Plastic KETER sheds

[fixitflav]

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?

I was thinking of what the pressure is when the compressor stops but the freezer is still at -18°. In your scenario my guess is the refrigerant is partly liquid. The instructions say position the thing and leave it 24 hours before switching on, presumably for the liquid to gravitate down. If it were all gas nothing would happen. If that's right, the pressure is the refrigerant vapour pressure at the ambient temperature.
My figures at ambient 20° are
R12 and R134a ~5.7 bara
R410a 14.7 bara

 

[fixitflav]

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!

Don't know whether you found these sites Design a Refrigeration System - The Engineering Mindset and Refrigerant R-134a

Not sure I have the inclination (or likely the ability) to follow all that, but at least the vapour pressure of R134a at various temperatures agrees with my calcs!

 

[JohnW2]

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?

We're obviously all aware of the fact that such is what is done, but that, in itself, does not mean that such is necessary for the refrigeration cycle to work. I still doubt that it is ('necessary') for the cycle to 'work' after some sort of fashion, since it seems to me that it probably inevitably will 'work' ('after some sort of fashion') when one starts compressing the gas, regardless its pre-compression pressure.

On reflection, I suspect that the answer/reason is if one were to simply 'fill' the system with refrigerant at atmospheric pressure (inevitably all gas, unless the temp was extremely low), one simply would not get anything like as much refrigerant into the circuit as was needed.

That (the amount of refrigerant in the system) is another factor which we have not been considering, but hich is presumably of some importance.

 

[JohnW2]

I was thinking of what the pressure is when the compressor stops but the freezer is still at -18°. In your scenario my guess is the refrigerant is partly liquid.

As implied in what I've just written to Sunray, is it not inevitable that a fair proportion of the refrigerant in the system will always be liquid - since, otherwise one would only have a very small amount of refrigerant in the system? ....

... I have a 2,000 litre propane tank in my garden. When "full" about 80% of the contents (1,600 L) are liquid and the remainder (400 L) gas. That means that, when "full" its contents can turn into about 432,400 litres of gas at atmospheric pressure (1,600*270 + 400) which, fortunately, lasts us quite a while. If the tank were 'filled' only with 2,000 L of gas, it would probably only last us for '5 minutes' (and, indeed, would presumably need a pump (or some other measure to 'get it out of the tank' if it were stored at atmospheric pressure!)

The instructions say position the thing and leave it 24 hours before switching on, presumably for the liquid to gravitate down. If it were all gas nothing would happen. If that's right, the pressure is the refrigerant vapour pressure at the ambient temperature.

The instructions for most domestic fridges and freezers seem have changed over the years, initially down to 4 or so hours and, most recently, I've seen 1-2 hours. Where an 'explanation' is given, it most commonly seems to relate to the need for 'oil' to gravitate from other parts of the system down to the compressor (which, it seems is always 'at the bottom'?)

 

[fixitflav]

As implied in what I've just written to Sunray, is it not inevitable that a fair proportion of the refrigerant in the system will always be liquid - since, otherwise one would only have a very small amount of refrigerant in the system? ....

As I said in #61, I think there is some liquid refrigerant when the whole thing is at ambient temperature. When the compressor has just stopped so the freezer is at -18° more of it will be liquid..

... I have a 2,000 litre propane tank in my garden. When "full" about 80% of the contents (1,600 L) are liquid and the remainder (400 L) gas.

I know how propane tanks work! It's a liquid at storage pressure and the pressure stays constant (at a given temperature) as the contents are used, until the liquid has evaporated. About 8 bara at 20°. Unlike an oxygen (or hydrogen or nitrogen) cylinder where it starts at 200 bar when full and falls steadily as the gas is used.

The instructions for most domestic fridges and freezers seem have changed over the years, initially down to 4 or so hours and, most recently, I've seen 1-2 hours. Where an 'explanation' is given, it most commonly seems to relate to the need for 'oil' to gravitate from other parts of the system down to the compressor (which, it seems is always 'at the bottom'?)

I said 24 hours from memory, the actual time isn't important. But re-reading my Bosch instructions, it says allow the appliance to sit vertically for about 1/2 hour.
Also says the refrigerant is isobutane (R600a), a hydrocarbon as I surmised in #16.

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?

It also says, below minimum design ambient temperature (+10° in my case) "functional irregularities may occur during automatic defrosting of the refrigerator interior". Maybe that's part of the explanation. I think it means the freezer, which has automatic defrost. The fridge has the usual condensate drainage.

 

[JohnW2]

As I said in #61, I think there is some liquid refrigerant when the whole thing is at ambient temperature. When the compressor has just stopped so the freezer is at -18° more of it will be liquid..

Indeed - but, as I said, unless the pressure 'when the whole thing is at ambient temp' is high enough for an appreciable proportion of the refrigerant to be liquid, there would actually be fairly little refrigerant in the system - and does not the ability of the system to 'move' heat efficiently (at least, the rate at which it can do that) depend upon how much (mass) refrigerant there is?

I know how propane tanks work! It's a liquid at storage pressure and the pressure stays constant (at a given temperature) as the contents are used, until the liquid has evaporated. About 8 bara at 20°. Unlike an oxygen (or hydrogen or nitrogen) cylinder where it starts at 200 bar when full and falls steadily as the gas is used.

I'm sure you do know how they work, but I suspect that some (including some who may be reading our mutterings!) may not.

I was trying to make the point that, like a propane tank, refrigeration appliances presumably contain a mixture of liquid and gas (with the liquid turning into gas 'as needed', whereas the other sorts of gas you mention (oxygen, hydrogen, nitrogen {hence all air} etc.) are stored entirely as compressed gas, presumably because of their very low boiling points at any credible pressure? However,as above, I suppose the major part of my point was that unless there an appreciable proportion of the refrigerant is liquid, there's probably not going to be much total mass of refrigerant in the system.

It also says, below minimum design ambient temperature (+10° in my case) "functional irregularities may occur during automatic defrosting of the refrigerator interior". Maybe that's part of the explanation. I think it means the freezer, which has automatic defrost. The fridge has the usual condensate drainage.

Maybe we're partially getting somewhere, since the potential problem they are mentioning is a lot more benign that the "freezer won't work in an outhouse" story we so often hear being thrown about. Mind you, I have a feeling that that story has been around for a lot longer than 'self-defrosting' freezers, so I'm not sure!

However, this does make me wonder whether (supported by my own long experience of 'no problem') my own suspicions may be right - namely that there is perhaps a fair bit of 'mythology' associated with all the stories/'warnings'?

 

[JohnW2]

Don't know whether you found these sites Design a Refrigeration System - The Engineering Mindset and Refrigerant R-134a

Those are much the same as all the others I've seen (there appear to be countless). I'm not sure what they think is the target audience, but I suspect (but, of course, maybe wrong!) that the great majority of non-Physicists who might read it probably don't even know what entropy and enthalpy actually are - and I've yet to come across any of the articles which attempt to explain that!

 

[SUNRAY]

... that the great majority of non-Physicists who might read it probably don't even know what entropy and enthalpy actually are - and I've yet to come across any of the articles which attempt to explain that!

Oh my giddy aunt, education from 50 years ago and I suspect I've not heard of them since, I suspect this is total bovine excreta but entropy (also spelt entrophy?) something like lack of energy or heat (And I'm struggling for words here) through failure of the process or confusion or something. Enthalpy is something like total energy or heat capacity of a system. B U T P L E A S E don't expect me to have a clue what I'm saying!

 

[JohnW2]

Oh my giddy aunt, education from 50 years ago and I suspect I've not heard of them since, I suspect this is total bovine excreta but entropy (also spelt entrophy?) something like lack of energy or heat (And I'm struggling for words here) through failure of the process or confusion or something. Enthalpy is something like total energy or heat capacity of a system. B U T P L E A S E don't expect me to have a clue what I'm saying!

Quite so - and you say that you are one of the (I would think very very few) people who have had at least some education about those topics, even if 50 years ago. I would imagine that most people have probably not even heard of the words, ever!

I therefore think that you underline the point I was making - that the vast majority of people who go to websites such as you recommended 'to learn' are probably going to have big problem doing that 'learning' since it would require them to understand graphs which have axes labelled 'entropy' or 'enthalpy', and Tables of data with columns labelled with those words, aren't they?

Rather than 'usefully educating', sites that do that seem be going more down the road of trying to 'blind with science'

 

[SUNRAY]

Quite so - and you say that you are one of the (I would think very very few) people who have had at least some education about those topics, even if 50 years ago. I would imagine that most people have probably not even heard of the words, ever!

I have to say that those were 'a blast from the past' and no way in my conscious knowledge, having not attempted to do a google search I have no idea if I am anywhere close. However as a follow up to this I had a dream last night of watching one of my physics teachers drawing graphs on the blackboard:

But again I have no idea if that's BS or what they are or what the axes are

I therefore think that you underline the point I was making - that the vast majority of people who go to websites such as you recommended 'to learn' are probably going to have big problem doing that 'learning' since it would require them to understand graphs which have axes labelled 'entropy' or 'enthalpy', and Tables of data with columns labelled with those words, aren't they?

I don't disagree with that

Rather than 'usefully educating', sites that do that seem be going more down the road of trying to 'blind with science'

I recently had to 'learn' how to use the software to control the lights for the AmDram show, the previous 'tech guy' who had been saying for several shows that he intended to retire from the group - but despite me asking he'd always kept everything very close to his chest - suddenly announced a month before this show that he wouldn't be there any more. (He turned up once to collect the remainder of his kit stored there)

I reckon I spent at least 50 hours on YT and other 'help sites'. The problem with all of them has been the expectation that one has some idea how it works (which isn't the case for a newbie) and one can follow the speed they run at - that may sound silly but one regularly did this sweeping movement with the mouse and clicked on a box as the end of it, however slowing it down to ¼ speed it appeared she clicked on 3 boxes before stopping on and clicking the 4th, which was wrong - it was only 3 but one changed state when clicking on the adjacent box.
The biggest problem I had with it was weeding out who was right and who was wrong as as there are different methods of achieving the same thing depending on where one is working in the software and they all share the same the same 'dashboard' but one can't mix the methods. As an example in one mode a green tick has to be clicked to access something (IE a dimmer setting), in another mode the green tick saves that something (IE the dimmer setting which can then be adjusted after saving it) making it very easy to save an incorrect setting.

However all of them go far too deeply into the software far too quickly, initially All I wanted to do was be able to 'play' previously saved shows to see if any saved scenes would be suitable... Most of this show was based on previous saved scenes but having some murder mystery content an element of accusatory spotlights were required. And of course being AmDram the suspects never stood in the same place twice!! so the spotlights were very hit and miss but with this director one has to do what "I'm the professional director" says.

EDIT: Oh dear waffle modeJust how far can it move from adding power to a shed?

 

[SUNRAY]

...I was trying to make the point that, like a propane tank, refrigeration appliances presumably contain a mixture of liquid and gas (with the liquid turning into gas 'as needed', whereas the other sorts of gas you mention (oxygen, hydrogen, nitrogen {hence all air} etc.) are stored entirely as compressed gas, presumably because of their very low boiling points at any credible pressure? ...

All 3 of those are regularly stored and transported in liquid form.

 

[fixitflav]

All 3 of those are regularly stored and transported in liquid form.

If you mean oxygen, hydrogen, nitrogen, air - only when cooled to below critical temperature, -140°C for air. Propane, eg has critical temperature 96°C so can be liquefied by pressure alone.
In a freezer, the refrigerant must be liquid at the P and T in the condenser (heat dump grid), and gas at the P and T in the freezer compartment.

 

[JohnW2]

If you mean oxygen, hydrogen, nitrogen, air - only when cooled to below critical temperature, -140°C for air.

Quite so, and a lot lower (about -240°C) for hydrogen (and, at about -268°C, pretty close to absolute zero for helium).
For anyone who doesn't know, the 'critical temperature' is the temperature above which it is impossible for a substance to exist as liquid, at any pressure.

Also probably worth saying for the sake of some readers .... when using them as 'fuels' (to burn), at atmospheric pressure the boiling point of propane (about -42°C) is much lower than that of butane (about -0.5°C) - which means that although propane is fine for the purpose, butane is not suitable for use in a significantly cold ('sub-zero') environment (e.g. 'outdoors'), since the stored liquid won't turn to gas.

Propane, eg has critical temperature 96°C so can be liquefied by pressure alone.

Again, quite so - and the point (distinction from the other gass) I was making.

In a freezer, the refrigerant must be liquid at the P and T in the condenser (heat dump grid), and gas at the P and T in the freezer compartment.

Exactly - which is why I have been repeatedly saying (essentially agreeing with you) that the choice of refrigerant is probably pretty importantl - and why I suspect that the same refrigerant would not be 'appropriate' (well, at least not 'ideal') for a freezer and for aircon, given that the target temps in the 'evaporator' are so different in those two scenarios.

 

[SUNRAY]

If you mean oxygen, hydrogen, nitrogen, air - only when cooled to below critical temperature, -140°C for air. Propane, eg has critical temperature 96°C so can be liquefied by pressure alone.

How do you think they cool the gasses then? Particularly Hydrogen to 30°K (hopefully correctly remembered from school)

In a freezer, the refrigerant must be liquid at the P and T in the condenser (heat dump grid), and gas at the P and T in the freezer compartment.

 

[SUNRAY]

Quite so, and a lot lower (about -240°C) for hydrogen (and, at about -268°C, pretty close to absolute zero for helium).
For anyone who doesn't know, the 'critical temperature' is the temperature above which it is impossible for a substance to exist as liquid, at any pressure.
....

Ah this is still going back to my school education (where it seems I may be wrong) but I recall it as using a membrane filter to separate the hydrogen and a compressor similar to a jet engine to compress it sufficiently to turn it to liquid.


I must remember to check for further replies before posting.