PDA

View Full Version : No Pressure drop in condenser







mrr
25-10-2016, 07:15 AM
Hi,

I have an air-cooled condenser with the following sensors:
1- Pressure sensor at the discharge line of the compressor, which is considered the input pressure of the condenser.
2- Pressure sensor on the top of a liquid line receiver, which is considered the output pressure.

The condenser is working in a vapor compression system with R134a.

I would expect some pressure drop. However both sensors give almost the same pressure value. My questions are:

- Is it normal to have almost zero pressure drop or is this indicative of non-condensables or another problem? Have experienced people seen this before?
- Having the pressure sensor on the top of the liquid line receiver (where there is a mixture between liquid and vapour) might give a different pressure value?

Regards

HVACRsaurus
25-10-2016, 08:09 AM
Howdy,

The pressure drop would "normally" be so small, that it would be "normal" to see the same sensor values.

Happy tinkering.

chemi-cool
25-10-2016, 03:19 PM
Hi,

I have an air-cooled condenser with the following sensors:
1- Pressure sensor at the discharge line of the compressor, which is considered the input pressure of the condenser.
2- Pressure sensor on the top of a liquid line receiver, which is considered the output pressure.

The condenser is working in a vapor compression system with R134a.

I would expect some pressure drop. However both sensors give almost the same pressure value. My questions are:

- Is it normal to have almost zero pressure drop or is this indicative of non-condensables or another problem? Have experienced people seen this before?
- Having the pressure sensor on the top of the liquid line receiver (where there is a mixture between liquid and vapour) might give a different pressure value?

Regards

Condenser is all about rejecting heat.
Pressure is almost the same from discharge valves to the TXV inlet.

mrr
26-10-2016, 08:28 AM
Hi,

Thank you HVACRsauris and chemi-cool,

I was reading this thread: http://www.refrigeration-engineer.com/forums/showthread.php?6017-Optimal-pressure-drop-in-condensers-and-evaporators , and I was thinking that some pressure drop should happen. However, I tested inlet and outlet pressures and I read the almost the same value.

I was wondering if in the liquid receiver a small "raise" of pressure could happen (the outlet sensor is on the top part of the liquid line receiver, after the liquid line part which outs from the condenser).

Regards,

mbc
26-10-2016, 09:14 PM
hi
certainly is drop pressure in condenser
and that is depend of condenser design . mostly about 2 to 5 psi.
might you sensor could not read this rang

Grizzly
26-10-2016, 09:55 PM
Sorry mbc.
I have to disagree!
I am used to seeing a pressure drop across an evap, but not across an condenser.
However condensers would operate at high temps where you are than our European (35 - 40C) Range
Although I might be reading the original post wrong!
Grizzly

cduque
27-10-2016, 10:56 AM
Hi,

I think that the question is not if there exists a pressure drop in a condenser (of course that with a flow always exists a pressure drop) but if that pressure drop is "almost zero" or "noticeable” and easily measurable.

Manufacturers don't usually inform us of this value but in the few information that I could collect I was able to see a type of condenser (NH3) with 5 kPa (0.05 bar or 0,7 PSI) of pressure drop at full charge. You can see that it is quite small and normal manometers or sensors may not identify clearly this differences.


In "real life" what really matters is the temperature difference equivalent to that pressure drop, and it is quite different depending on the fluid, for instance ASHRAE defines that for the discharge line from the compressor to the condenser it is admissible to have a 0.5C for 30 m (1F for 100 ft). The consequence for various fluids is:

NH3 (at 35C) – 19.3 kPa
CO2 (at 5C) - 50.5 kPa
R134a (at 45C) - 15.0 kPa
R407C (at 45C) - 22.0 kPa
R410A (at 45C) - 31.6 kPa

You can see that only for this run you can have four times the pressure drop of that condenser.

So the pressure drop at the condenser always exists but normally is neglectable and not easy to measure.

CDuque

mbc
28-10-2016, 11:17 AM
Hi Grizzly

OK what is that reason of moving gas from inlet to out let in condenser

there is drop pressure in every pipe is carrying gas - liquid

also it is one of the our function , when we are calculating our pipe size

mbc
28-10-2016, 12:06 PM
14396


hi in this picture you could see drop pressure in condenser

Grizzly
28-10-2016, 06:10 PM
Hi mbc.
Fair point, I was having a bad day and got it wrong.
You are correct.
There is a pressure drop. Just one I have never had to consider.
Grizzly

NH3LVR
28-10-2016, 08:17 PM
Let us consider if the Condenser has a equalizing line, running from the top of the receiver to the top of the Condenser. Then there would be no pressure drop.

RANGER1
29-10-2016, 01:06 AM
http://www.evapco.com/sites/evapco.com/files/131a_-_piping_brochure.pdf

Not sure what mbc referring to , as the Bitzer document is for thermosyphon oil cooling.

Segei
29-10-2016, 02:52 AM
Pressure drop always exist. Gas and liquid move because pressure in one part of the pipe is greater than in another part. Only question is how great is pressure drop. Equalizing line reduce pressure drop but not eliminate it. However, gas and liquid can be moved by gravity.

mbc
29-10-2016, 09:01 PM
HI
I was looking to some document to show for drop pressure in condenser. I saw that.
yes it is bitzer document for thermosyphon I was designing for our project in blast freezer.

Mr. segie
Equalizing line do this work

in winter when it is open reduce capacity of condenser ( liquid stay in condenser and reduce our heat surface area )
during liquid comes from condenser to receiver volume of that should go to condenser .

RANGER1
29-10-2016, 09:54 PM
HI
I was looking to some document to show for drop pressure in condenser. I saw that.
yes it is bitzer document for thermosyphon I was designing for our project in blast freezer.

Mr. segie
Equalizing line do this work

in winter when it is open reduce capacity of condenser ( liquid stay in condenser and reduce our heat surface area )
during liquid comes from condenser to receiver volume of that should go to condenser .


Page 21 is one way, thought whole Danfoss book was worth posting for general knowledge.

http://files.danfoss.com/TechnicalInfo/Dila/01/PA000C202.pdf


This is more conventional

http://www.achrnews.com/articles/93556-controlling-condenser-pressure

Balance line generally used on larger systems with more than one condenser.

mbc
30-10-2016, 04:09 PM
Hi Ranger

Thanks for your post.

sterl
02-11-2016, 05:52 PM
If your sensors are valve isolated: Swap them. This will indicate how much error you are dealing with....

And Segei is right: the reason that the drain legs on a gravity drained condenser-receiver arrangement are trapped and is to accumulate enough liquid head to overcome the pressure difference between an active and an inactive condenser circuit connected in parallel. So you will only read the pressure difference on the active coil if you read pressure above the liquid downcomer. And of course, the inactive circuit(s) will have no pressure difference at all....

With equalizing HPR to discharge main of sufficient size, every other location on the high side will read the same as the discharge main where it delivers to the condensers. Unless, of course, you invade the active condenser coil itself with a pressure instrument.....There may be a detectable difference between the compressor outlet and the condenser inlet....

botrous
26-02-2017, 11:34 PM
Hi Guys ,

Pressure drop in the condenser in determined by the condenser design, number of feeds (inlets/Outlets) and the subcooling circuit.

More feeds = less pressure drop = less capacity
Less feeds = more pressure drop = more capacity

The designer tries to balance the above equations to get the most of the condenser.
And note that the pressure drop is calculated based on 35C entering air temperature to the condenser, changes in temperature will cause a slight change in the effective pressure drop taking place.


Regards