PDA

View Full Version : How to check Flash gas?







Uni
14-12-2008, 11:00 PM
Hi guys,

How can I check if there is flash gas along the cap tube? Refrigerant is R407C, Evap in temp is about 8degC, water is condensed right before the evap inlet.

Where should I read the evaporating temperature? I have Evap In, Evap Center and Evap Out temperatures.

Thanks

Tesla
15-12-2008, 06:09 AM
At the outlet DIRECTLY with a yellow jacket richie superheat calculation kit - Directly measures temp and press of refrigerant (not the temp of tube outside. This way you get an accurate measure of superheat via 1/4" schrouder fitting. As for the first question without getting too long winded - should be checking for a liquid seal before inlet or subcooling via sightglass, electronic sightglass, or superheat calculation kit. I think the flashgas exists to cool refrigerant to evaperating temp. Check out a pressure enthalpy chart.

750 Valve
15-12-2008, 08:20 AM
if you plot your conditions on a pressure/enthalpy chart you will easily see the percentage of flash gas produced through the metering device.

expat
15-12-2008, 08:28 PM
Hi Uni, I'm not quite sure I understand what it is you want to measure.

Is it superheat and the efficiency of the metering device with the evaporator or are you experiencing a problem with flash gas in the liquid line?

US Iceman
15-12-2008, 08:46 PM
How can I check if there is flash gas along the cap tube?


I think we need to clarify a term here. What exactly do you mean by flash gas? This can be used several ways depending on what the person means.

Generally I use the term flash gas to be any vapor created by an expansion process of liquid refrigerant.

Therefore, in a cap tube the flash gas would start to form as the liquid pressure decreases. This would be along the entire length of the cap tube then.

Flash gas also forms in a liquid line that either:


absorbs heat causing the liquid to boil
or, when the liquid flows to a higher elevation with no subcooling
or, if you expand the liquid to a pressure in between the condenser and evaporator pressure

Uni
14-01-2009, 10:45 PM
Sorry for late response. I was out for a long time during the holiday.

My main problem is high discharge pressure, 2.6MPa (377psi) @ 98degC (208F), Condensing temp is 57degC (134F) @ 2.58MPa (374psi), about 5degC (9F) of subcool, Evap temp is 10degC (50F) @0.63MPa (91psi) and Suction temp is 27degC (80F) @0.54MPa (78psi).

Testing condition is 27degC (80F) indoor and 35degC (95F) outdoor, 50%RH.

Cond and Evap fans are working properly and giving sufficient airflows, 520m3/h (306cfm) and 480m3/h (282cfm) relatively.

The system uses R407C and was designed for 3KW Cooling and used to be effective at 2.6kW. Currently it can just provide no more than 2.1kW.

Could you please figure out what could be wrong with this circuit?

What can be improved or tried?

wambat
15-01-2009, 01:14 AM
Sounds like an over charge of refrigerant or lack of air flo accross condensersor a partial
restriction

Slatts
15-01-2009, 08:59 AM
Sounds like an over charge of refrigerant or lack of air flo accross condensersor a partial
restriction
can't argue with that wambat.
How about it uni? have you hosed out or blown out the condenser?
has anyone added gas to the system?
is the manufacturers charge weight on the machine? it may well be best to decant the charge, change the drier, vac the system and weigh in a new charge with fresh gas.

Uni
15-01-2009, 10:27 AM
1. The condenser & evaporator are totally clean and free from damage. The airflow was checked to be sufficient as well (designed criteria).

2. Gas was charged at just +/- 2g difference. I do recharge with fresh gas almost every week (for different tests).

And for this reason, I did refill the compressor oil once with 50ml after about 20 times of evacuating. Is it sufficient? I tested it again right after that and saw no gain in performance, just a slight reduce in temperature and pressure.

3. I was afraid of clogging so I changed over the cap-tube, removed the check-valve and reversing valve (for reverse cycle) and test the cooling cycle only. Should I remove the drier filter as well? What else can quite often be blocked, everyone? I'm not very experienced in this. Not sure how easy

4. It's not like being overcharged because it's comparing to itself at the same condition (2.6MPa (377psi) compared to 2.4MPa, 2.1kW compared to 2.6kW).

What do you think about the refrigeration cycle when looking at the figures? Too much superheat (to my calculation, it's more than 17K superheat; 4-5K subcool looks right)?

I'm stuck! :confused:

Slatts
15-01-2009, 11:23 AM
My main problem is high discharge pressure, 2.6MPa (377psi) @ 98degC (208F), Condensing temp is 57degC (134F) @ 2.58MPa (374psi), about 5degC (9F) of subcool, Evap temp is 10degC (50F) @0.63MPa (91psi) and Suction temp is 27degC (80F) @0.54MPa (78psi).

Testing condition is 27degC (80F) indoor and 35degC (95F) outdoor, 50%RH.

Cond and Evap fans are working properly and giving sufficient airflows, 520m3/h (306cfm) and 480m3/h (282cfm) relatively.

The system uses R407C

377PSI head pressure with 35C air on to the condenser on R407C tells me either the system is over charged, there's non condensable in the condenser or the condenser isn't getting rid of the heat due to being fouled either inside the coils or in the fins.
You say the evap temp is 10C at 91 PSI and 27Cat the suction line, Im assuming at the compressor and, if I understand your figures correctly youve got a suction line pressure drop of 13 PSI. A tad excessive, wouldnt you say?


1. The condenser & evaporator are totally clean and free from damage. The airflow was checked to be sufficient as well (designed criteria).

2. Gas was charged at just +/- 2g difference. I do recharge with fresh gas almost every week (for different tests).

And for this reason, I did refill the compressor oil once with 50ml after about 20 times of evacuating. Is it sufficient? I tested it again right after that and saw no gain in performance, just a slight reduce in temperature and pressure.

3. I was afraid of clogging so I changed over the cap-tube, removed the check-valve and reversing valve (for reverse cycle) and test the cooling cycle only. Should I remove the drier filter as well? What else can quite often be blocked, everyone? I'm not very experienced in this. Not sure how easy

4. It's not like being overcharged because it's comparing to itself at the same condition (2.6MPa (377psi) compared to 2.4MPa, 2.1kW compared to 2.6kW).

What do you think about the refrigeration cycle when looking at the figures? Too much superheat (to my calculation, it's more than 17K superheat; 4-5K subcool looks right)?

I'm stuck! :confused:
You replace the gas weekly? What on earth are you doing with this system?
Of course this opens us up to the possibility that youre not evacuating the system properly and were back to non condensables.
The oil you used to top up the system, was it clean and dry?
Did you use the correct capillary tube length and bore when you replaced it?
Could be that your capillary is too narrow or too long.
And yes, change the drier.

bill1983
15-01-2009, 06:23 PM
at the figures you are quoting, the pressure drop on the low side of the system is too high at 13 psi what componants are there in the suction line, drier, pressure regulator? or is there oil trapped in the evaporator?, reducing its capacity.
what is the distance between the 2 parts of the system? pipe sizes and no of bends etc. also how old is the system. the first thing i ask myself is what could have changed? do you know the plant history before you attended? by the way if you check the manufacturer's data you may find that he will quote up to 20k superheat at the compressor.
keep us informed please

Uni
16-01-2009, 12:09 AM
Thanks everyone for your interests in the topic,

I'm doing my study within this field so there are a lot of instrumentation and experiments with the poor heat pump. What I'm trying to do is to understand the fundamental and behaviour of the system responding to changes in conditions and components. I actually built up that heat pump based on a design, and luckily I've successfully sourced most of the components.

Back to the issue, I often evacuate the system for 30 minutes or more and recharge right away. Up to you, I will do a good vacuum and leave it over this weekend to see if there is any leak.

The coils and fins are free of damage anyway; I've checked the temperature profile of both of the EVAP and COND (each with two coils, from inlet - centre - last - outlet of both coils) and found no significant sign of fault.

So, we can say there should be no blockage inside the coils. I will remove the drier (the only accessory) and check the liquid and vapour lines, there are some brazing tees on them (for pressures reading, charging ports, etc.). Anything else that I should try?

The refill oil was supplied by the manufacturer but contained in kind of unoriginal plastic bottle. I'm not quite sure about the purity, no experience to realize.

I can be sure about the cap-tubes, I made some of them to change over (different length as well) and they seem not affecting the results. I will try bigger diameter cap-tubes next week as per your advices. However, before that we should work out what wrong with the system at the moment, shouldnt we?

Is it a good idea to try reducing the superheat by blocking a part of the EVAP?

Ill check the changes in air temp on/off the COND as well.

if you come up with any new idea, please keep me informed.

Thanks so much.

wambat
16-01-2009, 01:29 AM
changing cap tubes would require different charges so I suspect an overcharge
A system with an overcharge of refrigerant will have higher than normal condensing temperatures because liquid backs up in the condenser, robbing the condenser of useful condensing area. The elevated head pressure causes the volumetric efficiency of the compressor to decrease because of higher pressures of the re-expanding clearance volume vapors in the clearance pocket of the compressor. The amp draw of the compressor will increase from the higher head pressure. Higher head pressures also create higher compression ratios, and the entire system will have reduced capacities.
If the system has a TXV metering device, the TXV will still try to maintain its superheat and the evaporator pressure will be normal to slightly high, depending on the amount of overcharge.

The higher evaporator pressure will be caused from the decreased mass flow rate from the higher compression ratio, and the evaporator will have a hard time keeping up with the higher heat load of the warm entering air temperature. The TXV will have a tendency to overfeed on its opening strokes due to the high head pressures.

If we are dealing with a capillary tube metering device, the same symptoms occur with exceptions to evaporator superheat. Remember, one reason a capillary tube system is critically charged is to prevent flooding of the compressor on low evaporator loads. The higher head pressures of an overcharged capillary tube system will have a tendency to overfeed the evaporator, thus decreasing the superheat.

If the system is more than 10 percent overcharged, liquid can enter the suction line and get to the suction valves or crankcase. This will result in compressor damage and soon failure.

Uni
16-01-2009, 02:30 AM
changing cap tubes would require different charges so I suspect an overcharge
A system with an overcharge of refrigerant will have higher than normal condensing temperatures because liquid backs up in the condenser

The new captube is at the same bore and length so the charge amount should be maintained; and I did plug the old captube back in and recheck anyway.


The higher head pressures of an overcharged capillary tube system will have a tendency to overfeed the evaporator, thus decreasing the superheat.

As we mention before, the superheat seems to be too high.

icecube51
16-01-2009, 03:50 PM
try the free of charge Danfos program for capilairy tube calculation. "Dancap"

Ice

bill1983
17-01-2009, 02:41 PM
do i understand you correctly.? you said that you built the heatpump using a "design". who's design is it and has it been completely checked for accuracy as to componant size and pipe sizes?

Marc O'Brien
18-01-2009, 02:18 AM
Have you checked the standing pressure of the refrigerant storage cylinder's remaining charge. Refrigerant component proportions can change quite drastically when multiple charges are made from a single bottle. Of course more so with larger bottles.

wilsoncheung
18-01-2009, 02:54 AM
Sorry for late response. I was out for a long time during the holiday.

My main problem is high discharge pressure, 2.6MPa (377psi) @ 98degC (208F), Condensing temp is 57degC (134F) @ 2.58MPa (374psi), about 5degC (9F) of subcool, Evap temp is 10degC (50F) @0.63MPa (91psi) and Suction temp is 27degC (80F) @0.54MPa (78psi).

Testing condition is 27degC (80F) indoor and 35degC (95F) outdoor, 50%RH.

Cond and Evap fans are working properly and giving sufficient airflows, 520m3/h (306cfm) and 480m3/h (282cfm) relatively.

The system uses R407C and was designed for 3KW Cooling and used to be effective at 2.6kW. Currently it can just provide no more than 2.1kW.

Could you please figure out what could be wrong with this circuit?

What can be improved or tried?

_____________

The condensing temperature is at 57degc, I think is a little high, I suggest you check whether the condenser is dirty pr not.

wilsoncheung
18-01-2009, 05:46 AM
There are two problems in your system; one is as I have just said that your cond temp is too high; and the second is you superheat is also too high. As you told us that your condenser is clean and free from damage, so I guess your cap tube is too long or has too small diameter. Pls let me know cap tube configuration. Or you can change a expansion valve and test the difference.

Marc O'Brien
18-01-2009, 06:47 AM
There are two problems in your system; one is as I have just said that your cond temp is too high; and the second is you superheat is also too high. As you told us that your condenser is clean and free from damage, so I guess your cap tube is too long or has too small diameter. Pls let me know cap tube configuration. Or you can change a expansion valve and test the difference.

A good start would be to gather instantaneous readings:
Saturated condensing and subcool
Condenser Air on and off
Saturated suction and Superheat
Evaporator Air on and off

Condenser air dT will tell whether the condenser is fowled or coated with insulating deposits if there is any problem there at all.

I think, first off, standing saturated temperature readings compared to ambient temperatures should be taken of the unit and the refrigerant cylinder.

In the mean time the low subcool reading suggests the high saturated condensing temperature is not occurring on account of an overly resistive capillary tube and overcharge.

Brian_UK
18-01-2009, 04:56 PM
<snip>

Back to the issue, I often evacuate the system for 30 minutes or more and recharge right away. Up to you, I will do a good vacuum and leave it over this weekend to see if there is any leak.
<snip>
The time taken is not a factor to be considered.

What is the level of vacuum that you achieve at the end is the important figure.

Marc O'Brien
18-01-2009, 05:17 PM
The time taken is not a factor to be considered.

What is the level of vacuum that you achieve at the end is the important figure.

Leak testing should only really be done with nitrogen as in a tightness test after any necessary strength testing.

Triple evacuating removes air borne contaminants and deep vacuuming removes moisture. Water can only exist as ice under settled deep vacuum conditions and so warm nitrogen introductions periodically can assist in the course of deep vacuuming.

Essentially a pressure tightness test eliminates leaks and a vacuum rise test eliminates the presence of moisture. But, how long should you take to deep vacuum and when should you do a vacuum rise test? Well, Brian, I think these questions are extremely important and generally, if not universally, not known or understood in the industry or the industry schools and colleges. Lookout for my next article in the newly launched Service Engineer Magazine now to be called the RAC Engineer Magazine. I expect my article to make the industry rethink entirely the vacuum to remove moisture process.

El Padre
18-01-2009, 06:26 PM
Hi Marc,
I remember being told at college that a triple vac should always be performed, but only discovered why a few years after I left whilst chatting to someone at the RAC show, I shall look forward to your article.

Burrah Boy
22-01-2009, 07:43 PM
Silly question, have you been charging the R407 in vapour or liquid?