What is Harmful Volume in Reciprocating Compressors?

Harmful Volume or Harmful Space is the space between the piston face and the discharge valve plate at the upper dead center of the stroke. This clearance should be as small as possible, in order to force the compressed refrigerant vapor through the discharge valve. Any remaining steam will expand again in the suction stroke, partially filling the cylinder and reducing its effective volume, the volumetric efficiency of the compressor.