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The most simple way. with little protection but it is used
or has been used for years with most smaller commercial
freezers.

Run a hot gas line to the evap and tee it into the evap inlet
directly after the TEV. Put a mag valve on this line and when
it goes into defrost, it opens the hot gas line, hot gas passes
through the evap and the evap coil defrosts.

Trouble is, running in defrost will overheat the suction gas back
to the compressor and overheat it, so timing the defrost long
enough to defrost the coil and not too long so the compressor
overheats and fails.

After less than about 5 mins the heat from the discharge gas will
be lost and the defrost must be completed in this initial period.

Protecting the compressor with the use of a suction accumulator
would be an added precaution but this process would work with
little additional cost and works well.

There are drawbacks to using hot gas defrost though, clamping the
pipes to stop movement through expansion is a must and remember
the coil will be subjected to extreme temperature changes and that
can result in shock failures to evaporator pipe returns and fittings.

It is very crude, a bit agricultural in design, but it is the cheapest
option and it will work.

Rob

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