GEA Grasso 5 Installation And Maintenance Instructions Manual page 54

APPENDIX; Product Information (PI)
STARTING UP OF TWO-STAGE COMPRESSORS
Procedure to move on to two-stage operation and to increase capacity.
Once properly started, as indicated in the previous paragraph, the compressor
has to be switched over to two-stage operation with minimum capacity, followed,
if and when required, by gradual stepping up to maximum capacity.
The procedure thereby to be applied depends on the installation operating
conditions during starting which give rise to two distinct possibilities, viz.:
1.
2.
Fast pull-down part-load control steps
When using the standard capacity control steps of the two-stage compressor
types, the pull-down procedure to achieve full-load operation at design
conditions, as described in the previous paragraph, is rather often very time
consuming. This is due to the fact that all compressor types are always started
with only one HP cylinder in operation and that the minimum LP/HP swept
volume ratio for any part-load step is ϕ = 2.
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The compressor is started at low evaporating temperatures, this being
approx. the (design) value during normal (full-load) operation.
This situation occurs after the compressor has been stopped for a certain
period of time because of low capacity requirements of the installation. Under
these circumstances it is permitted to switch over to the two-stage part-load
step of minimum capacity immediately after proper starting with H.P. cylinders
in operation.
In the case of NH , this is even a necessity, for otherwise the compressor
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would be running in single-stage under two-stage conditions, resulting in too
high a discharge temperature.
The compressor is started at a relatively high evaporating temperature, that is
to say much higher than under design conditions and in any case not suitable
for two-stage operation.
Such a situation may occur after a prolonged period of compressor standstill
or when the compressor operates on a batch type freezing tunnel, just loaded
with warm products. Under these circumstances it is not permitted to switch
over to two-stage operation with minimum capacity until the H.P. cylinders
already in operation have lowered (in single-stage) the evaporating
temperature so that the corresponding working point at the condensing
temperature t , lies inside the field of application of the two-stage minimum
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part-load step concerned. Only then, after this step has been energized, the
saturation intermediate temperature t
Consequently, the maximum value of t
two-stage operation, is determined by the intersection of the near-vertical line
which represents the right hand limitation of the relevant field of application
and the horizontal line which represents the condensing temperature t
Likewise, during further stepping up to maximum capacity, the evaporating
temperature has to be pulled down by each intermediate part-load step so far
that the corresponding working point at given condensing temperature lies
each time just inside the field of application of the next part-load step of higher
capacity, before that step is energized.
will be below its maximum value.
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at which it is allowed to switch over to
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07.04.2021