Plastic KETER sheds

[SUNRAY]

Fair enough, but I still don't really understand where the estimates/guesstimates/whatever of pressures you've used have come from. Are they perhaps pressures which you 'know' (or, at least, 'believe') to be the range of pressures used in typical real-world domestic fridges and freezers? As I've said, I personally haven't got a clue about the pressures!

Domestic freezer compressors are commonly capable of 400PSI but that doesn't mean it's the pressure the system is running at as it's a leaky system.

 

[JohnW2]

This is all well beyond my experience but IIUIC there was only one refrigerant available in the early days.

I find it hard to believe that it has ever been possible for one refrigerant to be suitable for all refrigeration purposes

I may be wrong and will happily accept being corrected.

As I keep saying, I really don't know. I think the same is probably true of you, but yiou keep making suggestions and comments which may or may not be correct. If/when I have a bit of time, I might try to do some reading.

Where do you get +20ºC from for chiller temp?

I didn't - the +20°C figures relates to aircon.

 

[JohnW2]

Domestic freezer compressors are commonly capable of 400PSI but that doesn't mean it's the pressure the system is running at as it's a leaky system.

As I keep saying, I really don't know. I think the same is probably true of you, but yiou keep making suggestions and comments which may or may not be correct. If/when I have a bit of time, I might try to do some reading.

 

[SUNRAY]

I find it hard to believe that it has ever been possible for one refrigerant to be suitable for all refrigeration purposes

As I keep saying, I really don't know. I think the same is probably true of you, but yiou keep making suggestions and comments which may or may not be correct. If/when I have a bit of time, I might try to do some reading.

I didn't - the +20°C figures relates to aircon.

Presumably talking about return air temp and where the sensor will be working, the chiller part will be working at closer to freezing and the fan section will be working at a temp not far above that. Pipes between the units have to be well lagged to prevent the condensation forming on them and also to prevent it freezing on them.

 

[JohnW2]

[ I just wrote some nonsense, but realised quickly enough to be able to remove it. I need to do some re-thinking and then will probably try to write something a bit more sensible! ]

 

[fixitflav]

Yes, it could be any of those things, and I'm coming to think that (particularly at the extremes) the problem of ambient temp being too low is probably less 'fundamental' than it being too high. As above, if ambient temp gets too high, than that can become 'fundamental', since if it gets high enough the refrigeration process will simply stop - and I imagine that efficiency will reduced markedly as one approaches that situation.

I don't see why the process would just stop. At moderate ambient temperature the compressor runs On/Off. As ambient rises the On times get longer, till at ambient design limit it runs continuously. Above that the freezer temperature rises.

 

[fixitflav]

Domestic freezer compressors are commonly capable of 400PSI but that doesn't mean it's the pressure the system is running at as it's a leaky system.

I was wondering about that, as I don't have any figures. I checked vapour pressure of refrigerants using the Antoine equation and R410a is 22bar (320 psi) at 35°C, so 400 psi sounds about right (35° could be too low). Is that the compressor differential? Any idea what the pressure settles down to when the compressor is stopped?

 

[JohnW2]

I don't see why the process would just stop. At moderate ambient temperature the compressor runs On/Off. As ambient rises the On times get longer, till at ambient design limit it runs continuously. Above that the freezer temperature rises.

I'm currently trying to get my head properly (i.e. hopefully correctly this time!) around all this, so anything I say at the moment probably needs to be taken with a significant pinch of salt!

With that caveat, with my thinking as it is at present, I'm inclined to think that one (maybe the only one?) situation in which the process would presumably stop would be if the temp of gas returning to the compressor (from the in-cavity 'evaporator') was too high for the compressor to be able to liquify it - but, as yet, I have no idea as to whether that could happen with any refrigerant under any real-world circumstances.

In your subsequent post you've mentioned another matter which may be relevant - namely the 'baseline' pressure in the system when the compressor is not running (and has not been running for a good while). I think I've probably been tending to assume that it is petty close to atmospheric pressure, but I don't actually know whether or not that it true, nor whether that pressure has a significant bearing on the matters we're discussing.

Turning the clock back a bit, I don't think any of our discussion has so far got any of us any closer to understanding the nature of the alleged problem at low ambient temps, has it?

 

[fixitflav]

The 'target' temp for a domestic freezer is about -18°C, but for AC systems is more likely to be around +20°C - so I rather doubt that the same refrigerants would be appropriate (or, at least, 'ideal'/'efficient') in both applications.

It certainly seems reasonable that the refrigerant for a freezer taking in heat at -19° and rejecting it at ~ 35° would be different from an AC unit taking in warm air at 20° and returning it at say 15°. Whether that's the case in practice I don't know.

If the same refrigerant were (and could be) used in both cases, then the pressures would presumably be dramatically different in the two cases?

In any case, the pressure after the compressor must be high enough to liquefy the refrigerant.

 

[SUNRAY]

In your subsequent post you've mentioned another matter which may be relevant - namely the 'baseline' pressure in the system when the compressor is not running (and has not been running for a good while). I think I've probably been tending to assume that it is petty close to atmospheric pressure, but I don't actually know whether or not that it true, nor whether that pressure has a significant bearing on the matters we're discussing.

No it won't reach atmospheric pressure, the system is pressurised - again it's only a hunch but I'll make a stab at something over the LO pressure side of the system as, when running, the compressor will dip the static pressure.

Turning the clock back a bit, I don't think any of our discussion has so far got any of us any closer to understanding the nature of the alleged problem at low ambient temps, has it?

I thought this:

Had gone some way to an explanation as the gas is unable to vapourise.

 

[fixitflav]

I'm inclined to think that one (maybe the only one?) situation in which the process would presumably stop would be if the temp of gas returning to the compressor (from the in-cavity 'evaporator') was too high for the compressor to be able to liquify it -

I suppose that's possible, but my guess is it would still work, but the freezer going above design temperature in line with ambient above design.

In your subsequent post you've mentioned another matter which may be relevant - namely the 'baseline' pressure in the system when the compressor is not running (and has not been running for a good while). I think I've probably been tending to assume that it is petty close to atmospheric pressure,

This is interesting! If any of the refrigerant is to remain as liquid (in the heat dump grid) when the compressor is stopped, the pressure must be the vapour pressure at the room temperature.

Turning the clock back a bit, I don't think any of our discussion has so far got any of us any closer to understanding the nature of the alleged problem at low ambient temps, has it?

Only what we've already discussed.

Frustrating that we're guessing and struggling, but people who design these things must have the answers at their fingertips.

 

[SUNRAY]

FWIW Friday and Saturday evenings we ran our AmDram show and filled the tall fridge with prechilled drinks, the compressor was running but I suspect it wasn't chilling but the ambient around fridge was over 38ºC (Limit of the digital battery powered meter)

Apologies for the age related obscurring

 

[JohnW2]

It certainly seems reasonable that the refrigerant for a freezer taking in heat at -19° and rejecting it at ~ 35° would be different from an AC unit taking in warm air at 20° and returning it at say 15°.

Exactly my point. I would have thought that one could probably design/invent (on the basis of characteristics) a hypothetical refrigerant which would be fine in one of those scenarios but not the other - but I obviously would not know whether such an animal actually existed.

Whether that's the case in practice I don't know.

Same here.

In any case, the pressure after the compressor must be high enough to liquefy the refrigerant.

Quite so - and if it wasn't (high enough) then the cycle would simply 'not happen'

 

[JohnW2]

This is interesting! If any of the refrigerant is to remain as liquid (in the heat dump grid) when the compressor is stopped, the pressure must be the vapour pressure at the room temperature.

Yes, very interesting. Ultimately, if the compressor had not been running for a long time then all of the refrigerant in the system would be at ambient temp, so I wonder what is the refrigerant phase and pressure in that situation (e.g. when i'ts been sitting around in a showroom for weeks/months!). Again, maybe I'm missing something but, despite what Sunray has said, I'm not sure I can think of a reason why the pressure in that situation necessarily has to be above atmospheric?

Frustrating that we're guessing and struggling, but people who design these things must have the answers at their fingertips.

Yes, one would expect that. However, they don't seem to be great communicators. In the last few days I've read a lot on-line about how the refrigeration cycle works. What the different sources say all tend to be identical or near-identical (I think a lot of copying/pasting has probably been involved!), but none of it seems as clear or comprehensive as I would have hoped in relation to things I'm trying to learn about!

Someone seems to have created a 'rubber stamp' of a dumbed-down simplistic explanation (essentially for Joe Public) of how these things work, and almost everyone seems to be 'stamping' their website with it

I probably have some very old and very dusty Thermodynamics textbooks knocking around somewhere ( rarely throw anything out!), but I'm fairly sure that they would be too heavy going for me in my senior years

 

[SUNRAY]

Again, maybe I'm missing something but, despite what Sunray has said, I'm not sure I can think of a reason why the pressure in that situation necessarily has to be above atmospheric?
...

Simply that the system is charged and pressurised during the gassing stage

Why else would is escape and hiss when any part of it is damaged?