Two Approaches to Energy Savings
Recently, I reviewed operation of one refrigeration
plant. Chief engineer had concerns about raising suction pressure during hot gas
defrosting. Typically, this two stage refrigeration plant operates at vacuum - 5
in of Hg due to requirement of blast freezers. During summer operation suction
pressure can increase up to 0 psig if one evaporator goes on defrost. The reason
for suction pressure increase is that defrost ammonia condensate and defrost
blow by gas go to the suction line due to old design of the refrigeration plant.
Local contractor suggested installing separate ammonia condensate line from the
evaporators to the intercooler. This installation costs $20,000.
For
efficient hot gas defrosting, demand and supply should be balanced. Demand is
the heating requirement of the evaporator. This demand is unchanged all
year-round. We have the same evaporator, the same temperature around evaporator,
and probably, the same amount of frost. Supply is hot gas supply to the
evaporator. During summer operation condensing pressure of this refrigeration
plant goes up to 180 psig and it is significantly greater than winter condensing
pressure of 120 psig. At 180 psig hot gas will be oversupplied to the defrosting
evaporator. Uncondensed blow by gas will create significant parasitic load and
suction pressure will increase to 0 psig.
To
solve this issue, my suggestion was to install outlet pressure regulator for the
defrosting hot gas line. This regulator will keep pressure of 120 psig all year
round and summer oversupply of defrost hot gas will be prevented. The cost of
this installation is $1500. Certainly, additional ammonia
condensate pipe will save some energy, but these savings are not significant
(around $1500).
Similarly, we can find two different approaches in
energy savings. Usually, contractor suggests to end user energy saving measures
that require capital investments. Definitely, investments in VFDs, PLCs,
repiping, additional condensers will save you some energy, but it is a very
expensive approach. Optimization of the refrigeration plant operation does not
require significant investments and it has payback 10 - 20 times shorter than
capital investments. Many end users have done initial optimization, but a lot of
the refrigeration plants have an opportunity for final optimization. Do not miss
this opportunity.