Saturated Discharge Temperature

[nh3simman]

We refer to SST and SDT, being the saturated suction and discharge temperatures.

SST is the temperature of the vapor line at the suction pressure.

SDT should be the temperature on the vapor line at the discharge pressure.

But, with refrigerant temperature glide and component pressure drops, I'm not sure of the strict definition.

Can anyone give some clarify?

[lana]

Hi there,

SST is the evaporating temperature and SDT is the condensing temperature.

These temperatures are constant for "normal" refrigerants but they are not constant for Zeotropic blends. This causes major uncertainty for calculating the evaporator and the condenser.

In practice we need SST and SDT to measure superheat and sub cooling. Here we assume that the refrigerant dew point at suction pressure is SST and refrigerant bubble point at condensing pressure as SDT.

I completely understand your concern and there is nothing we can do about it.

Cheers

[NoNickName]

Being "saturated" in the boiling process it is equivalent to dew point of a zeotropic blend.
Being "saturated" in the liquefaction process it is equivalent to bubble point of a zeotropic blend.

[US Iceman]

I normally use SCT for the condensing temperature as discharge temperature is a separate point describing another condition. Likewise, SET is the saturated evaporating temperature.

You could say: SDT - SCT is the equivalent temperature loss for the discharge piping, which is similar to SET - SST is the equivalent temperature loss for the suction line piping. However, I tend to not like the term SDT as then people seem to forget what the actual discharge temperature is used for.

Actual discharge temperature - SCT = discharge superheat, which provides a value used for evaluating screw compressor operation or the degree of reasonable compression or oil injection or liquid carryover.

I think it is important to not only use the saturated temperatures but the actual temperatures as well to help make these distinctions.

[nh3simman]

I have actually been mis-using these terms in the past.

If you plot the cycle on the p-h diagram, I have assumed that the intersection of the evaporator process with the vapor line was the SST and the intersection of the condenser process with the liquid line was the SDT.

After writing the question and reading your comments, I realize why the definitions of SST and SDT have come about in the first place. Clearly from the refrigerant gauge set.

The gauge measures the pressure at the compressor suction and discharge. These gauge pressures can be interpreted as temperature. But, as lana points out, the question is complicated by glide.

So, do we use the vapor curve to get the define the SST and SDT or do we now have to specify DEW and BUBBLE point to be clear in the case of glide?

[lana]

Hi there,

If you plot the cycle on the p-h diagram, I have assumed that the intersection of the Evaporator process with the vapor line was the SST and the intersection of the Condenser process with the Liquid Line was the SDT.

In the two pahse region (P-h) , the temperature is constant. SO the evaporation and condensation precesses are happening with constant temperatures.

I don't understand why you confuse these temperatures ? OR DO YOU ?

SST = Saturated Suction Temperature = Evaporating Temperature = te
SCT = Saturated Condensing Temperature = Condensing Temperature = tc

Now when there is Glide then in the two phase region the temperature is not constant. So the Dew and Bubble definitions come to the game.

[US Iceman]

SDT = Saturated Discharge Temperature = Condensing Temperature = tc

I disagree with this statement. Sorry lana.

If you take the discharge pressure and convert it to saturation temperature, all you have is the equivalent temperature at the discharge pressure which is not the same as the condensing pressure/temperature relationship.

A subtle but important point.

[lana]

I beg your pardon.
I corrected the statment. It is not discharge, it is condensing.
Thank you for that.

[US Iceman]

I hope my statement was not taken as an offense lana. It was only my desire to point out a subtle topic often taken as meaning the same thing.

[lana]

NON taken.

But on second thought I think the statement:

"Saturated Discharge Temperature" is the same as the " Condensing Temperature". And that is because we say "Saturated".
On the P-h diagram the actual discharge temperature is point 2 which is after the compressor and its saturated temperature is the same as the condensing temperature.
Cheers

[US Iceman]

On the P-h diagram the actual discharge temperature is point 2 which is after the Compressor and its saturated temperature is the same as the condensing temperature.

True, since it is on the same isotherm. However, if you include the pressure losses in the piping then all of it changes slightly.

It is the pressure loss which changes the equivalent temperature losses that can affect component performance.

I think this is similar to some who use the glide temperatures as a benefit in selecting heat exchangers.

I believe NoNickName was on the right track when he said we should be using the dew and bubble points. Regardless of the refrigerant type the use of these terms should help to keep the focus on the proper criteria.

[lana]

It is good to be corrected by a friend than be embarrassed in the field.

Thanks a lot.

[The MG Pony]

It is good to be corrected by a friend than be embarrassed in the field.

Thanks a lot.

Now that is sig worthy!

[nh3simman]

I suspect that the terms SST and SDT will fall out of use because they are not clear when there is glide.

I agree, "Bubble" and "Dew" Point can not be confused.

[TXiceman]

NON taken.

But on second thought I think the statement:

"Saturated Discharge Temperature" is the same as the " Condensing Temperature". And that is because we say "Saturated".
On the P-h diagram the actual discharge temperature is point 2 which is after the compressor and its saturated temperature is the same as the condensing temperature.
Cheers

I use SDT as the equivalent temp/pressure at the compressor flange as most manufacturers rate the compressors as flange to flange. I use SCT as the condensing temp.

So in stating a rating you could say:
SST= 25 dF
SET= 27 dF
SDT = 112 dF
SCT = 110 dF
With this you are allowing the equivalnet of 2 dF line line loss in the suction and the discharge.

Ken

[US Iceman]

So in stating a rating you could say:
SST= 25 dF
SET= 27 dF
SDT = 112 dF
SCT = 110 dF
With this you are allowing the equivalent of 2 dF line line loss in the suction and the discharge.

That's exactly the same way I try to state these. The only ambiguity that remains is the actual liquid feed temperature, which could be stated as:

SCT - subcooling = Liquid feed temperature

And,

We also have the superheat for either evaporator, suction , or discharge.

I think it's important to delineate the exact point in the system with the correct terminology so I'm in complete agreement with Ken.

[nh3simman]

I use SDT as the equivalent temp/pressure

But that's exactly the problem. At the compressor discharge, you measure a pressure. If you look at the p-h diagam at that pressure, there are 2 temperatures at that pressure bubble and dew.

So you can't just quote SDT because both bubble and dew are saturated at the same pressure.

[US Iceman]

The SDT should be slightly higher in temperature because the discharge pressure should be slightly higher than the condensing pressure and the resultant SCT.

Therefore the SCT might be 35C for ammonia while the SDT should be around 36C or slightly higher. At the same time the actual discharge temperature might be close to say 110C (for a recip. compressor).

The problem with using only temperatures is that we need a way to describe their relative location on P-H diagram.

It's almost like we are dealing with glide temperatures, except they are only for the resulting temperature loss due to pressure loss. How's that for muddying up the waters?

[nh3simman]

The SDT should be slightly higher in temperature because the discharge pressure should be slightly higher than the condensing pressure and the resultant SCT.

Pressure drop is a different problem since pressure is a point function.

If you measure the pressure at the compressor discharge, it has a value that we relate to saturated conditions (irrespective of the line pressure drop). Just draw a horizontal line on the p-h diagram. It cuts the saturation at 2 points. When there is glide, these two points will have different temperatures.

Strictly in terms of definition, this means that we can't just say the SDT is x because you would not know if it was referring to the bubble of dew point.

This is why I think that SDT is a poor term.
SBP = Saturated Bubble Point
SDP = Saturated Dew Point
would be more appropriate.

[nh3simman]

We seem to have problems coming to agreement on some of the basic terms.



http://tinypic.com/view.php?pic=4hl78k5

The image above shows that the line pressure drops are not part of the discharge pressure reading.

[US Iceman]

I tried to open your link, but for some reason it would not open.

I think what we are seeing is getting tangled up in knots over semantics.

The problem by definition will not produce pure horizontal lines through the 2-phase region. They are tilted.

The discharge pressure at the compressor discharge service valve must be higher than the condensing pressure. Otherwise, we are discussing a pure theoretical process with no pressure losses. The only way this would really occur is fictitious.

If we do not include line losses the system is not operating, i.e., no flow, no capacity.

In practice, the discharge pressure is higher than the condensing pressure, just as the evaporating pressure will be greater than the suction pressure (at the compressor). If we are trying to convey this operation with saturated temperatures as the descriptive values this is what we would see.

SDT > SCT
SET > SST

This simple fact when plotted on a P-H diagram would imply the actual plots do not follow the isotherms. In fact, they would look like "an equivalent glide temperature" as a deviation from a theoretical cycle.

[wkd]

Dew point refers to the refrigerant in a gaseous state and since we require superheat at the compressor inlet when measuring the pressure at the compressor we should refer to dew temperature,the same applies to discharge gas leaving the compressor as this is superheated also.
I agree the confusion comes when in the saturated state however for both evaporators and condensers we can measure the pressure and then take a mean temperature of both dew and bubble points known as mean pressure.You will find most heat exchanger manufacturers refer to mean pressure and you can get some compressor manufacturers that refer to mid-point temperature,copeland select 6 comes to mind as this software enable you to size an evaporator based on expected line losses.
Hope this helps even if only a little.

[nh3simman]

...refer to mid-point temperature, copeland select 6 comes to mind as this software enable you to size an evaporator based on expected line losses...

Hi wkd, That's all we need now. Another definition of temperature.


USIceman, As I posted that comment, I knew you were going to refer to pressure drops.

But, you are not getting my point.

When you measure the pressure at the compressor discharge port, you are reading a single value.

The horizontal line on the P-h diagram is just to read the scale. It also indicates that at that pressure there are 2 temperatures (bubble- and dew-) that we use as indicators only.

Pressure drop doesn't come into the picture yet. We really need to get agreement on a single definition first before we get into the complications of pressure drop.

[NoNickName]

For me, the word "saturated" is meaningful and distinguishes the use of bubble and dew depending on what phase-change is in process.
For a liquefaction (condensation), saturated is bubble. For boiling (evaporation), saturated is dew.

[binman54]

According to Dupont at the site http://www.refrigerants.dupont.com/S...pdf/h45938.pdf
Saturated vapour is the dew point at the outlet of the evaporator. This would assume no superheating in the exchanger.
Saturated liquid is the bubble point at the outlet of the condenser. This would assume no subcooling in the exchanger.
The saturation point occurs when the refrigerant is changing state, boiling or condensing. This can also be called the co-incident pressure/temperature point. That's easy for R22 as there is no glide. So the condenser/evaporator pressure can be directly converted to temperature, as they do not change during the condensing or evaporating process.
With the 400 series refrigerants we still use SST and SDT because they are convenient.
Be careful when you use the handy charts. Dew point is now the SST and is measured at the outlet of the evaporator. Bubble point is the SDT and is only measured at the outlet of the condenser.
We often get people reading the pressure at the condenser inlet and converting that to temperature using the charts.
Better still throw away the charts and use the electronic temperature probes. They are cheap and reliable.

Binman54

[TXiceman]

I think we are overly com=licating this whole issue. The A/C folks and the refigeration folks speak a slightly different language when it comes to ratings. One thing to do here is get away from 400 series and the glide issues for this discussion. (I hate 400 series).

Lets look at a R717 compressor rating.

The namufacturer rates the compressor bare flange to bare flange so SDT is the ame as SCT as far as they are concerned. Rate your compressor for 100 dF condensing temperature and you know the compressor is rated at 211.9 psia and no discharge losses. To allow for discharge losses, rate the compressor for 101 dF SDT. This is th epressure equivalant to saturation at 101 dF or 215.2 psia which allows for 3.3 psi line loss to the cendenser which is operating at 10 dF CT or SCT.

With the SDT of 101 dF we do not know where the actual discharge temperature lies.

To do a preliminary design on a compressor I'd select it for 101 SDT and say 30 SST, but the system would design for 100 dF SCT and 32 dF SET. This allows for the equivalent of 1 dF discharge line loss (3.3 psi) and 2 dF equivalant suction line loss (2.55 psi). This also does not indicate any superheat in the suction.

Anyway, that is my nickles worth.

ken

[US Iceman]

The horizontal line on the P-h diagram is just to read the scale. It also indicates that at that pressure there are 2 temperatures (bubble- and dew-) that we use as indicators only.

If we consider the refrigerant as one not having a glide, there will be only one temperature on each isotherm. In other words, the dew point and bubble point are the same for each isotherm.

If the refrigerant has a glide, the deviation from a saturation temperature (re isotherm) is the offset due to the glide and would require reference to either the dew point or the bubble point. I think this is the way it is shown in the RefProp program. Anyway, that's my understanding.

TXiceman brings up some good points also. So far we have been discussing the nature of the refrigerants whether in a operating system or a theoretical cycle.

The real problem is being able to arrive at a common language to ensure proper application of components. This can be a whole new thread unto itself as there are differences between commercial refrigeration and industrial refrigeration or HVAC.

[Dan]

I vote for using "SCT" saturated condensing temperature, and against using "SDT". I don't think "discharge" adds anything but confusion. I also prefer "saturated evaporation temperature to saturated suction temperature. Regardless, the dew point and bubble point thing has always confused me. I think of "dew point" as when liquid condenses, and "bubble point" as when liquid boils. So why do we use dew point to judge the evaporation process and bubblepoint to judge the condensing process?

[US Iceman]

So why do we use dew point to judge the evaporation process and bubble point to judge the condensing process?

I have always referred to the definitions as you stated them Dan. But, the above comment kind of throws me.

The dew point curve is on the right side of the p-h diagram while the bubble point is on the left side. Everything to the right of the dew point curve is superheated, while everything to the left of the bubble point curve is sub-cooled liquid. And, everything between the two lines is a two-phase mixture of varying degrees of liquid and vapor.

I can't say I have ever heard of the the description you provided in the quote above. Has anyone else heard this before?

[Dan]

I can't say I have ever heard of the the description you provided in the quote above. Has anyone else heard this before?

I just think it odd.. almost backwards, and there is probably a reason that escapes me... to describe the important temperature/pressure measurement of the evaporation process as "Dew point" and the important temperature/pressure measurement of condensing as "Bubble point."

I have no problem remembering which is important. I just remember that they are intuitively backwards. But I bet there is a good reason out there that I am missing.

[US Iceman]

Here is some interesting info... This is a little bit of high brow reading material but it might be useful to some.

http://lorien.ncl.ac.uk/ming/distil/distilpri.htm

http://en.wikipedia.org/wiki/Vapor-liquid_equilibrium

http://www.wlv.com/products/databook...ta/db3ch15.pdf

This one has some decent information also (see page 10 for a description of bubble and dew points)
http://www.icorinternational.com/lit...e/PDF/tmes.pdf