How Much Heat is Required to Sterilize with Steam?

The people who sell these things will advise you on this. I do not suggest you build one of these.

They are called "autoclaves" and they are well insulated pressureizable boxes with safety features and fast open doors.

They will connect to saturated steam at a little above the required pressure and temperature (some are self contained and create their own steam) and modern ones are fully automatic. Dial in the time and temperature, load them and push start.

Older ones needed to be watched. Some disastrous explosions made the law and insurance deal with these as regulated pressure vessels that need to be made and inspected to standards and inspected annually. If you made one with that size and it blew up it would knock down the building and kill people nearby.

http://www.google.com/search?hl=en&q=autoclave+%2B%22inspection+standards%22

Aurizon, thanks for your reply...but let me clarify the question.

"I do not suggest you build one of these."

What I am considering is not to build an autoclave to place other items into, but to sterilize an existing cylindrical pressure vessel by applying steam. I intend to purchase an industrial grade pressure washer/steam cleaner to circulate steam through the vessel. I thought I might be able to install a relief valve on the bottom of the domed end cap to open at the required 2 Bar and just let the steamer continue to run to maintain the pressure and temp.

I am looking for a way to calculate the btu output requirements of the burner/coil to maintain the necessary tempurature.

Am I way off base with this idea???

WyoPatriot

if the vessel is not insulated you can assume it is a condensing surface at 135C fed by your washer.

The amount of steam might be quite a lot as any steam will condense on the inner metal and keep it at 135C. If it has a large area it might need more steam than your jenny can supply. Creating steam at 135C in quantity is energetically demanding. 4" of temporary fiberglass wrapping will cut this loss.

Wise to have a safety release valve. What is the pressure capability of the vessel you are pressurising? If it is good to 2000 PSI = less risk.

wyopatriot, hope, the term sterlisation belongs 2 biochemical/bioprocess engineers . for effective sterlisation,as u said it does not requires 135 deg, only 121deg @2bar for 30 min is enough,this is the precise figure which bioprocess industry follows,even USFDA recommends.

Calculation is:

heat rerequired = mass to be heated * specific heat of the material to be heated * temp gradient

vol to be heated is 65 cub . ft =1841 lit

consider water as reference liquid , 1841 lit = 1841 kg

Temp gradient = 135-18=117, as ambient temp is 18 deg

heat req = 1841 *1*117=215397 Kcal/hr

as it is to be achieved in 30 min , the output should be 215397*2=430794kcal/hr

confirm the sterlisation temp , because above 121 deg , no much, probably no change is going to happen except your process cost.

Hope this info would be enough for you.

gjprince -

Thank you. You're right on the 121 C temp requirement, the 135 C was a rule of thumb given to me by a vendor. This is to sterilize liquid holding containers similiar to what would be necessary in a dairy or brewery, although this is for oil/gas processing equipment.

In your calculation you state, "consider water as reference liquid , 1841 lit = 1841 kg",

You use the mass of 1841 liters of water, should this value not be the mass of the vessel itself plus the mass of the water vapor within the vessel. the way you have it figured the entire vessel would be full or liquid water at 121 C???

wyopatriot

usually what we do instad of empty vessel sterlisation , we do with water since it may touch all the part of vessel as air is a poor conductor , may form some cool pockets and does not allow the steam 2 heat @121deg and also if there is in experience handling in empty vessel ster ,chances for vaccum creation and temp gradient may even buckle the vessel(i had seen a 30 kL fermentor got buckled!!!).if u r confident enuf in empty vessel ster go ahead. otherwise go for water filled ster.

Also coming 2 water vap ,as it is formed due to condensation of steam it is 100%pyrogen free does not affect the process in the grounds of sterlity,provided vol is not a critical parameter .

Also during calculating energy the effective working vol alone cannot be considered , thats y , i took the entire vol.

hope this justification is enuf.i am also not very clear with the question u asked.

gjprince - I had not considered vacuum relief during the cooling time. I have need to keep the vessel anaerobic, so I may need to maintain the pressure by adding inert gas when cooling. Thank you for the practical advice.

WyoPatriot

Use heated nitrogen gas it is better and you don't have so many problems with very hot water/steam. it can just be bled off after use. You can heat it by almost any method and it is non poluting. It is non reactive in most situations does not cause static build up. It has so many advantages.