Sterilizer Steam Requirement

[balagast]

I have a project where I am sizing the steam inlet piping for steam sterilizers, autoclaves. I know how to get the appropriate diameter piping that I need using the mass flow rate through the pipes and my acceptable line velocity limits.

What I am hung up on is accurately calculating the amount of steam that is required to heat the loads. The maximum steam requirement will be in the initial heating phase, when the chamber is taken from approximately room temperature to its operating temperature. There is a set amount of time this must be completed in so I can calculate my heat requirement from the mass of the chamber and its properties (specific heat ect).

My first thought on how to get the steam requirement is to look at it as an open system and use an acceptable change in enthalpy of the steam. I know it is coming in at basically a quality of 1 (at the least its .98). Where I am stumbling is what kind of exit quality I can use to accurately depict what is happening.

Thanks.

 

[ione]

You have your heat load in W (J/s) or equivalent unit. Divide it by the specific enthalpy of evaporation of wet Steam (it depends on steam pressure and dryness). You can this calculator to find the value you need.

http://www.spiraxsarco.com/resources/steam-tables/wet-steam.asp

 

[balagast]

This would assume all of the steam is condensed correct? I was unsure I would be able to make that assumption.

 

[ione]

Right that was the assumption, and it is not that strange for an autoclave (and if you use suitable steam trapping station).

The effectiveness of steam as thermal fluid relies in its high evaporation enthalpy. During the heating process steam turns into condensate and evaporation enthalpy (or latent heat) is what is available for your heating process. It’s meaningless to use steam whether you think to use just its sensible heat.

http://www.spiraxsarco.com/resource...r/methods-of-estimating-steam-consumption.asp

 

[chicopee]

Wouldn't the manufacturer's installation manual give you the information that you desire?

 

[quark]

All governing values and equations (Sterility assurance level, F0 and decimal decay time etc.) of a steam sterilizing process presume complete phase change (i.e full availability of latent heat)

Check HTM 2010 series for requirements and guidance.

As a thumbrule, a 2'x2'x2' chamber steam sterilizer consumes about 50 kg/hr pure steam, when fully loaded.

 

[balagast]

"Wouldn't the manufacturer's installation manual give you the information that you desire?"

These are very large units, that are custom made. What I am doing is re-optimizing their system and replacing the cooling system and inlet steam piping. I suspected their steam supply rates were very high.

I appreciate the assistance. Knowing that the process should rely on complete phase change I will be able to approximate the steam requirements.

 

[speco]

Balagast,

I think you need to look at the startup case for heat exchanger that is condensing the steam. That will determine the worst case for the steam flow. Assuming you are using a steam trap on the outlet, the steam flow should be the instantaneous startup heat load divided by the latent heat. This also assumes there is no superheat.

Regards,

Speco.

 

[balagast]

Speco,
For the case of my steam requirements the H/E's are not doing any condensing until the process is completed, and the exhaust is regulated.

The steam is being condensed by the product inside the sterilizer.

I think I have this mostly sorted out, because I can approximate the loads (their thermal requirement) and the time they must be heated in.

The one last concern I would like verify is that steam will condenser at an appropriate rate so to speak. I know temperature the products need to achieve, but is there a point where the temperature of the steam and product get close enough to one another that the steam condenses at a very slow rate? I would assume steam at say 30PSIG (274F) will condense faster when the product is at room temperature that when it is say 200F. Is this rate of heat transfer simply the convective heat transfer?

 

[ione]

The driving force of the heat transfer process is the temperature difference, so you're right when you say that a beginning of the process, that is when the product to be heated is cold, the condensation rate is faster. Anyway it is up to the person who design the HE to ensure it has enough surface to meet the load requirements in the worst condition.

A marginal note. From your last post and if I'm not mistaken your HE is equipped with a control valve on the condensate side, so you're dealing with a variable area HE. In this case the effective area which gives the main contribution to the heat transfer process is the one where steam is condensing, whilst the submerged area gives just a trivial contribution. I personally prefer to deal with steam supply regulated via a control valve and a steam trap to drain condensate.