Original message
| jhammond601
 |  "why does the evaporator pressure drop"
, posted Fri 12 Aug 09:44:  
I cannot seem to get my head around what causes the evaporator pressure to drop as the air which is being cooled also drops.
I thought that the system was designed to have a set pressure in the Evaporator depending on the type of refrigerant used. This pressure would be constant and so the temperature would be constant inside the Evaporator. So does the Refrigerant inside the Evaporator determine the temperature of the air being cooled or does the air being cooled determine the pressure inside the Evaporator
John H
[this message was edited by jhammond601 on Fri 12 Aug 10:04] |
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| jhammond601
| "Re(1):why does the evaporator pressure drop" , posted Fri 12 Aug 20:18
Maybe I am not explaining the question properly.
I am not saying that the Pressure drop is a problem, as you cool down the Air inside a cold room, I just don’t understand why??
I have bought an Interactive CD on Air Conditioning and Refrigeration theory. On this CD it explains that "If you are cooling an area the temperature and pressure of the refrigerant inside the Evaporator slowly drops as the temperature of the air becomes cooler".
My question is: How can the Air cool down the refrigerant inside the Evaporator and drop its pressure. I know this is why you can use a LP Switch to control the temperature of a cooling space but I do not understand why the pressure would drop in the refrigerant.
When the refrigerant is metered into the evaporator the system is designed to produce a specific pressure temperature for the type of refrigerant used. If we are using r134a and the system is designed for 17.1 psi/18 deg f how can this pressure/temperature drop lower that 18 deg f if the system is designed to have this set pressure/temperature inside the evaporator.
Should the refrigerant inside the evaporator not have a constant pressure temperature regardless of the temperature of the air to be cooled as we are told the surrounding air gives up its heat to the refrigerant which boils at a steady pressure and temperature?
John H
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| | MasterTech
| "Re(2):why does the evaporator pressure drop" , posted Fri 12 Aug 21:50
The pressure drops because the temperature drops. So as the temperature in the house drops because of coolin there is less heat to pick up and the freon doesnt boil off as fast which creates less pressure.
Does that make sense?
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| | jhammond601
| "Re(3):why does the evaporator pressure drop" , posted Sat 13 Aug 08:23
Yes that makes sense!! But does the Refrigerant still enter the Evaporator at it set Pressure and Temperature regardless of the temperature of the air.
Using r134a as an example would the refrigerant still enter the evaporator at 17.1 psi/18 deg f (If that was the system design)
regardless of the Room Air temperature and then the pressure would drop in the evaporator because the refrigerant is not boiling off as fast due to the continual cooling of the room??
Or would the refrigerant enter the Evaporator at a continual lower pressure/temperature because of the effect the decreasing room temperature is having on the evaporator.
John H
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| | MasterTech
| "Re(4):why does the evaporator pressure drop" , posted Sat 13 Aug 13:24
The liquid pressure is driven by mainly from the outside temperature and minorly by heat load for inside temperature.
For the most part its just the outside temperature drives pressure to the metering device.
Got to go. Hope that helps.
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| beginner
| "Re(1):why does the evaporator pressure drop" , posted Fri 12 Aug 18:51
1. Restriction of air flow.
2. Restriction of the refrigerant flow at the metering device.
3. Low charge, hmm..maybe.
First check the evaporator then check the charge. What are the pressure readings?
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| | Superdave
| "Re(2):why does the evaporator pressure drop" , posted Sat 13 Aug 13:10
If you want to know the "exact" entering suction pressure, just beyond the metering device, "you" must know the following,
"Exact" entering temperature and pressure of the liquid Refrigerant and it's "exact" BTU content at those conditions.
Metering device flow rate at those "exact" conditions.
Pressure restrictions of the distribution assembly/tubes at "exact" conditions.
Compressor CFM at "exact" rated conditions.
Line set lenght and tube size and all "exact" related pressure losses and "exact" (total) heat loading of the evaporator.
This is all manufacturing engineering and not public information.
So the basic answer to your question is this, suction pressure/temperature is driven by heat(load)of the evaporator, if the house gets hot the suction temperature and pressure go up (loaded) if the house gets cold suction temperature and pressure go down (lower load). Remember why is the house getting hot, the outdoor temperature is going up, so the liquid pressure/temperature is going up, so the flow rate is going up, so the evap loading is going up, when the outdoor temp drops the reverse will happen.
Hope this helps..
Superdave
SuperDave
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| | jhammond601
| "Re(3):why does the evaporator pressure drop" , posted Sat 13 Aug 18:56
Yes..... That makes sense !!!
Thanks to all the people who helped with this query !!
I have one more question that I would appreciate to know the answer for:
Why does the pressure of a gas not change (Raise) when it is superheated or the pressure of a Liquid not change (Decrease) when it is subcooled
John H
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| | MasterTech
| "Re(4):why does the evaporator pressure drop" , posted Sat 13 Aug 20:21
The pressure of freon does change when super heated. Thats why there is so much pressure difference 38degree evaporator and 50degree evaporator.
Sub cool is cooling of freon below saturation point of freon and it does change pressure. But remmeber your dealing with a a/c system not just a lab experiment.
In a a/c system you are pressurizing freon until the coil temperature is above outside temperature so then you can run cooler outside air over coils to "desuperheat" gas and then sub cool liquid freon.
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| | jhammond601
| "Re(5):why does the evaporator pressure drop" , posted Sun 14 Aug 11:08
I am talking about the approx 10 deg f Superheat above saturation pressure on the Evaporator and the few degrees sub cooled at the evaporator. I am told the pressure gauge only reads Saturation Pressure/Temperature and that you need to use a thermometer to read the degrees superheat. Heat is pressure so why wont the Preasure gauge read it ??
John H
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| | MasterTech
| "Re(6):why does the evaporator pressure drop" , posted Sun 14 Aug 11:56
you pushing the limit of my knowledge lol.
Gauges read PSIG and have markings for saturated points of given freon. To determine if the freon is super heated or sub cooled is done with thermometer on the copper lines. Saturation point can be at different pressures, PVT, If u change one must change the other to compensate. Increase pressure then decrease temperature to keep saturation point. What im getting at is the only way to determine superheat or subcool is to take line temperature because volume is constant, pressure is constant on either side of metering device(roughly) so the only difference is temperauture. So by taking a temperature at any point on the copper and knowing the pressure on either side of the metering device you know if freon is super heated or sub cooled.
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| | jhammond601
| "Re(7):why does the evaporator pressure drop" , posted Mon 15 Aug 00:23
Thanks for the advice!!
I appreciate the time you and others have taken to answer my queries.
I have a better understanding of what is happening inside the Evaporator now. All the text book I have read tends to over simplify the process and as I am finding out it can be very complicated.
John H
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