Reciprocating compressors volumetric compressor

Because there is no clearance volume expansion, as in the reciprocating compressor, the volumetric efficiency is a function of the rotor slip. This is the internal leakage from the higher pressure to the lower pressure side, reducing potential volume capacity of the compressor. 

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... 

In general, only the reciprocating compressor allows for reliable flexibility in applying variable volumetric flowrate and variable pressure ratio in an operation. The rotary compressor does not allow for variation in either (except that of pressure through the decompression of the air or gas if the system back pressure is below the design pressure of the machine). The dynamic compressors are designed for specific volumetric flowrates and pressure ratios and are not very useful when these design limits are altered. 

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. 

Volumetric efficiency applies only to reciprocating compressors. A reduction in volumetric efficiency reduces the gas flow through the compressor. A reduction in volumetric efficiency need not reduce the adiabatic compression efficiency. 

A reduction in volumetric efficiency reduces the work required from the driver. For reciprocating compressors, we intentionally reduce the volumetric efficiency, to reduce the load on the driver. Sometimes this is done to save energy sometimes this is done to prevent a motor driver from tripping off on high amps. 

In the plant, we use the term unloading to indicate various ways of reducing the volumetric efficiency of a reciprocating compressor. There are two sorts of unloaders ... 

Equation (8.5) does not account for the volumetric efficiency (VE) loss of reciprocating-type compressors. Volumetric efficiency is simply the clearance allowed in the compressor cylinder head in which this compressed gas volume is allowed to mix with the inlet gas to be compressed in the next compression stroke. Thus this compressed gas in the cylinder clearance is recycling, which makes the output of the compressor less efficient, the larger this clearance volume. Volumetric efficiencies usually range fron 6 to 20%. An average reciprocating compressor VE should be 10%. For conservative cost calculating, use 15% of overall GHP for the VE efficiency. 

Reciprocating compressors (for air supply to air-diy a plant of small capacity) these are to be procured when substantially constant low volumetric capacity is required at higher discharge pressures. Vendor may be asked to indicate the type of capacity control being provided (variable frequency drive or by aity other means) as well as pulsation suppression device for the discharge side. This may be in the form of dampening pots. 

Reciprocating compressors may be limited by a third factor (in addition to engine horsepower availability and cylinder volumetric efficiency) rod loading. 

The best way to unload a reciprocating compressor is to increase the cylinder clearance. When the piston completes the end of its stroke, the space between the piston face and the cylinder head is called the clearance. Increasing the clearance reduces the scfd flow of gas to a compressor running at a constant speed and a fixed suction pressure. The effect of adjusting the cylinder clearance is proportional to the suction stroke volumetric efficiency (Evs). Evs is calculated as follows ... 

The required engine horsepower, but not the volumetric capacity, of a two-stage reciprocating compressor is affected by the intercooler outlet temperature. As a rule of thumb, each increment of 12 F increases the engine horsepower load by 1%. Frequently, all that is needed to minimize this problem is an occasional external water wash of the intercooler aerial fin-tube cooler. 

This means that a substantial volume of gas remains trapped between the cylinder head and the piston before the travel of the piston is reversed. This volume of gas is called the starting volumetric clearance. It determines the volumetric efficiency of the reciprocating compressor. We will discuss this later. 

Starting volumetric clearance The space between the piston and cylinder header at the end of the stroke in a reciprocating compressor. 

Volumetric efficiency of a reciprocating compressor is calculated using Eq. (17) with corrections mcntioiiied in section V.4. It represents the actual gas compression capacity to the piston dii lacemenL It is a function of comptession ratio, clearance, compressibility factor at suction and discharge, and isentropic exponent... 

For a reciprocating compressor, its effective capacity is calculated aa its piston displacement times its volumetric efficiency. Its piston displacement D) is calculated by Eq. (15a) for single-acting pislim and Eq. 

Volumetric efficiency (VE) of a reciprocating compressor is calculated by following equation by accounting 4% loss at suction and discharge valves ... 

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... 

The main disadvantages to multistage reciprocating piston compressors is that they cannot be practically constructed in machines capable of volumetric flowrates much beyond 1,000 actual cfm. Also, the higher-capacity compressors are rather large and bulky and generally require more maintenance than similar capacity rotary compressors. 

The first three factors are present in compressors, but they are small and on the whole can be neglected. The clearance volume problem, however, is unique to reciprocating piston compressors. The volumetric efficiency e estimates the effect of clearance. The volumetric efficiency can be approximated as... 

Figure 3-73. Volumetric efficiency for reciprocating piston compressors (with ciearance) [4].

For reciprocating positive pumps and compressors the fluid displacement per stroke depends on the geometry, the fluid compressibility, the polytropic coefficient, and the internal leakages and volumetric losses, but basically not on the speed of machine. 

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