Ambient and Condensing.

[kfjoe]

Just getting myself a little confused with the ambient/condesing/liquid level , and wondering if somone could perhaps help clear it up for me ?

The Ambient temp affects the condesing Temp , so if the ambient increases my condensing temp and pressure increase.: But there seems to be an increase in the liquid in the receiver, does this mean more of the condenser is filled with vapour (volume) so more liquid is pushed into the reciever ?

And on low ambients is there more Liquid in the coil and so less in the reciever ?

If the ambient changes and therefore the condesing pressure changes , doesnt the TD stay the same , so surely there is the same amount of heat rejection?

Would appreciate any help thanks
and apologise if I have got this totaly wrong.

Thanks

[Prof Sporlan]

If Marc's musings are less than clear to you, the Prof can be more succinct.

Your thinking ain't too bad...

[kfjoe]

Thanks for the reply Marc.
And I would certainly appreciate your explanation too Prof.

One of the things that is confusing me is the condensing temp / ambient relationship.
I understand that the condensing Temp is the product of the condensing medium(ambient) + the Delta T across the heat transfer surfaces .
But is it a case of the Pressure having to build up first and therefore the Temp , to a satisfactory temperature difference so that the refrigerant condenses?
And if the ambient was to rise or fall would this change the rate of heat transfer ? as the condensing temp(saturation) would alter too , so wouldnt the TD be the same and the rate of transfer be the same ?

I am basically trying to understand the effect it has in the condensor, would appreciate anyones input

Thanks .

[Peter_1]

I'm interested too Marc and have no access neither.
PLease mail me the webiste, perhaps I can make a subscription for it (if they will send it to Belgium)
Once paid, is downloading old articles possble?

[kfjoe]

Unfortunatley not.
Although I must subscribe for future issues.

[RogGoetsch]

Originally posted by kfjoe

One of the things that is confusing me is the condensing temp / ambient relationship.
I understand that the condensing Temp is the product of the condensing medium(ambient) + the Delta T across the heat transfer surfaces .
But is it a case of the Pressure having to build up first and therefore the Temp , to a satisfactory temperature difference so that the refrigerant condenses?
And if the ambient was to rise or fall would this change the rate of heat transfer ? as the condensing temp(saturation) would alter too , so wouldnt the TD be the same and the rate of transfer be the same ?

I find it helps to think of a refrigeration system as an equilibrium system at steady state, then from there look at changes.

Imagine your system is operating with unvarying conditions. From the condenser standpoint, this would mean that heat is being rejected from the air side of the condenser at the same rate it is entering from the refrigerant side. The condensing pressure and temperature would be at saturation at some TD over ambient.

If the ambient temperature were to change, the system would adjust until it reached a new equilibrium point. That is: until heat into the condenser again matched heat out. If the ambient temperature declined, we would expect heat flow out of the condenser to exceed heat entering and the surface temperature to begin immediately to fall. Hot gas impinging upon this surface would condense more rapidly, and saturation pressure would drop as the gas cooled and condensed more quickly.

But since it is the whole system in equilibrium, the rest of the system would adjust as well. With a lowering condensing pressure, the operating point of the compressor would shift along its performance curve and it would gain a bit in efficiency, slightly reducing the heat of compression, further reducing the heat into the condenser.

The final operating equilibrium point would be different in most aspects, and TD across the condenser would be no exception.

For a graphical illustration, take a pressure-enthalpy diagram for a hypothetical system, change the condensing temperature and redraw. It becomes more realistic when you add in adjustments for pressure drop, non-isentropic compression, subcooling and superheat, topics covered in almost any good refrigeration text in the P-h diagram chapter.

I also look forward to reading Marc's articles. (Haven't found them yet.)

Rog

[chemi-cool]

peter,

the web address is: http://www.achrnews.com/
to read the good stories, you will have to subscribe to the magazine and use your subscription no. to get access.

an american way to take your money. its not so cheap and the magazine could be better.

chemi

[Gary]

the web address is: http://www.achrnews.com/
to read the good stories, you will have to subscribe to the magazine and use your subscription no. to get access.

Nope, Marc writes for the ACR News. That link is for the ACH&R News, the rag I used to write for.

[rbartlett]

marc will have them on his pc so i'm sure he could email them to you..if he found time etc..;-)

cheers


richard

[Dan]

One of the things that is confusing me is the condensing temp / ambient relationship.
I understand that the condensing Temp is the product of the condensing medium(ambient) + the Delta T across the heat transfer surfaces .
But is it a case of the Pressure having to build up first and therefore the Temp , to a satisfactory temperature difference so that the refrigerant condenses?
And if the ambient was to rise or fall would this change the rate of heat transfer ? as the condensing temp(saturation) would alter too , so wouldnt the TD be the same and the rate of transfer be the same ?

I am basically trying to understand the effect it has in the condensor, would appreciate anyones input

Thanks .

The condensing temp/ambient relationship is simple... it works basically the same for evaporators and condensers. Heat moves from warmer to colder. If the warm thing is 1 degree warmer than the cold thing, it will move at a certain rate. If the warm thing is 2 degrees warmer than the cold thing, it goes twice as fast... and on and on. Simplistic, yes... but a great basic way to look at heat transfer.

Condenser TD's, for example. If you have a condenser sized for a 20 deg TD to achieve 20,000 btu'hr rejection, you double its size or double its airflow and achieve the same heat transfer with only a 10 deg TD... Thus lowering the saturated condensing temperature by 10 degrees. You could also double the airflow and reduce the size of the condenser by half and maintain the same duty.

In a practical sense,you will find that medium temperature condensers will be sized for a higher TD than low temperature condensers. There is a certain arbitrariness to this custom, but also a certain sense to it.

A condenser sized to do 20,000 btu/hr rejection at 20 degrees TD (typical air conditioning TD) will be half the size of a condenser sized to do 20,000 btu/hr rejection at 10 deg TD (typical low temperature TD). Half the cost, too.

But the difference between operating at a 120 deg condensing temperature compared to operating at a 110 deg condensing temperature is not a large power penalty at the lower compressor ratios of an a/c unit, yet is considerably more significant at the high compressor ratios of low temperature duty.

Argumentum adsurdum: You can maintain a freezer with a tiny heat exchanger operating 1000 deg TD and it could be the size of a marble with a really good blower, or you could maintain the same freezer with walls wrapped around pipe at 1/2 a deg TD.

The thing to keep in mind, is that TD translates into a higher condensing temperature relative to ambient in the condensing stages, and translates into a lower evaporating temperature relative to refrigerated space in the refrigerating stages.

In either case, larger TD's get more done with less surface(initial cost), but expend more energy to get the job done(operating cost.)