Refrigeration Insights series. Season 2: The enemy within, part 7.
In the first part of this season we saw that a badly sequenced defrost is the number one suspect behind hydraulic shock. Today we give it the full article it deserves, guided by a technical paper written from field experience: the hot gas defrost system with refrigerant return to condensers by Jorge Sifaqui Clausen, a Chilean consultant, presented at the IIAR 2014 conference. It is complemented by Bruce Nelson's paper (2015) on optimizing defrost frequency.
Why defrost exists
The paper's definition is clean: defrosting means removing the ice that forms on evaporators, especially finned ones, when operating at an evaporating temperature below 0 degrees. Ice is unavoidable; what gets managed is its build-up, so that the air cooler keeps delivering reasonable capacity.
And a piece of welcome honesty: defrost is a disruptive process. It generates unwanted thermal loads, inefficiencies and changes in the operating pressures of the system. There is no free defrost: there are well managed defrosts and defrosts that turn out very expensive.
The three families
The paper reviews the usual systems along with their application limits:
By air. Limited to rooms with an ambient above 0 degrees and evaporation close to 0: the sensible heat of the air, moved by the fans with refrigeration stopped, does the work.
By water. For ambients close to 0 degrees and evaporation no lower than -10, conditional on having enough water at a suitable temperature. The paper notes an efficiency idea: cover that water with heat recovery from oil coolers, water-cooled condensers or desuperheaters on the discharge.
By hot gas. The lead player in low temperature: it uses the refrigerant discharged by the compressors themselves as the heat source, giving the evaporator back, for a while, the role of condenser.
The sequence is everything
The variant Sifaqui proposes (with refrigerant return to condensers, through a servo valve that splits the discharge line into two sections) has one virtue: it does not disturb the evaporating pressure of the rest of the system while one evaporator is defrosting.
But the most transferable lesson in the paper is its sequence of stages, which includes steps that many plants skip:
The drip time, with refrigeration and defrost stopped, so the water runs off the fins. Equalizing the evaporator pressure with the suction through a bypass, before reopening the suction (the step whose omission produces the hydraulic shock we talked about six weeks ago). Restarting refrigeration with the fans off, so that water is not thrown onto the product in the room. And only then, normal ventilation.
And the pragmatic closing: the timing of each stage is experimental, it depends on the local climate, on the type of product stored and on the traffic in the rooms. There is no universal recipe: there is method plus fine tuning.
When to defrost: the other half of the question
Nelson's complement (2015) tackles the prior question: how many times a day it makes sense to defrost. Every unnecessary cycle is hot gas energy plus the thermal load of reheating the room; every cycle too few is an evaporator working blocked. Optimizing defrost frequency is one of the most profitable efficiency adjustments there is, because it requires buying nothing: only measuring and adjusting.
The practical takeaway
Audit the defrost sequence of your most troublesome evaporator against the list in the paper: is there drip time? is there equalization before reopening the suction? is the restart done with the fans off? And audit the frequency: who decided how many defrosts per day, when, and with what data? If the answer is that it has always been that way, there is money on the table.
Next week: the water in the evaporative condenser, scale, corrosion and biofilm.
Sources: J. Sifaqui Clausen, “Hot gas defrost system with refrigerant return to condensers with R-717 refrigerant”, Technical Paper #4, IIAR 2014; B. Nelson, “Optimizing defrost operating time and frequency”, IIAR 2015. Thermomac is a corporate member of IIAR.
