Saturated Suction Temperature and Superheat

[Slash]

In a system where the evaporating temperature is -8oC, and the suction line pressure drop is 2K, the saturated suction temperature (SST) will be -10oC. This is fine.

What I would like to know is why doesn't superheat affect the SST? In this same system, if the total superheat (evaporator plus suction line) were to be e.g. 10K, then why is this 10K not added into the equation to obtain the SST? Of course, it is not added, but I would like to understand why?

[frank]

Hi Slash and welcome to the foum.

The saturation temperature is the point at which liquid refrigerant turns to vapour for a given pressure. This temperature/pressure relationship is key to your understanding phase change. Do you have a Comparator?

Once the refrigerant has changed to a vapour (saturation point) any further heat added to the vapour is called superheat. By measuring the refrigerant pressure, you can find the saturation temperature on the comparator. Then, if you measure the pipe temperature and take away the saturation temperature, you are left with the 'Superheat' always quoted on the Kelvin scale (K)

Superheat, i.e. heat added to the vapour, happens after phase change from liquid to vapour.

Have a read here for further enlightenment http://www.refrigeration-engineer.com/forums/showthread.php?14830-how-to-use-a-refrigerant-comparator

[Slash]

Hi, thanks for the reply. I'm definitely still missing something here though.

I just can't get why the 2K suction line pressure drop is subtracted from the evap temperature to find the compressor suction condition (SST), but the suction Superheat is not factored in to the equation (or at least doesn't seem to be)...

Feel like there is an obvious answer to this that I am somehow just missing...

[RANGER1]

http://www.danfoss.com/NR/rdonlyres/F35695FE-F4A2-40B9-847F-E81E20A5EEA8/0/PF00F202.pdf

This link is pretty good in general

[Tesla]

Hi Slash Ranger has provided good data and a lot of guff to understand. The suction pressure drop is like you said very basically but we are taught a very basic concept of the enthalpy chart first to understand the basics then we are taught all the other real things that come into it like pressure drop and the superheat you are talking about. In the real world I simply measure it at the compressor but most would take approximate or indicative measurements. As for calculations unless we use sophisticated software we can only calculate a static moment in time of the system where the software could plot the dynamic situation as the parameters are always changing and it's a real mind spinner just thinking about it.
In looking carefully at your post I think you may be confusing pressure with temperature or equivalent of. Pressure is pressure and superheat of temperature difference is K. I was taught in refrigeration as a rule of thumb to allow around 2k temp increase in superheat in the suction line and 1.5 - 2 psi pressure drop, and the pressure drop is a negative flow goes from positive to negative - hope I have not confused you any more.

[Slash]

Aha! I think I've got it. Tesla I believe you are correct and I am indeed confusing pressure and temperature. The 2K suction line pressure drop was the confusing factor here. As its expressed in Kelvin I'm thinking temperature, but it is simply a reference to a PRESSURE in terms of a temperature equivalent, and is unrelated to the separate process of Superheat.

Feels like something of a eureka moment as this has been bugging the sh*t into me for absolutely ages. Got there in the end! Thanks for all the comments folks.