Pneumatic and hydraulic equipment

Q1. (a) Draw a pV diagram to illustrate the operation of a two-stage compressor with inter-cooling.

(b) List the advantages to be gained by the use of a multi-stage design compressor.

Q2. (a) With the aid of a sketch., describe briefly the operating principle of a rotary vane compressor.

(b) State the reasons why oil must be injected into this machine for its efficient operation.

Q3. (a) A mass of 400 kg is to be raised by the actuation of two identical hydraulic cylinders with a piston diameter of 120 mm. Calculate the required system pressure to just raise the load.

(b) If the load is to be raised 600 mm in 10 seconds, what will be the required flow-rate (Q) in I min^-1?

Q4. Describe the operation of a variable displacement axial piston (swash plate) pump, indicating the means by which its output is varied.

Q5. With reference to basic constructional features, contrast the methods of compression employed by positive displacement and dynamic air compressors.

Q6. An item of plant requires 2 m³ min^-1 of pulsation-free and oil-free compressed air supplied at a pressure of 7 bars. Select and size a suitable type of machine with regard to output FAD (free air delivered) and quality of air supply.

Q7. Air is drawn into a compressor at normal temperature and pressure (N.T.P.) mid compressed to a pressure of 6 bar gauge. After compression the air is delivered at 1.2 M³ min^-1 and cooled to a temperature of 30°C, at which point condensate is collected at the rate of 2 litres per hour. Estimate the FAD (N.T.P.) of the compressor, and the relative humidity of the air entering the compressor.

Q8. Describe the difference between regenerative absorption and chemical absorption air drying.

Fluid power diagrams

1. FIGURE 1 shows a pneumatic circuit diagram used for air supply and exhaust in an automatic control system.

(a) Name all symbols (including pipelines) used in the system.

(b) Number the ports of each DCV.

(c) Classify the sections of the components in the circuit.

(d) State how the system works.

(e) State the functions of valves 1 and 2.

Q2. FIGURE 2 shows a pneumatic circuit in which four actuators are controlled.

(a) State the sequence in which the cylinders operate on the operation of the start valve.

(b) Show, with the aid of a schematic diagram, how an electronic controller could be included in the circuit to control the sequence of cylinder operations.

Specification, selection & maintenance

Q1. Using the nomogram and charts (FIGURES 1 and 2 on pages 4 and 5, determine the diameter of a. suitable air main for the distribution of 6 m3 min-i FAD. System pressure is 6 bars, the length of the pipe run is 175 metres, and a maximum pressure drop of 0.3 bars is allowed.

The distribution main must also include the following:

• 4 bends (r = 2d)
• 2 elbow fittings
• 6 tee connectors
• 2 diaphragm v aloes

Q2. Describe the relevant advantages and disadvantages of using steel, copper or plastic pipe for a distribution main.

Q3. What problems may occur if a pipe is specified:

• larger
• smaller

than the optimum value?

Q4. A compressor delivers 300 l s^-1 of free air into a pipe at a pressure of 6 bar gauge. Using the pressure drop formula:

Pressure drop = 800lQ²/Rd^5.31

Calculate the minimum diameter of pipe if the pressure drop in a system is to be limited to 0.3 bar when is delivered through a pipe of equivalent length 160 m.

5. Explain the difference in operation between a 'macro' and a 'micro' oil mist lubricator. State where you would use a 'micro' in preference to a 'macro' lubricator.

6. List four possible causes of overheating on a multi-stage reciprocating compressor.

7. Produce a system monitoring chart that could be used by a machine operator to maintain a daily record of hydraulic system performance details.

8. State the precautions to be taken when changing the fluid in a hydraulic system from a mineral oil based hydraulic fluid to a tire resistant fluid.