Condensing preasures?

[Rowan]

Hi, sorry for my ignorance but could somebody please explain.
I was recently at an interview and were discussing the basic fridge circuit when the questions started about condensing presure/temps/ambient temp. The theoretical system had a ambient temp of 25'c and ran on r404a. The interviewer then asked 'what presure and temp would you see coming out the condenser?'. My answer was 'hopefully 12 bar and 26'c in a purfect world'. This was wrong and you should aim for 15'k above ambient (@ 17bar & 40'c) as a rule of thumb. Could somebody please explain the rule of thumb as i have always assumed lower heads meant less work for the comp and better economy. Or have i just totaly misunderstood the statement?
Regards Rowan:)

[marc5180]

I dont really work much with 404a but as a rule if a system is working ok and has got down to temp,Not too sure about pressure but i would expect to see the condensing temp between 38-42 DegC.

[US Iceman]

If the ambient dry bulb temperature is 25°C and the condenser is designed to handle the necassary heat rejection of the system at a 15k TD, then the condensing temperature is 25+15 = 40°C, so the pressure would be whatever it is for the saturation temperature of 40°C for the specific refrigerant in question.

You are correct on the rule-of-thumb. As the discharge pressure decreases the energy input required by the ompressor is reduced. Using the example above... If the ambient air temp. falls to 20°C, then if you add 15k to this, the new condensing temp. is 35°C.

The trick is to have the system work at low condensing temperatures as the ambient temperature decreases (for air-cooled systems).

Hope that helps.;)

[Rowan]

If the ambient dry bulb temperature is 25°C and the condenser is designed to handle the necassary heat rejection of the system at a 15k TD, then the condensing temperature is 25+15 = 40°C, so the pressure would be whatever it is for the saturation temperature of 40°C for the specific refrigerant in question.


The trick is to have the system work at low condensing temperatures as the ambient temperature decreases (for air-cooled systems).

Hope that helps.;)

Would it not be better to size the condenser to work at a lower TD then 15K??
Just in my mind this would reduce head/work load or is this not possible?
My question is sort of why 15K above surely this could be reduced??
Cheers Rowan

[Billy Ray]

Rowan,

Yes off course it would be better for a lower TD.

Therefore this means a larger condenser.

This means more cost on installation. Generally end users (customers) will look for the lowest quotation, therfore alower TD is more expensive to install.

Unless requested, companys will tend to offer in the region of 15K TD.

As Iceman points out, Ambient + condenser TD = condennsing temperature. Assuming worst case senario, 35 + 15 = 50 deg C, thats getting pretty high discharge pressures etc & thats based on a new clean condenser!!

Note: There comes a point at a reasonable minimun TD.. This i would suggest is 8K TD.

Billy Ray

[Rowan]

Rowan,

Yes off course it would be better for a lower TD.

Therefore this means a larger condenser.

This means more cost on installation. Generally end users (customers) will look for the lowest quotation, therfore alower TD is more expensive to install.

Unless requested, companys will tend to offer in the region of 15K TD.

As Iceman points out, Ambient + condenser TD = condennsing temperature. Assuming worst case senario, 35 + 15 = 50 deg C, thats getting pretty high discharge pressures etc & thats based on a new clean condenser!!

Note: There comes a point at a reasonable minimun TD.. This i would suggest is 8K TD.

Billy Ray

So the 15k rule is more of a practical/costing rule than an ideal. Just couldnt understand while you would want to run 15'k above ambient in an 'ideal world sustem'.
Surely in this day of enery efficency/high energy costs the 15'k rule is out of date?
Your views would be much appreciated, sapose it depends on your enery costs?!
Cheers Rowan:rolleyes:

[Billy Ray]

Yep, 15K TD you can consider a general guidline.

Remember 15 KTD is OK, untill the ambient gets above 30 degC or so.

Then the poor sytstem will struggle to achieve setpoint.

So 51 weeks of the year its works, one week you get plenty of service calls.

Billy Ray

[US Iceman]

For the point of discussion you could use 5K.

The problem becomes the size of the condenser to accomodate the heat rejection required. As the size/cost of the condenser increases you need to compare it to the cost electricity saved to see if it is economically sensible.

At some point you will find you are using more fan power on the condenser than what you save on the compressor. That would be hard to explain away to a customer.:eek:

[PaulZ]

Hi Rowan
Another thing to consider if you drop the discharge pressure is the capacity of the TX valve. The reduced discharge would mean a bigger TX to get the capacity. Have a look at a TX selection chart and see how much the capacity of a valve changes as the pressure drop decreases. This also would increase the initial install costs.
I would suspect over the years it has been found 15k is the most economical when all things are considered.
Paul