Desiccant

ryszard · 28th July 2020, 19:00

Hi,anyone recently purchased the desiccant/dryer gel that fits in the a/c condenser to absorb moisture,if so where did you get it from.Am about to fit my new compressor but have been advised to change the dryer as it has been exposed too long to the outside moisture.Have only found them on the bay,ironically the slightly cheaper ones are from outside the UK.They seem to be half the price of a complete condenser.My condenser was a new one fitted a couple of years ago,so really just need the pack. Regards Ryszard....

rab60bit · 28th July 2020, 23:24

Quote:

Originally Posted by ryszard

Hi,anyone recently purchased the desiccant/dryer gel that fits in the a/c condenser to absorb moisture,if so where did you get it from.Am about to fit my new compressor but have been advised to change the dryer as it has been exposed too long to the outside moisture.Have only found them on the bay,ironically the slightly cheaper ones are from outside the UK.They seem to be half the price of a complete condenser.My condenser was a new one fitted a couple of years ago,so really just need the pack. Regards Ryszard....

Doesn't help you much but all the new (SIAC OEM?) condensors being offered by well known trader on here no longer have replaceable dryer cartridges despite still illustrating them as such - sealed for life on the presumption that when the fins start to rot/fall off the desiccant has had it anyway. I guess the refills are now obsolecent?

SD1too · 29th July 2020, 08:18

Hi Ry,

As John indicates, all the condensers I've come across in recent years have sealed driers. I'm no chemist but I believe that silica gel is extremely stable and is unlikely to break down. It's common for people to recommend drier replacement but really all that's necessary is a 30 minute evacuation which will extract the stored moisture and rejuvenate the drier. [Edit: see following posts for a correction.]

By the way, when installing a new compressor make sure that the correct amount of refrigerant oil is added for the 75/ZT system. They're often supplied with some but it might need topping-up. An independent self-employed specialist will be your best bet.

Simon

T-Cut · 29th July 2020, 10:41

Quote:

Originally Posted by SD1too

I'm no chemist but I believe that silica gel is extremely stable and is unlikely to break down. It's common for people to recommend drier replacement but really all that's necessary is a 30 minute evacuation which will extract the stored moisture and rejuvenate the drier.

'Fraid not. Silica gel won't give up its absorbed water under vacuum. Indeed, vacuum desiccators containing silca gel are found in most chemistry labs. The standard way to regenerate silica gel is to heat it in an oven at 120C or so for a few hours. For lab driers, the gel is often dyed with a material that changes colour (blue-pink) when the gel's saturated with moisture. The color reverts after dehydrating. However, i doubt that the aircon driers contain an indicator so it's pot luck. Assuming the desiccant pack isn't made from a thermoplastic, it might be worth cooking it for a while. The problem is, you're just guessing when it's ready.

TC

SD1too · 29th July 2020, 12:39

Quote:

Originally Posted by T-Cut

'Fraid not. Silica gel won't give up its absorbed water under vacuum.

I've always been guided by my automotive air conditioning text book which says that if the system vacuum is, for example, 29.4 inches of mercury, then any moisture inside will boil at an ambient temperature of 16 degrees C and be drawn out as a vapour. Please can you explain where I'm going wrong with my understanding of that?

Simon

T-Cut · 29th July 2020, 16:45

Well, there's water and there's water. The 'moisture inside' you refer to is free water, like dampness you can see, touch. This will indeed evaporate at ambient temperatures if the ambient pressure is reduced. Ideally, the pressure should be reduced to or below the water's vapour pressure. Around 20C, water has a vapour pressure of approx 15mm Hg, which explains your figures. So, if the surrounding pressure drops to that point, the water will 'boil' (water boils when its vapour pressure equals/exceeds the atmospheric pressure - it's the definition of 'boiling point' and explains why a kettle boils at lower temperatures when you're up a mountain).

The 'other' water is chemically bonded to the desiccant - silica gel for example. Here the water isn't palpable, it becomes part of the silca gel structure, called hydration. Because of this, the vapour pressure is exceedingly low. It's far lower than a typical mechanical vacuum pump can achieve and trying to dry it by this method would not achieve very much. However, if you put extra energy into the hydrated silical gel, the water will slowly be released from the matrix and it will evaporate. The standard method therefore is to put the hydrated gel in an oven at about 120C. Heating it fir two to four hours will liberate most of the hydration. However, it necver removes it all, so repeated regeneration is a diminishing return.

Chemistry labs use silca gel desiccators for drying stuff, in gravimetric analysis for example. These desiccators are typically thick glass containers with a slide-on lid. Silica gel is placed in the bottom section, under a perforated shelf.

The lid has a valved connector onto which a vacuum system is attached. The container is then pumped down to half an inch Hg or so. The water in the test sample evaporates and the sample will dry. The lberated water vapour becomes bonded to the silica gel, which slowly changes colour over weeks of use. It then goes into the 120C oven for regeneration.

Hope that clarifies.

TC

SD1too · 29th July 2020, 17:34

Quote:

Originally Posted by T-Cut

The 'other' water is chemically bonded to the desiccant ...

Thank you for the explanation. So in summary a chemical reaction has taken place (hydration) between moisture and desiccant which requires heat to reverse. Practically speaking this isn't possible to achieve, hence the need to renew a condenser/drier if water has entered an open and uncapped system. I've read that silica gel can "absorb" large amounts of water so:

The open condenser/drier would perhaps have to be in a high humidity atmosphere and for how long?
The amount and condition of desiccant in the drier and hence its maximum capacity is unknown.

There are three difficult questions to answer there which perhaps explain the precaution of renewing the condenser/drier if its service history is unknown. On the other hand, if the condenser/drier is kept capped and "free" moisture is extracted for 30 minutes whenever the system's been opened, the conclusion would appear to be that renewal is most likely unnecessary.

i hope that I have a better understanding now but if you can identify any serious misconceptions please let me know!

Simon

T-Cut · 29th July 2020, 18:50

I think you've got the gist of it. The absorption capacity of silical gel is around 20-25% by weight in a typical environment in the low 20s Centigrade and 40-50% RH. If you knew the 'dry' weight, a simple weigh scale would tell you how much water they'd absorbed. The unknowns in the aircon situation are unknowable without somebody doing the basic investigations. Clearly not worth the effort since silica gel is the cheapest desiccant there is. There are many much better (and more expensive) ones of course. If you want to experiment recovering exhausted drier packs then cooking them in the oven for a few hours at regulo 1-2 might be interestng, but as said, you'll never know without measurements.

TC

SD1too · 29th July 2020, 19:52

OK so it's all guesswork as the necessary measurements are impractical or even impossible as far as a car is concerned.

Ryszard; the question seems to be how "long" was your two years old condenser exposed to "outside moisture" and just how moist was it?

These are impossible questions to answer.

If I were you I would see how it goes with your existing condenser and if you have problems, renew it. How does that sound for a cost effective solution?

Simon

ryszard · 30th July 2020, 00:46

Hi,thanks all for the feed back.Having just fitted the new compressor,I will bite another bullet and order a new condenser.The a/c guys recommend a replacement condenser particularly if the dryer has been exposed to air after 7 days.My saga has been dragging on for over two weeks,the air con pipes have been disconnected from the compressor in this time,shame the previous a/c guy did not warn me otherwise I would have avoided this.I considered taking a chance on using the present condenser but the problem would be having to pay another refill price later if it still needed to be replaced and still end up forking out for the new condenser.All compressor suppliers advise to fit new condesets in their literature.Will keep you advised on my outcome.Regards Ryszard....

28th July 2020, 19:00	#1
ryszard Gets stuck in Moonstone diesel,X351 Jaguar and now MG ZT 260 Join Date: Dec 2013 Location: Northampton Posts: 682 Thanks: 288 Thanked 118 Times in 96 Posts	Desiccant Hi,anyone recently purchased the desiccant/dryer gel that fits in the a/c condenser to absorb moisture,if so where did you get it from.Am about to fit my new compressor but have been advised to change the dryer as it has been exposed too long to the outside moisture.Have only found them on the bay,ironically the slightly cheaper ones are from outside the UK.They seem to be half the price of a complete condenser.My condenser was a new one fitted a couple of years ago,so really just need the pack. Regards Ryszard....

29th July 2020, 08:18	#3
SD1too Doesn't do things by halves Rover 75 2.5 Connoisseur Auto (1999) Dealer launch model. Join Date: Mar 2007 Location: Former Middlesex Posts: 20,344 Thanks: 1,587 Thanked 3,749 Times in 3,181 Posts	Hi Ry, As John indicates, all the condensers I've come across in recent years have sealed driers. I'm no chemist but I believe that silica gel is extremely stable and is unlikely to break down. It's common for people to recommend drier replacement but really all that's necessary is a 30 minute evacuation which will extract the stored moisture and rejuvenate the drier. [Edit: see following posts for a correction.] By the way, when installing a new compressor make sure that the correct amount of refrigerant oil is added for the 75/ZT system. They're often supplied with some but it might need topping-up. An independent self-employed specialist will be your best bet. Simon __________________ "Whatever is rightly done, however humble, is noble." Sir Henry Royce. Last edited by SD1too; 30th July 2020 at 07:45..

29th July 2020, 16:45	#6
T-Cut This is my second home Rover75 and Mreg Corsa. Join Date: Nov 2006 Location: Sumweer onat mote o'dust (Sagin) Posts: 21,752 Thanks: 341 Thanked 3,660 Times in 2,924 Posts	Well, there's water and there's water. The 'moisture inside' you refer to is free water, like dampness you can see, touch. This will indeed evaporate at ambient temperatures if the ambient pressure is reduced. Ideally, the pressure should be reduced to or below the water's vapour pressure. Around 20C, water has a vapour pressure of approx 15mm Hg, which explains your figures. So, if the surrounding pressure drops to that point, the water will 'boil' (water boils when its vapour pressure equals/exceeds the atmospheric pressure - it's the definition of 'boiling point' and explains why a kettle boils at lower temperatures when you're up a mountain). The 'other' water is chemically bonded to the desiccant - silica gel for example. Here the water isn't palpable, it becomes part of the silca gel structure, called hydration. Because of this, the vapour pressure is exceedingly low. It's far lower than a typical mechanical vacuum pump can achieve and trying to dry it by this method would not achieve very much. However, if you put extra energy into the hydrated silical gel, the water will slowly be released from the matrix and it will evaporate. The standard method therefore is to put the hydrated gel in an oven at about 120C. Heating it fir two to four hours will liberate most of the hydration. However, it necver removes it all, so repeated regeneration is a diminishing return. Chemistry labs use silca gel desiccators for drying stuff, in gravimetric analysis for example. These desiccators are typically thick glass containers with a slide-on lid. Silica gel is placed in the bottom section, under a perforated shelf. The lid has a valved connector onto which a vacuum system is attached. The container is then pumped down to half an inch Hg or so. The water in the test sample evaporates and the sample will dry. The lberated water vapour becomes bonded to the silica gel, which slowly changes colour over weeks of use. It then goes into the 120C oven for regeneration. Hope that clarifies. TC Last edited by T-Cut; 29th July 2020 at 16:47..

29th July 2020, 19:52	#9
SD1too Doesn't do things by halves Rover 75 2.5 Connoisseur Auto (1999) Dealer launch model. Join Date: Mar 2007 Location: Former Middlesex Posts: 20,344 Thanks: 1,587 Thanked 3,749 Times in 3,181 Posts	OK so it's all guesswork as the necessary measurements are impractical or even impossible as far as a car is concerned. Ryszard; the question seems to be how "long" was your two years old condenser exposed to "outside moisture" and just how moist was it? These are impossible questions to answer. If I were you I would see how it goes with your existing condenser and if you have problems, renew it. How does that sound for a cost effective solution? Simon __________________ "Whatever is rightly done, however humble, is noble." Sir Henry Royce.

29th July 2020, 18:50	#8
T-Cut This is my second home Rover75 and Mreg Corsa. Join Date: Nov 2006 Location: Sumweer onat mote o'dust (Sagin) Posts: 21,752 Thanks: 341 Thanked 3,660 Times in 2,924 Posts	I think you've got the gist of it. The absorption capacity of silical gel is around 20-25% by weight in a typical environment in the low 20s Centigrade and 40-50% RH. If you knew the 'dry' weight, a simple weigh scale would tell you how much water they'd absorbed. The unknowns in the aircon situation are unknowable without somebody doing the basic investigations. Clearly not worth the effort since silica gel is the cheapest desiccant there is. There are many much better (and more expensive) ones of course. If you want to experiment recovering exhausted drier packs then cooking them in the oven for a few hours at regulo 1-2 might be interestng, but as said, you'll never know without measurements. TC

30th July 2020, 00:46	#10
ryszard Gets stuck in Moonstone diesel,X351 Jaguar and now MG ZT 260 Join Date: Dec 2013 Location: Northampton Posts: 682 Thanks: 288 Thanked 118 Times in 96 Posts	Hi,thanks all for the feed back.Having just fitted the new compressor,I will bite another bullet and order a new condenser.The a/c guys recommend a replacement condenser particularly if the dryer has been exposed to air after 7 days.My saga has been dragging on for over two weeks,the air con pipes have been disconnected from the compressor in this time,shame the previous a/c guy did not warn me otherwise I would have avoided this.I considered taking a chance on using the present condenser but the problem would be having to pay another refill price later if it still needed to be replaced and still end up forking out for the new condenser.All compressor suppliers advise to fit new condesets in their literature.Will keep you advised on my outcome.Regards Ryszard....