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Posted By Kan
Perhaps a bit technical, but I've been trying to solve a psychrometrics problem for quite a while now.
It concerns a dryer in which a substance is dryed by air. The substance is added and removed continuously. The air is cooled between states 2-3 and heated between 1-3. The drying chamber is between 1-2, and is considered adiabatic.
State 1: T1 (dry bulb) = 65C
State 2: relative humidity = 85%
State 3: T3 (dry bulb) = 2C, relative humidity = 100%
Pressure is constant at 1 bar, airflow is 10kg/s and circulates in a closed process.
I'm trying to sketch the process in a Mollier-diagram; Any tips/ideas on how to determine the temperature at state 2?
Thanks!
Kan
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Posted By Keith32
Assuming this is not one of the usual wind up questions...
But you state the process/air is cooled between states 2 -3 and then heated between 1 - 3.
Therefore doesn't 1 - 3 cover stages 2 -3?
That's how I read the question, therefore overall the process is being heated....
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Posted By Kan
Thanks for the reply.
The problem is to determine state 2 in a psychrometrics chart.. I think a solution is to consider the enthalpy to be constant from 1-2. The process line follows the lines of constant enthalpy until it reaches the .85 saturation.
Hope this is a good enough approx.
Kan
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Posted By Keith32
I never did Mollier diagrams/air conditioning design calculation in my engineering studies.
I did expansion/compression of gasses in compressors/combustion chambers and expansion across turbines - as used in jet engines - tends not too be too much moisture inside such engines.
Do you not need to consider Boyle's Gas Equations P1 V1/T1 = P2 V2/T2 or whatever it is.
What about the other thermodynamic laws/formula which slip my memory now that the grey matter is fading away.
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Posted By Psycho
Kam
Try this
CDB RH CWB CDP W H ALT POINT
[degC] [%] [degC] [degC] [kg_WV/kg_DA] [kJ/kg_DA] [m]
2.0 100.0 2.00 2.000 0.00438 12.989 0 3
65.0 2.8 25.50 2.000 0.00438 76.870 0 1
27.5 85.0 25.50 24.796 0.01992 78.528 0 2
DESCRIPTION OF PROCESSES IN THE CYCLE
3-1 SENSIBLE HEATING OF COLD AIR OFF OF COOLING APPARATUS AT POINT 3. CONSTANT HUMIDITY RATIO
1-2 COUNTERFLOW OF AIR AND PRODUCT IN PRODUCT DRYING TUNNEL. THIS IS AN EVAPORATIVE COOLING PROCESS AT CONSTANT WET-BULB TEMPERATURE
2-3 COOLING AND DEHUMIDIFYING AIR AS IT PASSES ACROSS A COOLING COIL
STEPS IN SOLVING THE PROBLEM
DESCRIBE THE PROBLEM COMPLETELY AND IN ITS ENTIRETY
• START FROM KNOWN POINT 3 AND HEAT AT CONSTANT HUMIDITY RATIO TO KNOWN 65°C. THIS STATE POINT IS NOW KNOWN FROM CDB, W AND PBAR
• FOR THE ADIABATIC EVAPORATIVE COOLING PROCESS PROCEED FROM POINT 1 UP THE 25.50°CWB LINE TO 85% RH. THIS STATEPOINT IS NOW KNOWN FROM CWB, RH AND PBAR
• THE AIR IS NOW COOLED AND DEHUMIDIFIED FROM POINT 2 TO POINT 1 AND CYCLE IS COMPLETE
CHECK YOUR WORK. DOES IT MAKE SENSE? ARE THE ANSWERS REASONABLE?
(from author of ASHRAE's Understanding Psychrometrics.
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Posted By Psycho
Kam
The table did not paste well
It is a 5 row 8 column table. The last 3 rows are for state points 3, 1, and 2 in that order
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