Page 32 - Coulson Richardson's Chemical Engineering Vol.6 Chemical Engineering Design 4th Edition

INTRODUCTION TO DESIGN
SI unit
British
Quantity Table 1.1. Approximate conversion units 15
Eng. unit approx. exact
Energy 1 Btu 1 kJ 1.05506
Specific enthalpy 1 Btu/lb 2 kJ/kg 2.326
Specific heat capacity 1 Btu/lb ° F 4 kJ/kg ° C 4.1868
(CHU/lb ° C)
2
2
Heat transfer coeff. 1 Btu/ft h ° F 6 W/m ° C 5.678
2
(CHU/ft h ° C)
Viscosity 1 centipoise 1 mNs/m 2 1.000
1lb f /ft h 0.4 mNs/m 2 0.4134
Surface tension 1 dyne/cm 1 mN/m 1.000
Pressure 1 lb f /in 2 7kN/m 2 6.894
1 atm 1 bar 1.01325
5
10 N/m 2
Density 1 lb/ft 3 16 kg/m 3 16.0190
1g/cm 3 1 kg/m 3
Volume 1 imp gal. 4.5 ð 10 3 m 3 4.5461 ð 10 3
3
Flow-rate 1 imp gal/m 16 m /h 16.366
Note:
1 US gallon D 0.84 imperial gallons (UK)
3
1 barrel (oil) D 50 US gall ³ 0.19 m (exact 0.1893)
1kWh D 3.6MJ
American catalogue in US gallons or gpm (gallons per minute) will have only 80 per cent
of the rated capacity when measured in imperial gallons.
The electrical supply frequency in these two countries is also different: 60 Hz in the US
and 50 Hz in the UK. So a pump specified as 50 gpm (US gallons), running at 1750 rpm
(revolutions per second) in the US would only deliver 35 imp gpm if operated in the UK;
where the motor speed would be reduced to 1460 rpm: so beware.

1.9. DEGREES OF FREEDOM AND DESIGN VARIABLES.
THE MATHEMATICAL REPRESENTATION OF
THE DESIGN PROBLEM
In Section 1.2 it was shown that the designer in seeking a solution to a design problem
works within the constraints inherent in the particular problem.
In this section the structure of design problems is examined by representing the general
design problem in a mathematical form.

1.9.1. Information flow and design variables

A process unit in a chemical process plant performs some operation on the inlet material
streams to produce the desired outlet streams. In the design of such a unit the design
calculations model the operation of the unit. A process unit and the design equations