2021 RETA Breeze Jan-Feb

is found that “works” consistently there would be a reluctance to investigate any tuning up. If reducing the evaporator pressure and the supply pressure is tried — things may happen. - Say that the supply pressure is dropped from 110 to 90. This means there are fewer BTU’s per pound or per cubic foot of vapor provided the evaporator. That can have the result of needing to extend the defrosting time – which could cancel any gains of “efficiency” by dropping the supply pressure. - Same sort of consideration for the defrosting pressure. My training (factory recommended settings) from the mid-1980’s was to set the defrost regulator at 90 psig. We did not have supply pressure control on the inject side of the circuit (kind of dicey – now that we know about hydraulic shock events). So – the rule of thumb was to defrost at 60°F. That was fine for a 33°F vegetable cooler – freezer plants that tried to operate like that in their -20 freezers would put up a fog from the evaporator being defrosted. The fog would get drawn to the closest evaporators still in refrigeration and rapidly ice up as the fog condensed and froze to the surface. If I worked there at your plant, I would generate a written table of the operating parameters including the timed observations of how long of a pump-down is needed before injection starts, how long injection lasts, the depressurize time, the drying time for the surface, etc. The operating pressures would affect the density of the gas and how many BTUs is in the flow of the gas on a per-minute basis. Material of construction would be noted for the evaporators, including wall thickness the distance from the prime surface of the evaporator tubes to the edges of the fins. Some recent evaporators

have sheet type fins where the furthest distance from the prime surface is over 1-1/4” - that is a long way to have to conduct heat energy, especially if the room environment is freezer conditions and not vegetable holding conditions. The goal is to clear the evaporator surface as swiftly as possible AND have a graceful entry and exit from defrost conditions. If the pipes and evaporator do not sway when refrigeration is restored following defrost, that is a good thing. If they go bump in the night – closer examination is needed. It could be that everyone is “right” regarding their preference for control settings. The type of defrost configuration comes into play too. Evaporators that use a ‘float drainer’ device to hold flow back from the evaporator to the engine room – until it is passing liquid – will reduce that false loading effect seen in systems that have a regulator passing refrigerant of any form – at a prescribed pressure. See – there is no easy answer for what is optimum. I would say that a study of all the factors surrounding defrost and evaporator design could lead to a consideration to reduce a defrost operating pressure. The consequences, or effect from the change, would need to be evaluated. If defrost time is extend 15 minutes because the defrost temperature was reduced – was anything improved in the first place? Guess what – our conversation here is going to be incorporated into a RETA Breeze article. I’ve been asked by HQ to write an article that expresses the value of association to members. The existence of the association set up the opportunity for us to meet and to learn from one another. The review course gave the opportunity to open the discussion of the numerical

factors involved in operating a system. The conversational exchange (Q/A) during class allowed for further opportunity for students to ‘get it’ and to consider their own operations. I won’t be mentioning names or companies – just that a student followed up on the comment of the instructor and asked the deeper question about why different operators would have differing opinions about the right settings for defrosting. Understanding why a certain set of parameters is selected for an operational purpose is as important as those values being right for the conditions at the time. By understanding what is happening in the system – a system upset in the making may be recognized and resolved before a bad thing takes place. I hope this helps – I’d love to see your reference table of evaporators, materials of construction, operating parameters, and what would be predicted if a control setting were changed from one value to another. Taking such a step would certainly give defensibility for the requirements of a management of change. I like to have a 15psi differential between the controlled supply gas pressure and the condensing pressure in the evaporator. So, 85/70 is a value set I look for. This is for evaporators with aluminum surfaces. The 15 psid gives pretty steady flow both in the going in and coming out valves. This also means that the pipe diameter and distances run are favorable as well. Much to consider when evaluating these operational conditions. To me, the importance of the association is that the association exists, and the staff and volunteer leadership understand the mission and work tomeet the mission with each contact with a member, or non-member.

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