Clearance volume

Variable Flow Rate Conventional variable clearance volume and valve lifting devices are impracticable at high pressures and, should it be necessary to vary the flow rate, use has to be made of variable speed electric drives or magnetic clutches. Integral steam and gas engines have been used and Burckhardt (168) developed an hydrauhc drive to provide an integrated variable capacity machine, but its efficiency is less than that of a straight mechanical drive. 

The valve porting influences volumetric efficiency by contributing to the minimum clearance volume. If the porting must be enlarged to reduce the flow loss, it is done at the expense of minimum clearance volume... 

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. 

More flexibility can be obtained with a variable volume clearance pocket such as that shown in Figure 11-20. This is a plug built into the outer cylinder head. When moved, the clearance volume of the outer end of the cylinder changes. 

Four-stroke cycle. TDC and BDC = top dead center and bottom dead center positions of the piston, respectively. Vj = displacement. V, = clearance volume. Compression ratio = (Vj -I-... 

Double-deck valves reduce valve velocities in large diameter cylinders. With these valves, high clearance volumes and... 

Piston displacement is the actual volume of the cylinder displaced as the piston travels its stroke from the start of the compression (condition (1)) to the end of the stroke (condition (e)) of Figure 12-12 expressed as fF of volume displaced per minute. Displacement values for specific cylinder designs are available from the manufacturers, Table 12-6. Neerken is a useful reference. Reciprocating compressors are usually rated in terms of piston displacement, which is the net volume in ft per minute displaced by the moving piston. Note that the piston does not move through the clearance volume of Figure 12-12 therefore this volume is not displaced during the stroke. 

Figure 12-17B. Effects of clearance volume on performance efficiency of reciprocating compressor cylinder (valve design effect). (Used by permission Livingston, E. H. Chemical Engineering Progress, V. 89, No. 2, 1993. American Institute of Chemical Engineers, Inc. All rights reserved.)...

Percent clearance is the volume % of clearance volume to total actual piston displacement. ... 

For double-acting cylinders, % clearance is based on total clearance volume for both the head end and crank end of the cylinder X 100 divided by the total nel piston displacement. The head and crank end % clearance values will be different due to the presence of the piston rod in the crank end of the cylinder. The % clearance values are available from manufacturers for their cylinders. The values range from about 8% for large 36-in. cylinders to 40% for small 3-and 4-in. cylinders. Each cylinder style is different. 

The compressor manufacturer can control items a-c, e, f, and h however, the control of clearance volume at high compression ratios for gases/vapors with low specific heat ratios is of great concern. Compression efficiency is controlled by the clearance volume, valves, and valve pocket design. A decrease in compression efficiency leads to increased power requirements. ... 

The nominal compression ratio (which is usually specified) is the displacement volume plus the clearance volume divided by the clearance volume. Because of the mechanics of intake value closing, the actual compression ratio r is less than the nominal. Thus, the compression pressure p (psia) may be estimated by... 

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... 

The screw compressor can be visualized as a development of the gear pump. For gas pumping, the rotor shapes are modified to give maximum swept volume and no clearance volume where the rotors mesh together, and the pitch of the helix is such that the inlet and outlet ports can be arranged at the ends instead of at the side. The solid portions of the screws slide over the gas ports to separate one stroke from the next, so that no extra inlet or outlet valves are needed. 

Screw compressors have no clearance volume, and may work at high compression ratios without loss of volumetric efficiency . In all screw compressors, the gas volume will have been reduced to a pre-set proportion of the inlet volume by the time the outlet port is uncovered, and this is termed the built-in pressure ratio. At this point, the gas within the screws is opened to condenser pressure and gas will flow inwards or outwards through the discharge port if the pressures are not equal. 

In practice, it is not possible to expel the whole of the gas from the cylinder at the end of the compression the volume remaining in the cylinder after the forward stroke of the piston is termed the clearance volume. The volume displaced by the piston is termed the swept volume, and therefore the total volume of the cylinder is made up of the clearance... 

A typical cycle for a compressor with a finite clearance volume can be followed by reference to Figure 8.40. A volume V i of gas at a pressure P is admitted to the cylinder its condition is represented by point 1. 

Thus, theoretically, the clearance volume does not affect the work done per unit mass of gas, since Vi — V4 is the volume admitted per cycle. It does, however, influence the quantity of gas admitted and therefore the work done per cycle. In practice, however, compression and expansion are not reversible, and losses arise from the compression and expansion of the clearance gases. This effect is particularly serious at high compression ratios. 

The value of V4 is not known explicitly, but can be calculated in terms of V3, the clearance volume. 

The effect of clearance volume may now be taken into account. If the clearances in the successive cylinders are C], c2, cj,. .the theoretical volumetric efficiency of the first cylinder... 

When multiple compressors are specified, only those required for the initial plant should be purchased. Adequate space must of course be allocated to meet expansion needs. If only one compressor is specified, one large enough to meet expansion needs should be specified. For centrifugal compressors the capacity can easily be varied over a wide range. The designer should, however, check to see that the initial requirements exceed its minimum capacity, which is called the surge point. In the case of reciprocal compressors, either a clearance volume can be added to the compressor cylinders or the suction valve can be removed. See reference 39 for a discussion of this. 

Also included in in vivo data is a set of human (90% of drugs) and animal pharmacokinetic (30% of drugs) data. While the in vitro data are generated in-house (Cerep), pharmacokinetic data are gathered from the literature. A variety of different parameters are covered including absolute bioavailability, oral absorption, clearance, volume of distribution, half-life, protein binding and excretion information. 

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