# Construction

Service Plant
1a.
State the two main groups of compressors in industry and explain the differences between them with respect to method of compression, compression ratio and capacity.
Quote an example from each group.
1b.
Discuss the factors which affect the choice of a compressor type for particular applications.
1c.
Select a suitable compressor for each of the following application and state whether single or multi-stage is indicated. The below figure shows the broad pressure and capacity ranges over which the various types of compressor normally operate.

1ci
A capacity of 1000m3 h-1 at a pressure of 9bar at a steady non pulsating flowrate
1cii.
A capacity of 10 000m3 h-1 at a pressure of 100 bar.
1ciii.
A capacity of 100m3h-1 at 100 bar.
2a.
Using simple sketches describe the construction and operating principles of:

i.                     Reciprocating compressors
ii.                   Centrifugal compressors

2b.
Explain the meaning and importance of the phrase ‘multi-stage compression with interstage cooling’
2c.
A three-stage compressor takes air at a pressure of 0.96 bar and delivers it at 12 bar. Assuming the intermediate pressures are in geometric progression find the two intermediate pressures. Take the gas constant R for air as 0.287kJ kg-1 K-1
2d.
Discuss 4 examples of the use of compressed air in industry.
3a.
Briefly explain the functions of the following components which make up a steam production and power plant system:

i.                     Generator
ii.                   Condenser
iii.                 Turbine
iv.                 Superheater
v.                   Steam Drum
vi.                 Boiler
vii.               Economiser
viii.             Combustion air pre-heater
ix.                  Feedwater pumps
x.                    De aerator
xi.                  Water treatment plant
3b.
Construct a simple block diagram to show these major components of a steam production and power plant system in their correct sequence.
3c.
A surface condenser is to be designed to condense 150 000kg h-1 of dry saturated steam at 0.05 bar. If the overall heat transfer coefficient is 11.5kW m-2 K-1 and the inlet and outlet water temperatures are 14deg C and 24 deg C respectively, calculate the heat transfer surface are required.
4a.
Explain the differences between fibre-tube boilers and water-tube boilers with respect to the following:

§  Construction
§  Operation
§  Main mechanisms of heat transfer
§  Properties and uses of steam produced
4b.
Discuss the major factors which affect the choice of boiler type for particular applications
4c.
A boiler produces superheated steam at 40 bar and 350°C from feedwater at 30°C. The higher heating value (calorific value) of the fuel is 45 MJ kg–1 and the heat from each kg of fuel generates 13.1 kg of steam. Calculate the boiler efficiency based on the higher calorific value of the fuel
5a.
Write brief notes to explain the function and importance of the following with respect to steam distribution systems:

i.                     Thermal expansion devices
ii.                   Air vents
iii.                 Steam traps
iv.                 Pressure reducing valves
v.                   Pressure release valves
vi.                 Insulation (lagging)
5b.
Explain why it is important to remove water from steam distribution systems and describe two ways in which it may be achieved.
5c.
It is proposed that an existing pipeline which has a nominal diameter of 100mm will be used to return condensate from a new piece of process plant. Operating considerations dictate that the pressure in the pipeline must not fall below 2 bar and the condensate velocity must not exceed 30ms-1. If the steam pressure in the process plant is to be 8 bar what would the maximum condensate flow rate in the return line be?
6a.
Explain the function of the following with respect to the production and distribution of compressed air:

i.                     Air intake filter
ii.                   Air silencer
iii.                 Air compressor
iv.                 Intercoolers
v.                   Aftercoolers
vii.               Ring Main
viii.             Auto drain trap
6b.
Draw a block diagram to represent the essential stages in the production and distribution of compressed air to users.

6c.
Compressed air is delivered through a pipe which is 60m long and has a nominal bore of 80mm. If the air is at 5 bar and 20 deg C and experiences a drop in pressure of 4000Nm-2 as it flows through the pipe find its mass flow rate.  Take the friction factor as 0.007 and the gas constant for air as 0.287kJ kg-1 K-1.
7a.
Explain the function of the following with respect to refrigeration using the vapour compression cycle:

i.                     Compressor
ii.                   Oil separator/filter
iii.                 Cooler
iv.                 Expansion valve (throttle valve)
v.                   Refrigerator/evaporator
vi.                 Connecting pipework
7b.
Draw a block diagram to represent the essential stages (in the order they occur) in the vapour compression cycle. Indicate where energy enters and leaves the cycle.
7c.
State two industrially important refrigerants used in industry, stating one advantage and one disadvantage for each.
7d.
Suggest two signs which would indicate that a vapour compression refrigerator has an insufficient charge of refrigerant.
7e.
Explain the operation and uses of heat pumps.
8.
A vapour compression refrigerator, with ammonia as refrigerant, works between 2.908 bar and 12.37 bar. The temperature after isentropic compression is 82degC and the condensate is undercooled by 8degC in the condenser. Determine the coefficient of performance of the cycle.
9.
Suggest a suitable pressure at which dry saturated steam should be supplied to a calorifier (heater) to maintain a chemical process at 140degC and calculate how much steam would be condensed to provide heat at the rate of 7000MJh-1. Allow a temperature difference of 25degC between the steam and the process fluid for effective heat transfer.
10.
What two features of the planned drainage provisions for the vessels A,B,C are incorrect from the figure below?