Plastic KETER sheds

[JohnW2]

This isn't something I'd ever thought about as it's not my boat. However I don't know how dependant the process of liquification of gas is on temperature.

This subject gets very complex, and way over my head, but in simplistic terms, over ranges of temp and pressure likely to be relevant in context, it's generally true that, for any given pressure (within that range), there is a temp below which the substance will inevitably be liquid and, for any given temp (within that range), there is a pressure above which the substance will inevitably be gas. A simplistic 'phase diagram' for an HFC refrigerant may help you to understand ...

You need to concentrate on the regions labelled "Liquid" (light green) ans "Vapor" (light pink). We're not interested in the solid phase (white) and, as for the other three (when pressure and/or temp is/are above the values at the "Critical point"), they represent weird states ('above my head'!) where liquid and gas are not really distinguishable from one another!

so, simplistically ..... for any temp within that region, increasing the pressure will eventually cause the gas to become liquid, but with an increasing amount of presure required to achieve that at higher temps. Conversely, for any pressure within that region, decreasng the temp will eventually cause gas to become iquid but with an increasingly low temp required to achieve that at lower pressures.

Does that help at all?

 

[JohnW2]

I assume it's that with the installed compressor power there is a maximum heat flow, at design maximum ambient temperature, and minimum freezer temperature. If the ambient temperature goes higher, the heat flow falls, because the grid cannot dissipate the heat. But it would be nice to think there's a bit of a margin in the stated performance figures.

Indeed. If the ambient temp were to rise above the temp of the grid, the whole process would presumably grind to a halt - but, as you say, I imagin there are proably some fairly generous margins built into the 'spec'.

However, I still haven't really got my head properly around the 'problem' if/when the ambient temp is too low - any ideas?

 

[fixitflav]

Indeed. If the ambient temp were to rise above the temp of the grid, the whole process would presumably grind to a halt

There has to be a temperature difference between the grid and the air in order to dissipate the heat. The maximum grid temperature must be in the design.

However, I still haven't really got my head properly around the 'problem' if/when the ambient temp is too low - any ideas?

Not really. Maybe it's down to type of refrigerant - perhaps some will freeze solid at too low, not a good idea! Perhaps types that can't handle low temperature have benefits in other ways, like higher efficiency, or cheaper.

 

[JohnW2]

There has to be a temperature difference between the grid and the air in order to dissipate the heat. The maximum grid temperature must be in the design.

Quite so.

Not really. Maybe it's down to type of refrigerant - perhaps some will freeze solid at too low, not a good idea! Perhaps types that can't handle low temperature have benefits in other ways, like higher efficiency, or cheaper.

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. However, once I have got my initial incorrect thinking sorted out, I don't think there is any comparable 'show stopping' ('fundamental') issue with a low ambien temp (unless, as you say, the refrigerant solidifies!) - so the problem with low ambient temps may only be manifested in terms of reduced efficiency.

That being the case, if the consequence of a lower ambient temp is simply that the compressor has to work harder to maintain the desird cavity temps, then that would not be apparent to the user unless they monitored the energy usage of the appliance (which very very few would), If they did monitor energy usage, they might find that it rose when ambient temp was low, in contrast to the falling at lower ambient temps that one wouldexpect if efficiency remained roughly unchnged?

I think I'll try to find phase diagrams for some real modern refrigerants and see if that sheds any light on this question!

 

[SUNRAY]

Indeed. If the ambient temp were to rise above the temp of the grid, the whole process would presumably grind to a halt - but, as you say, I imagin there are proably some fairly generous margins built into the 'spec'.

However, I still haven't really got my head properly around the 'problem' if/when the ambient temp is too low - any ideas?

I hadn't expected to be going down understanding more of AC but here goes:
First this isn't my field but I have interface many AC units to BMS so yes I do se pressure gauges etc and yes I communicate with the AC guys.

I believe typical HI pressure range is 200 to 250 PSI depending on refrigerant and I'll go a bit further and say probably more like the top half of that and LO pressure around 25 to 40 PSI
Scrawling on your curve and hoping my guesstimations are somewhere nearly close please allow ±½ a house brick:

200 PSI shows a max temp of 35ºC and 40 PSI shows a minimum temp of -40ºC

I think those pretty much sum up what questions have been asked although to the uninitiated (Especially Sunray) it does appear different pressures could be used to move the cutoff points.

EDIT: TYPO the red box has been drawn as -10 to 50ºC not -20 as marked

 

[JohnW2]

I believe typical HI pressure range is 200 to 250 PSI depending on refrigerant and I'll go a bit further and say probably more like the top half of that and LO pressure around 25 to 40 PSI ... 200 PSI shows a max temp of 35ºC and 40 PSI shows a minimum temp of -40ºC ... I think those pretty much sum up what questions have been asked ....

Fair enough - but such exercises are largely 'self-fulfilling', in that the answers you get will depend entirely on what pressures you choose to feed in - and I haven't got a clue as to whether your 40 - 200 PSI assumption is 'reasonable' or 'wildly incorrect'

 

[SUNRAY]

Fair enough - but such exercises are largely 'self-fulfilling', in that the answers you get will depend entirely on what pressures you choose to feed in - and I haven't got a clue as to whether your 40 - 200 PSI assumption is 'reasonable' or 'wildly incorrect'

I don't believe they are wildly incorrect - although well out of date possibly.

All I have done is calculate the pressure readings into PSI (Units I'm more comfortable with), add the red box for -10 to 50°C and black line for 0ºC and the green and orange markings for pressures I hope I remember then estimate the limits. No intention of being self fulfilling and if anything came up with figures I didn't expect.

 

[JohnW2]

I don't believe they are wildly incorrect - although well out of date possibly. ... All I have done is calculate the pressure readings into PSI (Units I'm more comfortable with), add the red box for -10 to 50°C and black line for 0ºC and the green and orange markings for pressures I hope I remember then estimate the limits.

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!

 

[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!

I hadn't expected to be going down understanding more of AC but here goes:
First this isn't my field but I have interfaced many AC units to BMS so yes I do see pressure gauges etc and yes I communicate with the AC guys.

I believe typical HI pressure range is 200 to 250 PSI depending on refrigerant and I'll go a bit further and say probably more like the top half of that and LO pressure around 25 to 40 PSI...

Sorry about the typos.

AIUI all chiller systems using a refrigerant work in fairly similar conditions

 

[JohnW2]

AIUI all chiller systems using a refrigerant work in fairly similar conditions

Fair enough again. As I said, I haven't got a clue, so I'll have to take your word for it unless/until someone provides contradictory information.

On the face of it, AC systems are AC systems, and often have pressure gauges, whereas domestic fridges and freezers are domestic fridges and freezers, which very rarely have pressure gauges, so I personally can't be sure (from what you've said) as to how similar they are in terms of either refrigerant or pressures.

 

[SUNRAY]

Fair enough again. As I said, I haven't got a clue, so I'll have to take your word for it unless/until someone provides contradictory information.

On the face of it, AC systems are AC systems, and often have pressure gauges, whereas domestic fridges and freezers are domestic fridges and freezers, which very rarely have pressure gauges, so I personally can't be sure (from what you've said) as to how similar they are in terms of either refrigerant or pressures.

Quite so and I'll be very happy to be updated.

I did a very little google search last night and saw various figures within the ranges 0f 2-5 and 12-24bar (~30-75 and 180-360 psi) and a target figure of 1250KPa (181psi) in cars