Superheating in Autoclaves 1

[PharmBoy]

I am new to the industry and my thermo is a little rusty, but I have read that it is very important that all the air is removed from the autoclave chamber for the steam sterilization process to work correctly. In addition, I also have read that air leaks can cause superheating. Can someone explain this to me or point me to a link that might have additional info on Autoclave operations?

Thanks!

 

[Joesteam]

The air acts as an insulating layer. If it pockets in a particular area of the autoclave, that area will not get to the correct temperature.
As for superheating, I don't believe the air has anything to do with that. However, when the steam goes from a high pressure to the much lower pressure in the autoclave, superheating is possible. It is not usually a problem in autoclaves as the superheat level is low and there is usually enough risidual water in the autoclave to quickly desuperheat the steam. It is a problem in wood drying kilns where the residual superheat reduces the humidity level in the kiln that is important to prevent warping and cracking, but this is another application altogether.

 

[PharmBoy]

Joesteam.....Here is the whole story. We use our autoclave to sterilize tools and containers to manufacture pharmaceuticals. During validation, we are seeing higher temperatures on the top shelf....0.5°C to 1.0°C higher. This shelf is has small items that are double bagged and very tightly packed on the shelf. My thoughts were that if we were not getting air out of the bags or if there was air infiltrating the chamber at the top, we would see the temperatures lower since the air (as you stated) would act as an insulator. However, a number of people, on-site, have brought up a superheating state that is occurring. I personally do not currently understand this phenomenon. I’m reviewing my thermodynamics, especially mixtures. But what I have reviewed still does not point me to a temperature increase. Still looking for additional help! Thanks for your thoughts!

 

[PharmBoy]

Oh! I forgot to add that the supply clean steam to this autoclave is dropping from 45 psi to 30 psi approximately every 2 minutes for about 15 seconds. I have a feeling this might have something to do with the superheating.

 

[Joesteam]

There is a potential for some superheat with a reduction from 45 to 30 psig, however not likely and not very much. Unfortunately I forgot how to read the Mollier chart that will tell us how much potential superheat you could get with this pressure drop. Even so, any presence of liquid either in the steam or the autclave will quickly desuperheat the steam.
The next question is what are the actual temperatures you are measuring in this top portion (or the bottom). Is this temperature above that of saturation at 30 psig (274 deg F)? If it is not, there is little chance of superheat present.
What may be happening is that the steam is condensing on the top portion and subcooled condensate then drips down cooling the lower section a bit. Or a longshot is that there could be an air pocket at the outlet because the steam trap cannot handle air well.

 

[PharmBoy]

Thanks for the info and discussion Joesteam. I will look at the Mollier Chart. If some revelation pops up or I figure something out, I will post it.

Thanks again!

 

[quark]

Pharmboy,

You are right that the temperature should reduce upon air ingress and it takes more time to get the sterilization temperature. However, HTM2010(the best guide on sterilization process) allows you a temp. difference of 3⁰C from the cold point to the maximum temperature. Further, you should check for proper draining of condensate from the autoclave. One more thing is to check the accuracy of temperature probes. Calibrate the cold point probe and the top probe with same reference standard at the same time. You can get good information from

http://www.fedegari.com

Joesteam,

With the advent of software, the things become very easy and Mollier Charts are passe. You can download steamtab from

http://www.chemicalogic.com/steamtab/downloads.htm

Go to the saturated tab and get the properties at the required pressure. Entlapy of steam remains same during the expansion. Now go to the Superheated/Subcooled tab and choose the new pressure and enthalpy. This will give the actual temperature. Again calculate the saturation temperature and you will get the degree of superheat. For example, enthalpy at 45psig(or 59.7psia) pressure is 1177.71btu/lb. With 30psig(or 44.7psia) and 1177.71btu/lb enthalpy, the temperature is 284.616F. The saturation temperature at 44.7psia is 273.999F. Steam gets superheated by 10.617F.

Regards,

 

[PharmBoy]

Thanks "quark" for the link and the info. Your info and some of the other things I have found have been very useful. I guess what I have discovered is that our autoclave is pretty sick and is in need of a good old tune-up.

Thanks again!

 

[lilliput1]

The sterilizer (autoclave) should have a vacuum pump to draw air out. Check with the manufacturer of your sterilizer and get a copy of its testing & validation procedure. Sterilzation should use saturated steam for full effectivity of sterilzation. Get also their clean steam requiremt and check loads on the clean steam generator. It may not have enough capacity to meet all the loads simultaneously. You can also do a google search on sterilizer cycles, Primus sterilizer models etc.

 

[Jigneshpadia2002]

Can we get superheating in OVEN as well. I mean the instrument used as DRY Autoclave ! No steam ! What will be the case there?

 

[Joesteam]

Only steam will give you superheat

 

[quark]

That is called overheating in Dry Heat Sterilizers and is basically due to improper heat distribution and penetration. Things to take care or, even distribution of load in the sterilizer, uniform air flow in the sterilizer and good accuracy temperature sensors. There will be no problems with the process but you will be wasting energy.