Graph of Pressure against volume in a reciprocating compressor
Volumetric efficiency Vh = Actual suction volume Vx/
Theoretical suction volume Vs
For greater efficiency air compression should be isothermal as this requires the minimum work input. In practice Isothermal compression is not possible, an ideal Isothermal cycle requires sufficient time to allow all the required heat to be transferred out of the cylinder, practicality dictates that the piston must have a relatively high speed to give a reasonable output,
Cylinder cooling on a single stage compressor gives better efficiency but there is a limitation in the surface area to cylinder volume that can be used for cooling effect, but multistage compressors with an efficient extended surface interstage cooler gives cycle improved compression efficiency better approaching that of the isothermal. In theory the greater the number of stages the closer the curve will approach the ideal isothermal compression curve, however there is an increase in cost, complexity, and the law of diminishing returns limit the number.
Compression in stages has the following advantages;
The compression ratio at each stage is lower and so the final temperature is lower. This reduces problems with lubrication
. The machine is smaller and better balanced
water can be drained off at each stage
Compression better approaches the ideal isothermal
It is important that the compressor clearance volume is kept small as possible in order to improve overall volumetric efficiency as the air trapped in this space must expand to below suction pressure before new air can enter, this is an effective loss of stroke.
A clearance is required in order to prevent the piston striking the cylinder cover when starting or stopping off load. The clearance volume is sometimes referred to as the 'Bump Clearance'.
Lubrication of the crankcase in a compressor does not pose any specific problems and normally consist of splash lubrication with pressurised oil being fed to shell bearings. Where drip cylinder lubrication is used, this should be kept to a minimum conducive with liner wear. A standard mineral oil similar to that used in the main engine may be used, although due to carbon deposits, higher quality oils are generally used with the most effective being specifically designed synthetics which have allow a considerable reduction in maintenance but are costly.
Mineral oils contain a blend of lighter elements such as paraffin's, and heavier elements such as asphaltenes. During compression the lighter elements are vaporised leaving the heavy ends, these coat the piston rings and discharge valves in combination with oxidised oil deposits. These deposits also coat passage ways and coolers resulting in higher interstage air temperatures. Deposits on discharge valves cause them to become sticky and leak resulting in hot air being drawn back into the cylinder for recompression. This increases the temperature and hence causes greater oxidation and deposits, and so the condition deteriorates with increasing rapidity.
Temperature can become very high, this may result in oily deposits at the discharge valves carbonising. Eventually this carbon could glow red and cause detonation. It is more likely, however, that oily deposits will be carried over to the air receiver and air start manifold to be ignited by blowpast at the cylinder air start valve.
Deposits at piston rings cause leakage allowing oil to enter the cylinder from the crankcase thus increasing the danger it is essential that crankcase lubrication be kept to a minimum compatible with an acceptable wear rate. Regular maintenance will minimise oily deposits build up and hence the risk of explosion
Materials and design of a reciprocating compressor
The compressor casing, cylinder covers and piston rings are generally of cast iron. Pistons may be of cast iron, steel of aluminium. Aluminium being the preferred material for use on the LP piston due to its larger diameter. Valves are usually made so that parts can be interchanged between the suction and discharge valves. Seats are of mild steel with small diameter air passages to prevent the fragments of broken valve plate from entering the cylinder. Valve plates are of vanadium steel heat treated and ground to provided the required hardness and surface finish. Springs should be arranged such that they lift and seat squarely. Uneven spring force or deposits on the seat cause valves to bend resulting in fatigue cracking.
For compressors designed for starting air requirements a water jacket relief valve is fitted.
The rotary compressor may be of the impeller type similar to that used in the turbocharger , scroll, twin rotating lobes or of the sliding vane type similar to the one shown above. In practice there would be several more vanes than shown.
Rotary compressors are capable of handling large quantities of low pressure air much more efficiently than a reciprocating compressor. In order to produce increased pressures it is possible to stage rotary compressors but leakage problems increase at higher pressures as well as stress on the vanes.
The sliding vane compressor consists of a slotted rotor with its axis offset from that of the cylindrical casing. Vanes fit in the slots and have contact with the casing
On the suction side the space contained between the casing, the rotor and an adjacent pair of vanes is gradually increasing allowing air to be drawn in.
On the compression side this same space is gradually reduced causing the pressure increase. When the leading vane uncovers the discharge port air will flow to outlet. Larger compressors of this type are water-cooled, smaller compressors tend to be air cooled.
The main problems related with sliding vane compressors concern wear at the vane tips and sealing of the ends
Rotary compressors in general do not require internal lubrication but they are not suitable alone for providing air at a pressure for starting duties. They can, however, be linked to reciprocating stages to produce a hybrid compressor.
The compressor is lighter, more compact and better balanced than an equivalent all reciprocating unit. In basic terms the rotary first stage supplies air to the reciprocating second and subsequent stages. All stages being driven by the same shaft
Cast iron-Casing, Liners, Pistons( the LP piston is sometimes made from an aluminium alloy, Cylinder covers Steel- Crankshaft, Conrods, Pistons, Valve seats Vanadium Steel- Valve plates
Starting air compressor circuit
Starting and stopping sequence is adjustable, the magnetic valves are open when the compressor is stopped so any residual pressure is blown off. On starting the magnetic valve are sometimes delayed to close so as to allow the compressor motor to reach full speed before the compressor is loaded up.
The non-return valves prevent HP air leaking back from the receiver on which the filling is also of the non return type.
Calculation of required cylinder compression for a multistage reciprocating compressor
r = stage pressure ratio
R = compression pressure ratio
for a two stage
r = R1/2
for a three stage
r = R1/3
for example, a 3 stage compressor requiring a final pressure of 64bar would have
the following interstage pressures 1st stage 1bar compressed to 4bar
2nd stage 4bar compressed to 16bar
3rd stage 16bar compressed to 64bar
It would appear that most of the work is being carried out in the final stage, this is untrue as with the increase in pressure is a complimentary reduction in volume, if the temperature conditions remain the same then work will be equally divided between the stages.
By reducing the suction pressure, the cylinder is required to do more work on the air before the discharge valve opens. This means that the air will be delivered at a higher pressure. The higher temperature can lead to problems with the cylinder lubrication as well as a drop in efficiency as well as carbonising of the oil and increased deposits on the valves and piston rings and interstage passages. In the extreme it can lead to seizure and possible diesel detonation of the oil laden air.
The reduction in pressure at the suction can be due to a partially blocked suction filter or partially choked suction valve. The lower pressure conditions in the cylinder at the start of compression can cause oil laden air to be drawn from the crankcase up the liner. This oil can lead to increased deposits in the compressor as well as further downstream in the distribution system
(P1.V1)/ T1 = (P2. V2)/T2
(P1/P2).(T2/T1) = (V2/V1)
P1.V1g = P2.V2g
P1/P2 = V2g/V1g From these we get;
T2 = T1. (P2/P1) (g -1)/g
g = 1.4
and if we take for and example
P1 = 0.4 bar
P2 = 1 bar
Pf = 5 bar
Tinitial = 300 K we end with final temperatures for the two compression's of
T1= 617 K and T2 = 475 K
from the graph it can be clearly seen that losses due to the bump clearance has increased and the period of constant pressure delivery has been reduced.