Question about Bubble/Dew Points

[gfgordon]

Hello.

I am a student currently attending a refrigeration technical training class in Las Vegas, and I'm confused by an apparent contradiction I have found between various sources about using Bubble Point and Dew Point values.

In our textbook, Heating and Cooling Essentials by Jerry & LaDonna Killinger, Chapter 14 discusses zeotropic blends, fractionation and temperature glide, etc.  In that chapter, Bubble Point is described as the saturated liquid pressure, or the pressure/temperature at which the first bubble of vapor appears in a liquid refrigerant.  Dew Point is described as the saturated vapor pressure, or the pressure/temperature at which the first drop of liquid condenses from a vapor.

The point in the system where a saturated liquid refrigerant begins to boil into a vapor is in the evaporator, on the low-pressure side of the system.  The saturated refrigerant changes state to become a superheated vapor before going into the compressor.

The point in the system where a saturated vapor refrigerant begins to condense into a liquid is in the condenser, on the high-pressure side of the system.  The saturated refrigerant changes state to become a subcooled liquid before going to the metering device.

So, in essense, Bubble Point refers to superheating and Dew Point refers to subcooling.  A simple internet search for "zeotropic blends" led me to a number of sites discussing this topic, and offering the same explanation for Bubble Point and Dew Point.

Now, here's where I'm getting confused.

Our classrooms have wall-mounted pressure-temperature charts produced by Sporlan, and we are given pocket-size PT charts also produced by Sporlan.  On the bottom of these PT charts is a line which reads, "To determine superheat, use Dew Point values.  To determine subcooling, use Bubble Point values."  Sporlan's web site also says this, as does the PT chart at ICOR International.  This appears to me to be the complete opposite of the definitions described in our textbooks and on the various web sites I've looked at.

My instructor in the refrigeration class initially explained Bubble Point and Dew Point the same way I understood them from reading the text, and indicated that Bubble Point is for superheat and Dew Point is for subcooling.  However, when I questioned the apparent contradiction on the Sporlan PT chart, he consulted with another instructor who said that the statement on the PT chart is correct and the textbook is wrong.

I'm stumped.  Can anyone please help me understand this?

[Admin]

You're not alone, it's all quite confusing.  Both the Sporlan PH diagram and your textbook are correct.

Here's the Sporlan PH diagram that tells us to use Dew Point values to determine superheat and Bubble Point values to determine subcooling.




Here's an example of a refrigeration cycle, showing us where to find the superheated and subcooled regions.




The Dew point refers to when the last drop of liquid evaporates and any temperature rise afterwards is called superheat.

The Bubble point refers to when the last bubble of vapor condenses and any temperature drop afterwards is called subcooling.

[gfgordon]

Thank you for responding.

I'm still confused, though.  I don't understand how the Sporlan chart and the textbook can both be correct when they seem to be saying opposite things.

Superheat is taking place in the low side.  Liquid refrigerant at its saturation point in the evaporator begins to boil off into vapor.  The Bubble Point is where the first bubble of vapor appears in the liquid refrigerant during the change of state.

Subcooling is taking place in the high side.  Vapor refrigerant at its saturation point in the condenser begins to condense into liquid.  The Dew Point is where the first drop of liquid condenses from the vapor during the change of state.

That being the case, why would I use the Bubble Point value to determine subcooling and the Dew Point for superheat as it says on the Sporlan PT chart?  Shouldn't it be the other way around?

Sorry if I'm being thick-headed.  This is all new to me and I'm trying my best to understand it.  It just seems contradictory.

[Admin]

In our textbook, Heating and Cooling Essentials by Jerry & LaDonna Killinger, Chapter 14 discusses zeotropic blends, fractionation and temperature glide, etc.  In that chapter, Bubble Point is described as the saturated liquid pressure, or the pressure/temperature at which the first bubble of vapor appears in a liquid refrigerant.  Dew Point is described as the saturated vapor pressure, or the pressure/temperature at which the first drop of liquid condenses from a vapor.

I think the bolded parts of the textbook make sense.  After the bold it sais OR the pressure/temperature... So maybe it's another condition that doesn't actually exist in our example.  I find it confusing too.  Your textbook refers to the bubble point as starting with the first bubble, whereas Sporlan refers to the bubble point as starting with the last bubble... 

 

I know there's confusion when you compare azeotropic to near-azeotropic/zeotropic.  In single component mixtures the dew point and bubble point are the same thing, called the boiling point.  Now with blends you have to deal with two or more components with different boiling points.

Superheat is taking place in the low side.  Liquid refrigerant at its saturation point in the evaporator begins to boil off into vapor.  The Bubble Point is where the first bubble of vapor appears in the liquid refrigerant during the change of state.

Superheat takes place on the high pressure side, the Dew point refers to when the last drop of liquid evaporates and any temperature rise afterwards is called superheat.  (Enter Compressor)

Subcooling is taking place in the high side.  Vapor refrigerant at its saturation point in the condenser begins to condense into liquid.  The Dew Point is where the first drop of liquid condenses from the vapor during the change of state.

Subcooling takes place on the low pressure side, the Bubble point refers to when the last bubble of vapor condenses and any temperature drop afterwards is called subcooling.  (Enter TXV)

[gfgordon]

No, you've got it backwards there.  Superheat is what's taking place in the evaporator, on the low-pressure side of the system.  The saturated refrigerant boils into a superheated vapor before going through the suction line into the compressor.  Subcooling is what's taking place in the condenser, and that's the high-pressure side of the system.  The refrigerant enters as superheated vapor, goes through the de-superheat, saturation, and then subcooling phases, and enters the liquid line as a subcooled liquid on its way to the metering device (TXV or fixed orifice or whatever).

[Admin]

To be accurate superheat can exist in the evaporator and the compressor hot discharge line before the condenser.  There is evaporator superheat and compressor superheat.  They always refer to superheat as being low pressure, because we measure our pressure at the compressor suction valve.

As far as dew point / bubble point go, it's confusing depending on what you read.  Some sources say,

The dew-point is the temperature at which the saturated vapour starts to condense. (start of condensation)
The bubble-point is the temperature at which the liquid starts to boil. (start of evaporation)

Other sources say,

The Dew point refers to when the last drop of liquid evaporates. (end of evaporation)
The Bubble point refers to when the last bubble of vapor condenses. (end of condensation)

Let me know what you find out, but I think as far as central air conditioning is concerned, we would go with the second source.  If manufacturers have marked the dew point just before the compressor, we know we are dealing with vapour.  I think both sources are right, they just define the end and start differently.  Look at the bold text above, it might help understand how they are looking at it.

When you look at it this way, is there really a difference between the start of condensation and end of evaporation, or the start of evaporation and end of condensation?