in pump, lbs cp = specific heat of liquid in pump 

Applied Process Design for Chemical and Petrochemical Plants 

The validity of the method has not been completely established, although it has been used rather widely in setting approximate values for proper operation [10]. For multistage pumps use only the head per stage in temperature limit by this method. 

Hso = head at no flow or shutoff, ft cp = specific heat of liquid, BTU/lb/°F ATr = temperature rise in liquid, °F 

A pump may overheat if it is required to operate at very low or zero flow for any significant period. Overheated pumps can create serious suction problems, mechanical problems, and safety problems. The time a pump may operate at zero flow without overheating should be established. The minimum safe flowrate for continuous heat removal during extended operation at low or zero flow should be determined. 

If heat removal problems cannot be avoided in the pump system, circulation through an external cooler may be required. 

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Compute the temperature rise for the operating NPSH. An NPSH of 18.8 ft is equivalent to a pressure of 18.8(0.433)(0.995) = 7.78 psia at 220°F, where the factor 0.433 converts feet of water to pounds per square inch. At 220°F, the vapor pressure of the water is 17.19 psia, from the steam tables. Thus the total vapor pressure the water can develop before flashing occurs equals NPSH pressure + vapor pressure at operating temperature = 7.78 + 17.19 = 24.97 psia. Enter the steam tables at this pressure and read the corresponding temperature as 240°F. The allowable temperature rise of the water is then 240 — 220 = 20°F. Using the safe-flow relation of step 2, the minimum safe flow is 62.9 gal/min (0.00397 m3/s). 

We noted earlier in this chapter that many reactions in the chemical industries are exothermic and require heat removal. A simple way of meeting this objective is to design an adiabatic reactor. The reaction heat is then automatically exported with the hot exit stream. No control system is required, making this a preferred way of designing the process. However, adiabatic operation may not always be feasible. In plug-flow systems the exit temperature may be too hot due to a minimum inlet temperature and the adiabatic temperature rise. Systems with baekmixing suffer from other problems in that they face the awkward possibilities of multiplicity and open-loop instability. The net result is that we need external cooling on many industrial reactors. This also carries with it a control system to ensure that the correct amount of heat is removed at all times. 

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