Efficient Capacity Control in Reciprocating Compressor
Puneet Kumar, Associate Design Engineer II, Mechanical Fluor Daniel India Pvt Ltd

This article delves on the basic differences and advantages of all the capacity control system in reciprocating compressors enabling the customers to use optimum reciprocating capacity control system for speed, energy savings and accuracy.

Compressor capacity control is the method by which the quantity of gas supplied by the compressor is regulated to provide, as efficiently as possible, only that amount of air or gas required at a particular instant. The ideal governing arrangement would be to obtain such infinite control that the compressor capacity would be exactly equal to the fluctuating demand of the plant at all times.

It is of paramount importance that the control system should be such that the compressor power is almost proportional to the quantity of the gas delivered.

Introduction
Compressors, as a part of process plants, refrigeration plants, oil & gas refinery facilities, etc are required to have regulated output for different plant conditions. For the efficient solution of the capacity control problem, certain points must be taken in to consideration at the planning stage of compressor plant so that maximum saving in power consumption is achieved with minimum initial cost and without impairing the safe running of the compressor plant.

Capacity regulation can be divided into three basic modes:
1. Stepped Control
2. Step less Control
3. Composite Control

These will be examined from point of view of the plant requirement, efficiency, and construction and control systems so as to enable their efficient use for a particular application.

Stepped Capacity Control
In this method, the capacity is regulated in steps. For example, three steps control means that the compressors can deliver 100 per cent, 50 per cent & 0 per cent of its capacity. Because of its simplicity and reliability in use, this method is the most common choice.

Stop-Start Control: Start/stop is the most efficient control mode for small compressors. The compressors in this process shut itself down when the demand is low & automatically restarts on an increase of air requirement. This is advantageous in case where demand for compressed gas fluctuates widely, particularly, when there are sudden large demands followed by periods of very small demand or no demand at all. This method provides completely no loss capacity control and thus almost 100 per cent energy is saved. This system can also be used in large installation when there are several units working in parallel. When an increase in output is required, one or more unit is brought into operation. (The article continues on page number 54)

Since large compressors can seldom be started other than off-load, bringing in compressor does has some difficulties and these also possess limitation on this use.

Suction Valve Unloader Control: In this system, the valve lifting device keeps the suction valves open during the compression stroke. By keeping these valves open by means of valve lifter, the cylinder capacity is zero during this stage. And when the valve lifter withdraws, suction valve again comes to normal position and capacity rises to maximum. (See figure 1)

This method is generally used in all process industries.

From the point of view of efficiency, the power reduction is almost directly proportional to the capacity reduction. If two double acting cylinders are available, capacity regulation of 100 per cent, 75 per cent, 50 per cent, 25 per cent & 0 per centcan be achieved.

Fixed Volume Clearance Pocket Control: A FVCP is used to add additional clearance volume to the head end of a cylinder. The clearance pocket, when open, increases the cylinder clearance volume, which causes the compressor capacity to decrease. An indicator card taken from a cylinder end with a 50 per cent capacity reduction (see figure 2) shows this effect more clearly. From the point of view of efficiency, the power reduction is almost directly proportional to the capacity reduction if allowance is made for losses due to throttling through the valves. (See figure 2)

Stepless Capacity Control
This method allows the continuous regulation of the capacity of the compressor. The following methods of Stepless Capacity Control are widely used.

Speed Control: It is the simplest method of controlling the capacity of the compressor. If the speed of the driver can be varied, this will give an excellent means of loss free capacity control of compressor. The diesel engines, gas engines, seam engine, steam turbine and gas turbine are best suited to speed control.

It must be noted that while in reciprocating compressors, the torque is independent of the speed (process condition constant), these are generally not true for those prime movers whose speed can be varied. This limits these prime movers to not allow speed reduction of more than 70 per cent, thus imposing the range of continuous capacity variation between 100 per cent to 70 per cent only.

Reverse Flow Control: Stepless capacity control by Reverse flow through suction valves is one of the most efficient means of capacity control while operating the Compressor with a constant speed driver in this method, Capacity control is achieved by unloading device, fitted to each suction valve, which delays the closing of the suction valve in a controlled way during compression stroke. Thus gas already filled in the cylinder will flow back into the suction piping. The amount being proportional to the length of the compression stroke during which the suction valves remain open. With the help of this method, it becomes possible to vary the compressor capacity continuously. From the point of view of efficiency, the power reduction is almost directly proportional to the capacity reduction apart from the losses due to the flow of gas in both the directions through the suction valves.

The only problem with this system was that of actuating the inlet valves, since that must match the speed of the compressors. And it has been overcome by Hoerbiger_Hydrocom. This utilises the principle that a dynamic pressure builds up in front of a body placed in flow of gas, directly proportional to the square of gas velocity. The disc of the suction valve acts like this body.

With the help of Hydrocom, we can achieve regulation to any desired capacity between 30 per cent and 100 per cent of total load, and in some special cases capacity reduction down to 15 per cent also.

How HydroCOM works: At full load gas is compressed during the entire compression stroke. At part load HydroCOM holds the suction valve open at the start of the compression stroke and automatically closes the valve when the desired volume of gas is in the cylinder is reached. Only the gas required is compressed to discharge pressure. (See figure 3)

By-Pass Control: Stepless control by using a by-pass from discharge to suction is one of the most common and easy means of start-up and stepless capacity control. This control is based on the principle of connecting the discharge side of the compressor with the suction side via a throttle valve.

However, by-pass control is a 100 per cent loss system. This method involves wastage of power and as such should be used preferably in combination with other means of stepped control, such as, valve unloaders and clearance pockets.

By-pass Control may also involve a by-pass cooler to ensure that suction temperature does not go beyond acceptable limits.

Composite Capacity Control
It is clear from the preceding that each type of control method has got its limitations. Therefore, best results are achieved by combining two or more methods so that each type of control can be used most effectively to get maximum advantage from the point of view of reliability, absorbed power reduction, simplicity in operation and initial installation costs. Of course, now-days, with ever increasing energy costs, initial extra investments are not of much relevance as they are paid back by energy savings in the very first year of compressor operation. Some of the combinations in practice are:

  1. Valve unloaders with reverse flow control along with fixed or continuous clearance pocket control.
  2. Valve unloaders with start-stop control along with the reverse flow control.
  3. Valve unloaders with speed control.
  4. Valve unloaders with fixed or continuous clearance pocket along with By-pass control.
References:
  1. Compressors: Selection & Sizing, 3rd edition (2005), Royce Brown, Elsevier
  2. Compressor and Modern Process Application (P. Hentz)
  3. Reciprocating Compressors-Operation & maintenance-Heinz P. Bloch