Reciprocating compressors discharge

Motor-driven compressors usually operate at constant speed, and other methods of controlhng the capacity are necessary. On reciprocating compressors discharging into receivers, up to about 75 kW (100 np), two types of control are usually available. These are auto-matic-start-and-stop control and constant-speed control. 

The Steam Injection Cycle Steam injection has been used in reciprocating engines and gas turbines for a number of years. This cycle may be an answer to the present concern with pollution and higher efficiency. Corrosion problems are the major hurdle in such a system. The concept is simple and straightforward Steam is injected into the compressor discharge air and increases the mass flow rate... 

Compressors are usually high-cost items, but easily correlated by brake horsepower vs. S/horsepower. Variations in engine-driven reciprocating compressor prices can be caused by the type of driver, the speed (the slower the speed the more costly, but the more reliable), the total discharge pressure, and the size. 

The reciprocating compressor is generally in the lower flow end of the compressor spectrum. Inlet flows range from less than 100 to approximately 10,000 cfm per cylinder. It is particularly well-suited for high-pressure service. One of the highest pressure applications is at a discharge pressure of 40,000 psi. Above approximately a 1.5-to- i pressure ratio, the reciprocating compressor is one of the most efficient of all the compressors. 

For positive displacement compressors, discharge PR valves are nearly always required. Reliance on stalling of a reciprocating compressor is generally not economically attractive, since driver stalling pressures are usually quite high in comparison to operating pressure. 

Non-lubricated screw compressors have very close clearances and thus they are designed for limited ranges of discharge temperature, temperature rise, compression ratio, etc., all of which can cause changes in thesc clearances. Lubricated compressors have a somewhat broader tolerance to changes in operating conditions, but they are still more limited than reciprocating compressors. 

Reciprocating compressor capacity may easily be adjusted by changing compressor speed, changing compressor cylinder clearance, unloading compressor cylinder inlet valves, recycling gas from unit discharge to unit suction, or a combination of these methods. All these methods may be accomplished either manually by the operator or automatically by the control panel. 

Figure 11-21D. Helical rotors refrigerant compressors. (1) Cutaway of a 100-ton intermediate compressor. The intermediate Helirotor compressor has only three moving parts the two rotor assemblies and the capacity controlling slide valve. The general purpose Helirotor compressor has only four moving parts two rotor assemblies, the variable unloader valve, and the step unloader valve. Unlike reciprocating compressors, the Trane Helirotor compressor has no pistons, connecting rods, suction and discharge valves, or mechanical oil pump.

In many instances, the design of suction and discharge pulsation dampening drums (or bottles) for reciprocating compressors is based on piston displacement and volumetric efficiency, and this design normally will suffice to reduce peak pulsation to approximately 5% of the line pressure. In special or other cases, experience has shown that operational difficulties (vibrations, meter pulsations, etc.) may indicate that the peak pulse pressure of 5% line pressure is inadequate. Thus, the pressure in pulsation-reduction design selection is... 

A V-belt-driven, force-fed oil lubrication system is used on water-cooled compressors. Oil goes to both bearings and to several points in the cylinder. Ten times as much oil is recommended to lubricate the rotary cylinder as is required for the cylinder of a corresponding reciprocating compressor. The oil carried over with the gas to the line may be reduced 50 per cent with an oil separator on the discharge. Use of an aftercooler ahead of the separator permits removal of 85 to 90 per cent of the entrained oil. 

The only other variables that affect performance are the inlet-discharge valves, which control flow into and out of each cylinder. Although reciprocating compressors can use a variety of valve designs, it is crucial that the valves perform reliably. If they are damaged and fail to operate at the proper time or do not seal properly, overall compressor performance will be substantially reduced. 

All reciprocating compressors must be fitted with pressure relief devices to limit the discharge or inter-stage pressures to a safe maximum for the equipment served. 

Reciprocating Reciprocating pumps are more difficult to monitor because of the combined rotational and linear motions that are required to increase the discharge pressure. Measurement-point location and orientation should be based on the same logic as that of reciprocating compressors. 

The recommendations of the International Standards Organization (ISO) covering mineral-oil lubricants for reciprocating compressors are set out in ISO DP 6521, under the ISO-L-DAA and ISO-L-DAB classifications. These cover applications wherever air-discharge temperature are, respectively, below and above 160°C (329°F). For mineral-oil lubricants used in oil-flooded rotary-screw compressors the classifications ISO-L-DAG and DAH cover applications where temperatures are, respectively, below 100°C (212°F) and in the 100-110°C range. For more severe applications, where synthetic lubricants might be used, the ISO-L-DAC and DAJ specifications cover both reciprocating and oil-flooded rotary-screw requirements. 

Figure 4.1 Reciprocating compressor, (a) Suction stroke, (b) Discharge stroke...

Where multiple compressors are involved in the same service or duty, separation should be provided between compressors to reduce mutual exposure. Compressor buildings housing flammable service compressors should be provided with a combustible-gas detection system. The system should alarm at a concentration of 20-25% of the LFL and shutdown the compressor at 40-50% LFL (see Section 8.1.3). The shutdown should include closing all inlet and discharge process lines. For reciprocating compressors in flammable service, explosion vents on the crankcases should be provided. 

How about the compressor discharge pressure What is this controlled by Answer—the condenser. The compressor discharge pressure has nothing whatsoever to do with the compressor. And this statement is true regardless of whether the compressor is a reciprocating or centrifugal machine, or fixed- or variable-speed. It is the condenser temperature that determines the compressor discharge pressure. 

For instance, suppose we are working with a reciprocating compressor. This particular machine has a very small volumetric clearance and a very high volumetric efficiency (these mysterious terms are fully explained in Chap. 29, Reciprocating Compressors ). With this compressor, a significant decrease in discharge pressure will result in only a tiny increase in vapor flow. Therefore, the load on the driver will decrease. 

A reciprocating compressor is a direct volume-reduction machine. The gas is simply squeezed out of a cylinder by a piston and pushed into the discharge line. The molecular weight of the gas does not influence the suction or discharge pressure of the compressor. The gas density does not influence the compressor performance or the work required by the driver. 

The piston rod used in a reciprocating-compressor cylinder can fail if overstressed. The stress experienced by the piston rod is a direct function of the AP developed by the cylinder. That is because on one side of the piston, we have the suction pressure and on the other side of the piston, we have the discharge pressure. The AP times the area of the piston is therefore the force acting on the piston. Excessive force will break the piston rod. 

Figure 3.24. Control of positive displacement compressors, rotary and reciprocating, (a) Flow control with variable speed drives, (b) Pressure control with bypass to the suction of the compressor, (c) Reciprocating compressor. SC is a servomechanism that opens some of suction valves during discharge, thus permitting stepwise internal bypass. The clearance unloader is controllable similarly. These built-in devices may be supplemented with external bypass to smooth out pressure fluctuations.

Figure 3.24 shows control schemes for rotary and reciprocating compressors. Vacuum pumps are compressors operating between a low suction pressure and a fixed discharge pressure, usually... 

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