| Author | Message | ||
     Bob uk Unregistered guest Posted From: 94.197.122.83 |
All refrigerants except two have environmental concerns. The two that don't damage are air and R718 water. Water has 4 times the heat capacity of air by weight and 600 times by volume. Water has about the highest heat capacity of anything else. I have been playing around with water in a vacuum. I found that at 29 inhg the water boils as the liquid water boils it takes a jolt of heat out of the liquid water. Latent heat of vaporization and all that. The water was 10c and it went to 9c. Unfortunately the compressor I was using as a vacuum pump is not efficient and uses 2 hp and it busted a reed valve when the vapour condensed into liquid inside the valve chamber due to heat of friction in the compressor. My idea is a closed system that has a static pressure of less than atmosphere. The idea is that the suction of the compressor lowers the pressure in the low side and raises the pressure on the high side. Trouble is that nature abhors a vacuum and trying to keep the static pressure at 27inhg and working low side at 29inhg is difficult and if left for even 10 mins the pressure goes up and the system won't work. It sounds all wrong that steam is cold. If I could get a better vacuum then the water in the vacuum pot which is a foot lower than the compressor will freeze with water vapour above it at the triple point of water. It was a useful experiment to see if the theory worked and it sort of worked but used a lot of energy to do 1 degree. Mercedes VW and Audi are trying CO2 as a refrigerant. They have the opposite problem of too higher pressure. Also I have designed a system using lpg R290. This system uses water and glycol to lpg heat exchangers to move the heat around the car. The condenser and evaporator are close together next to the compressor thus making for short pipe work. Inside the car is a radiator that runs off the evaporator heat exchanger and at the front of the car is another rad that runs off the condenser heat exchanger. The system is indirect. The two radiators are normal size because air is heated or cooled. The two heat exchangers are small because water and glycol is cooled or heated. No lpg inside the passenger compartment or in the front of the car to be damaged in an accident. Short pipes mean less gas to escape and maybe explode( unlikely) That should keep the safety people happy. The famous five in the fridge world are Air Water CO2 HC(lpg) and Ammonia. If I charge a air receiver with say 90 psi and allow to cool. Then bleed off the air at a reasonable speed ice will block the bleed. It freezes the water in the air. CO2 is weird because it goes immediately from gas to solid and absorbs heat fast. It misses out the liquid stage so the latent heats of fusion and vapouristation happen together. Triple point. Careful with frost bite with CO2. Ammonia is nasty stuff and I won't muck about with the stuff, and I already know it works. I get into the science stuff I find it interesting. (Message approved by david_gore) | ||
| Jan Forrest Grand Master Username: got_one Post Number: 704 Registered: 1-2008 |
Although CO2 does sublimate straight from the solid to the gaseous phase, or vice versa, at normal atmospheric pressures, it can still be made to become liquid (and kept there) by simply pressurizing it to 5 BAR or so. That's how you can get so much of the gas inside a fire extinguisher. As an airgunner I have an 'air' rifle and an 'air' pistol, both of which use the tiny CO2 cartridges rather than compressed air or spring pressure to accelerate the pellet down the barrel. Just one cylinder can give me 50 or more shots before it needs to be replaced. It's also how those 'Sparklets' carbonaters work. | ||
| Bob uk Unregistered guest Posted From: 94.197.122.73 |
Gas temperature and pressure are connected. when gases are compressed they eventually go liquid. The pressure they go liquid at varies with temperature. Butane is a good gas to play with because at minus 0.5c it goes liquid in open air. At 20c it needs 32psig to turn liquid. When it's in a can if vapour above the liquid is used the liquid boils until the pressure goes back to where it was, it does this immediately. Boiling means vapour. This means that pressure does not indicate how much gas. CO2 does the same so that your gun gets the same power each time. When it runs out the pressure drops very fast. If a can of butane is put in the freezer it will pour like water. Pour a small amount into a balloon and tie off. As the balloon warms the butane liquid it will boil inside the balloon. The balloon will wobble around as the butane boils and turns to all gas. The balloon will expand. Then listen through the balloon at sounds and they sound weird. Always wear eye protection and gloves because liquid butane can frostbite your eyes. And definitely do not try propane because it boils at -30 and will put you in hospital. (Message approved by david_gore) | ||
| Randy Roberson Grand Master Username: wascator Post Number: 325 Registered: 5-2009 |
You might try putting the vacuum pump suction through a container of concentrated sulphuric acid. The acid will absorb most of the water vapor and make it possible for you to get a great vacuum. There was a hand-cranked ice machine in the late 19th-early 20th century: the Rapplin. It was a vacuum pump with a jar of acid on the suction side. It would chill a bottle of wine or make ice in a small tray. I was amazed when I ran across it. Most likely what broke your reed valve was high differential pressure across it, generating high force. Don't know, but it happens with compressor valves in natural gas service. Also, as you say, liquid will break valves post haste. propane is a great refrigerant, widely used in refineries. Problem is the flammability. Refineries are able to handle it; consumers are not (to put it mildly). The first refrigerated shipment of meat was on a ship (the Great Eastern? I forget...) using steam-driven, air-cycle refrigeration. Worked great until it broke down and the meat spoiled. Not too efficient, but it got cold and they used coal to drive the steam engine. The history of refrigeration is amazing... | ||
| Norman Geeson Unregistered guest Posted From: 81.97.26.213 |
Bob Quote “Always wear eye protection and gloves because liquid butane can frostbite your eyes. And definitely do not try propane because it boils at -30 and will put you in hospital” Good advice about gloves…… but in the interests of being factual, I think propane, boils about – 42 to -44C. My memory is not as good as it was but I used tons of this gas in road vehicles, not just recently but 35 years ago and I do not think I have that fact wrong. I can also advise you from my experience on national engineering committees that its use in any form of AC plant is a none starter. A number of tunnel, bridge and ferry operators would not accept the situation to name a few. Quote “Ammonia is nasty stuff and I won't muck about with the stuff, and I already know it works” It does indeed work extremely well and most probably the best of the bunch, but few mechanics or technicians understand the problems with its use and you are wise not to “muck about with the stuff”. No doubt it will never make automotive use. Knowing how it works is not the same as knowing the problems and safety issues of this gas…..it has some surprises for the innocent. Oh, yes, I know Ammonia, for 8 years I had responsibility for a 100 ton plant, no not its weight, but its rating ability to turn 100 tons of water into ice in one hour! That also leads me to another point, I see a variety of gases mentioned but I have serious doubts whether any proper study has been made of their ability to present very, very nasty surprises. I do however wish you the best in your endeavours. We always need people who think outside of the box. (Message approved by david_gore) | ||
| David Gore Moderator Username: david_gore Post Number: 1488 Registered: 4-2003 |
Randy, I suspect your information may not be correct, viable mechanical freezing plant became available in the 1870's and the "Great Eastern" had already been converted to a cable-layer after its failure as a passenger/cargo carrier. The "Great Eastern" could have carried ice-chilled meat from the USA to the UK. Previous unsuccessful shipments from Australia using mechanical refrigeration included the "Northam" [1877], the refrigeration equipment was functional but refrigerant leaks could not be rectified before the ship had to sail and the meat cargo was left behind. Australian Thomas Mort who was the driving force behind the shipment died not long after this failure and never saw the later success of the frozen meat trade that he promoted. The SS "Paraguay" was the first successful international shipment of fully frozen meat [not ice-chilled meat] in 1876/1877 from Buenos Aires to Le Havre.This shipment was followed by the successful "Strathleven" shipment from Australia [1880] and the "Dunedin" shipment from New Zealand [1882] to the UK. These shipments established the important frozen meat trade between Australia/New Zealand and the UK. | ||
| Bob uk Unregistered guest Posted From: 94.197.122.79 |
I chose butane to play with because it's about the least dangerous. And does things without extreme cold or pressures. Of course the flamabity is a concern but I use very small amounts in a drafty old garage with the doors open. Ammonia is used in heat treatment of metals and I have had a niff which was enough for me. I am cautious with stuff because for instance there are several easy ways to make phosgene. It's a shame that lpg won't pass the safety bit because it's such a good refrigerant that doesn't react with stuff to form acids in the system. My idea is make a mobile system that uses the smallest amount possible. One way is to move the refrigerant fast and keep pipework short, so everything happens faster but on a smaller scale. Ideally 200cc of lpg only. I think the co2 system from Germany will be abandoned because of its high pressures. I have a small caravan fridge that works by burning propane which heats up a water ammonia mixture which mixes with hydrogen and gets colder. (Message approved by david_gore) | ||
| Randy Roberson Grand Master Username: wascator Post Number: 328 Registered: 5-2009 |
David is correct; I recall that the first machine was an air-cycle, though. Compress air; cool it back to ambient with sea water, then expand it and it gets very cold. They used a steam-driven double-acting compressor, except one end of the compressor cylinder was the compressor and the other end was the expander. The same effect makes your pneumatic impact wrench get cold when you are getting on it. I could always be wrong. Nowadays one would use a turboexpander, like the driver side of a turbocharger. The history of refrigeration is fascinating. |
