Hot Gas Defrosting

[AC_Hacker]

I've noticed that a lot of conversation has been going on regarding defrosting strategies for heat pumps that are operating in cold, high-humidity environments.

I just came across a Danfoss paper called, "It Pays to Identify the Best Defrost Technique".

The lead paragraph reads:

As far as air cooled evaporators are concerned, defrost is the “inevitable evil.” Frost forms and needs to be melted, or it will severely impact the performance of an evaporator and eventually block air flow.

Unfortunately, hot gas defrost techniques for melting frost are not problem free. Additional compressor energy is required to melt the frost/ice layers formed around the evaporator’s fins and tubes. At least a part of this energy is transferred back to the refrigerated space or heats up the evaporator -- and eventually needs to be removed during the cooling process.

Another drawback: the time used for defrost is not used for cooling. This could be an important factor in food processing plants, where defrost can significantly limit productivity levels. Another important but less obvious consequence: system integrity may be harmed when key components are subject to mechanical stress.

The main source of mechanical stress is the combination of high pressure coming from the condenser side, high discharge temperature, and high pressure differential. When combined, these factors can be quite dangerous and even destructive. Nevertheless, hot gas defrost is being used by more companies, often in CO2 low temperature plants. CO2 control is more complex than with ammonia, due to higher pressure levels and pressure differentials. The complications of hot gas defrost with CO2 may cause customers to avoid this method and look for alternatives, such as electrical or brine defrost.

Yet, hot gas defrost is one of the most efficient ways to melt the frost formed on an evaporator (see: Pearson A., Defrost Options For Carbon Dioxide Systems, 28th Annual IIAR Meeting, March 2006). In fact to reduce energy consumption, performing defrost quickly and efficiently is key to achieving overall energy consumption goals of the refrigeration system. In most cases, it is more cost effective than brine defrost. Several valve and control configurations are examined to find the best technique for optimizing the hot gas defrost process and cutting energy consumption.

There are links to additional aspects to the Hot Gas Defrosting issue on site.

Best,

-AC

[jeff5may]

In the smaller, home-sized or romm-sized units, there are 2 basic modes of defrost:

1. Reversing valve - more predictable, costs more, allows unit to provide heating and cooling.

2. Direct feed - must be tuned to work in a variety of conditions, less expensive, works with heating - (or cooling -) only units.

With both types of defrost, the unit needs to have some sort of control to perform defrosting when necessary. These defrost controls range from the dead simple to the overly complicated. The more complicated they are, the less energy they are supposed to waste.

[NiHaoMike]

And when the outdoor temperature is above freezing, it's also possible to defrost by turning off the compressor but leaving the outdoor fan running.

There's also electromagnetic defrost that has a wire running through the coils carrying a high frequency AC signal in order to flash the frost into steam and blow out remaining frost. It just hasn't become cost effective yet.

[AC_Hacker]

Yes, the paper is interesting to me, as I have a 1-Ton non-inverter mini-split ASHP that I will be incorporating into my system.

I only have the outdoor unit, and no info on the existing serial control signal scheme, which is a real problem.

I have closely followed the work of some vapor-compression hackers who are very experienced and skilled at decrypting serial communications protocols. Their lack of success has not been at all encouraging.

Acuario, in a previous post suggested abandoning the whole existing controller board, with its encrypted protocol, and building my own controller board from scratch, and then another person suggested to use as much existing circuitry and hardware as possible, and use some common microprocessor to piggy-back onto the controller board (for me that would be a Teensy) to send out control signals through the existing opto-isolators & to the existing SCRs, etc.

Initially, I was planning on removing the reversing valve for simplicity, but then I realized that I could use it for defrost cycles.

So this paper came at a very relevant moment, and it seems to suggest optimization parameters that could be incorporated into program statements..

-AC

[WyrTwister]

I am not an expert , but the quoted text sounds like defrosting for a freezer ?

When I run my mini split A/C - HP in cool , no need for defrost . When in HP mode , it does its own thing to defrost & since I can not change it , I try to ignore it .

Since mine is a non inverter unit , I doubt if it has one of the " smarter " electronics packages / defrost senerios ?

I am sure it used hot gas to defrost the out door unit ?

God bless
Wyr

[jeff5may]

I just scored another window unit off craigslist for $20. Haier 12kbtu 10 seer unit, owner said it just quit after the 90 day exchange warranty expired. No love from Wal-mart, so he sold it for double the scrap value. I'm in the process of restoring control board operation, it seems it got power surged and killed the buffer chip between the brain and muscles. Once i make it operate, I will be turning it into a "puppet on a string" by way of the shorted buffer IC. Dead simple to do as far as electronics goes.

[nokiasixteth]

I found that sprayin a small mist of well water over the outside unit on defrost . It defrost a lot faster.

[jeff5may]

Standard splits and mini splits pretty much use method #1 in my previous comment.

Something calls for defrost, and a few things happen. First, the outdoor fan shuts off. Then, the reversing mode spools to cooling mode. This heats the outdoor coil, stealing heat from your warm indoor air. When the oudoor coil has defrosted, the unit may or may not switch right back to heating mode. Some units have a delay built in, so they sit around (like a clothes washer) and wait for any water to drain before going back into heating mode. Like usual, the "smarter" the defrost control is, the more your unit will act different from a kitchen fridge/freezer combo unit from the 70's.

Like WyreTwistr said, most kitchen fridges operate on timed resistance defrost. Somewhere in the cabinet, there is a timer that runs whenever the compressor does. When the run time hits a certain running total (usually many hours), it leaps into action. It locks out the compressor and freezer fan and energizes a heating element wound into the evaporator coil. After running a predetermined amount of time (a few minutes), it shuts down the heating element and releases control of the compressor and cooling fan.

[nokiasixteth]

Ive noticed that the few times my mini has went into defrost . If you stick your hand up close to the exhaust its cold. That little red light comes on. The compressor kicks on 2. Then slowly ramps up to 5 the highest it wil go. Then drops down 4 3 then 2. Then comprssor will kick off and all that ice turns to water and then back to heating the house.