Question regarding evaporator/condenser designs 1

[eaboujaoudeh]

1.superheat
Psig Saturated Temp Suction line temp superheat temp.
58 32 44 12
64 37 47 10
70 41 50 9

can some1 help me understand the ratings above..
Saturated temperature is easy ok.
Suction line temperature is the temperature before or after the evaporator? my guess is after the evaporator..
As for the superheat temperature: they say it defines the liquid content of R-22 in the system at the exit of the evaporator, but i dont understand the principle..plz help with this.


2. Does anyone know a good program for coil design? i'm using a program called evap-cond but we just dont seem to get along very well.

3. Any good books or references?

4. About the compressor to be chosen for refrigeration application. i'm finding that their is a conception that if u need to transfer 10 tons of cooling to a room, u choose a compressor that is rated at 10 tons. but shouldn't a smaller one do?

Thank you guys


Elie Abou Jaoudeh

 

[Yorkman]

There are several different ideas on where to measure suction superheat. They all are measured on the suction line (after the evaporator) the location can very depending on the type of system your dealing with. On a close coupled system say an RTU package unit the suction superheat is usually measured about 6-12 inches from the compressor. On a split system, remote condenser it is usually measured near the TXV bulb at the evaporator outlet.
As far as compressor selection, the tonnage of the compressor is going to be determined by the design load and the specific volume of the suction gas your trying to move. In general terms this is based on suction pressure and the pounds per minute you need to circulate to meet design load. The horsepower is determined by the pounds per minute of refigerant you need to circulate and the heat of compression. I'm sure someone will go into further detail but in simple terms that about covers it.
I'm stating it pretty simply here. ASHREA has some good materials when it comes to system sizing.

I'm not a real engineer, but I play one on T.V.
A.J. Gest, York Int./JCI

 

[Yorkman]

I neglected to answer your first question regarding what suction superheat was indicating. Simply put all pure refigerants and azeotropic blends have one boiling point at any one given pressure. Suction superheat is a way to quantify how close to the saturated liquid temperature the suction line is running the smaller the superheat value the closer to the saturation temperature your running. The primary purpose of maintaing suction superheat is to protect the compressor from liquid refrigerant damaging compressor valves and bearings. It also is an indication in a sense, of how much of the coil is being used for the vaporization of liquid refrigerant. High superheat reading mean that much of the coil is not being used for the vaporization of liquid refrigerant, too little superheat can lead to compressor damage. I hope that helps.

I'm not a real engineer, but I play one on T.V.
A.J. Gest, York Int./JCI

 

[eaboujaoudeh]

it does help and thnx..but its still not what i'm confused about..i know the superheat uses and things like that..but the superheat temperature here is not a real temperature if what u say is true. cause the superheating a gas at a certain pressure means taking it over its saturation temperature and not below( at least as i remember).
As for the compressors i have a brochure for a certain line of compressors, but they don't give the mass flow rate of anything else, they just give max. suction pressure, and Tons of heat..the selection is being done according to that tonnage ! my question is because i always designed using mass flow rate !!

Elie Abou Jaoudeh

 

[Yorkman]

"cause the superheating a gas at a certain pressure means taking it over its saturation temperature and not below( at least as i remember)". You are correct superheating of any gas describes the number of degrees above the saturation temperature you have heated the gas, for that given pressure.
I'm not sure I fully understand where your confusion lies.
On the second note it sounds like the manufacterer has saved you the some of the calculations for finding the required displacement. When they state the max. operating pressure, they are saying at this suction/discharge pressure they guarrenty xyz tons. It sounds like it might be possible to oversize the compressor using max. tonnage delivered.

I'm not a real engineer, but I play one on T.V.
A.J. Gest, York Int./JCI

 

[eaboujaoudeh]

my confusion lies in the table at the beginning.
Saturation temperature = 32 for 58psig
while the superheat temp = 12 !!!!
what does that mean, and how can i find it for a system without physiscal measurement

Elie Abou Jaoudeh

 

[Yorkman]

You can't. Superheat is calculated like this; The pressure of the suction line is taken using a pressure gauge. Then using a refrigerant saturation tables the saturation temperature is found for that corresponding pressure. Next an actual temperature reading is physically taken of the suction line. The saturation temperature is then subtracted from the temperature reading this will be the amount of superheat. Superheat is just the delta T between saturation temperature and suction line temperature.

I'm not a real engineer, but I play one on T.V.
A.J. Gest, York Int./JCI

 

[eaboujaoudeh]

i see..thnx for bearing with me here.i think i know what i have left to do..

Elie Abou Jaoudeh