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CLERANCE VOLUMETRIC

EFFICIENCY

Clearance is a must in a compressor. It

does not allow the piston to  strike the

cylinder head. Clearance does not permit

to such full amount of gas. Exhaust gases

left in clearance space occupy more

volume during suction stroke.

NEED OF CLEARANCE IN THE CYLINDER

Piston is moving in the cylinder. It is desirable that the piston does not strike the cylinder head cover. To achieve this there is a small space between the cylinder head and the extreme position of piston. The volume of this space is called clearance volume or clearance. It is represented by symbol ‘c’ which is normally a few percent of stroke volume, say 4 %, then c=0.04 . Based on this clearance volume there is a clearance volumetric efficiency of the compressor.

 Clearance volume and  clearance volumetric efficiency

Clearance

It is the space between the extreme end of piston and the cylinder head.  The piston does not strike with the cylinder head. It is represented by the symbolic efficiency ‘c’ and expressed as percentage of stroke volume. Volumetric efficiency is a ratio of actual volume sucked to the stroke volume.

Clearance volume efficiency  is of two types:

(i)                 Clearance volumetric efficiency

η_cv =actual volume sucked/stroke volume

       = (V₁—V₄)/ (V₁ –V₃)

Write in the following form

        = (V₁— V₃ + V3 –V4)/ (V1 –V₃)

We get

         = 1 + V₃/ (V1 –V₃) –V₄/ (V1 –V₃)

Simplification gives

          = 1 + C – (V₄/V₃) V₃/ (V1 –V₃)

η_cv = 1 + C –C (p_high/p_low)^1/n

Where C is clearance expressed as percentage of stroke volume say 4 % then C= 0.04

(ii)               Total clearance volumetric efficiency

It includes the followings:

(i) Effect of clearance

(ii) Pressure drops

(iii) Leakage across the piston walls

(iv) Leakage across the suction and discharge valves

(v) Superheating on entry to cylinder

It is given by the following empirical equation

η_tcv = η_cv (p_cyl/p_suction) (T_suc /T_cyl)

Firstly  p_cyl   pressure in the cylinder

Secondly p_suction  pressure at entry to suction valve

p_cyl  < p_suction

(p_cyl/p_suction) <1

T_suc   is the absolute temperature at the inlet of suction valve

T_cyl is the absolute temperature on entry to cylinder

The cylinder wall is at high temperature due to the just earlier discharge

T_suc  <  T_cyl

(T_suc /T_cyl) < 1

Hence η_tcv < η_cv

where

1. (p_c/p_s) is the ratio of pressure in cylinder to pressure in suction line.

2. This factor will be less than 1 since the pressure in cylinder is less.

3. (T_s / T_c) is the ratio of ABSOLUTE temperatures in suction and cylinder.

4. This factor will be less than 1. The temperature in suction is less than the temperature in the cylinder. Cylinder is hot because of discharge before suction.

The above expression is an empirical relation for which there is no proof. It is verified experimentally and has the experience considerations