STEAM THERMODYNAMICS AND STERILIZATION

                             STEAM THERMODYNAMICS AND STERILIZATION

TEMPERATURE AND HEAT: Temperature is a measure of thermal energy, while heat is energy that is transferred as a result of a temp diff bet object and surrounding.

MECHANISM OF ENERGY TRANSFER:

1)   Conduction:  Transfer of energy through molecular agitation without any required motion of material as a whole.

2)   Convection: Transfer of energy resulting from contact with a moving fluid.

3)   Radiation: Transfer of energy through electromagnetic waves.

There is significant difference in energy content at a given temperature in various heating media. Heating media like super heated water, saturated steam, and steam air mixture contains different amount of thermal energy.

THERMODYNAMIC CHARACTERS OF STEAM: 

Saturated steam contains 2675 J/g at 100^oC, which consists of energy in the water 419J/g and energy required to create steam 2,256 J/g (heat of vaporization/condensation at 100 ^oC).

Hence The condensation of 1 gram of steam imparts 2, 256 joule to the object at 100 ^oC. 

-At 25 ^oC and 1 atm pressure, 4.1 Joule is required to change the temp of one gram of liquid water by 1 ^oC. The temperature will increase with input of energy until reaches 100 ^oC.  No further temp change will occur at 1 atm pressure  until an additional 2,256 joule have been absorbed by the water  to convert it to steam. The same amount of energy is imparted to an object when the process is reversed and steam is condensed to water.

Similarly at 2 atm pressure, the temperature will increase with input of energy until reaches 121 ^oC,  No further temp change will occur at 2 atm pressure  until an additional 2199 joules have been absorbed by the water  to convert it to steam. The same amount of energy is imparted to an object when the process is reversed and steam is condensed to water.

There is only one pressure that corresponds to a specific temperature on the curve when steam is saturated for example : at 1 atm saturated steam obtained at 100^oC, while at 2 atm it is at 121^oC. 

The energy properties contained by water and saturated steam at a definite temperature and pressure are already well known and constitutes steam table (Refer ASME International Steam table). Thus sterilization cycles are designed by considering these saturated steam temperature and pressure relations.

 If the values of Pressure and temperature are not in general agreement with established steam table then this may be an indication that the sterilization cycle is taking place without the full effect of the heat.

Also Flow of heat from heating media to a sealed container depends on:

1)   Temp difference  between container and heating media

2)   Geometry and characteristic of container

3)   Overall heat transfer coefficient: Overall heat transfer coefficient: is a complex function that includes the thermodynamic characteristics of the heating media. Apart from energy content difference of steam at difference temperature and pressure, there is difference in heat transfer rate of alternate Medias.

-  Saturated Steam: Heat Transfer rate –High

-  Steam Air Mixtures: Heat Transfer rate –Function of steam to air ratio and Flow velocity.

-  Super heated water :

Water Spray with air over pressure: Heat Transfer rate – Moderately High, Function of Flow velocity

Water Submersion with air over pressure: Heat Transfer rate - High but function of flow velocity.

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Article Reference:  PDA Technical Report No.1 “Validation of moist heat sterilization process. Cycle design, development, qualification and ongoing control”

#1.2 STERILIZATION- F VALUE

Continued from #1.2

Lethal Rate:  can be calculated by

L_(Tref,Z)=10 ^(T-T_Ref^)/Z

Where

T= Temperature of item being heated

T _ref = Reference Temperature

Z = Z value of Challenge organism (10^oC if unknown)

Lethal rate is an exponential function; hence a small change in temperature difference can have a significant effect on delivered lethality.

For a BI system witha z value of 10^oC and T _ref -121 ^oC, decrease in lethality by decrease in 1 ^oC temperature can be calculated as:

L=10 ^(120-121)/10= 10^-0.1 = 0.79

It means for a BI with a Z value 10 ^oC one minute at 120 ^oC is equivalent to 0.79 minute at 121 ^oC in terms of lethality.

Microbial destruction at any temperature (120 ^oC in example) can be related to equivalent minutes at a reference temperature (T _ref ; in example 121 ^oC).

F_physical /F value: means the equivalent amount of time, in minutes at T _ref, which has been delivered to a product by the sterilization process".

F value is a measurement of lethality of a process.

F value is the calculated equivalent of time in terms of lethality at a reference temperature T _ref and a temp coefficient Z that is delivered to item being sterilized.

The F value is the integration of the lethal rates throughout the process.

F_ref = d(∑L)

Where:

d= the time increment between each temperature reading

L= Lethal rate calculated for each temperature reading.

F₀ Value:  is the number of equivalent minutes of steam sterilization at a temperature of 121 ^oC delivered to a container or unit of product using Z value of 10 ^oC.

F₀ of 8 minutes means sterilization effectiveness of that cycle is equivalent to 8.0 minute at 121 ^oC.

F _Bio: The term F _bio represents the delivered lethality measured by the actual kill of microorganism or in a BI System

F _bio = D_T×LR

D_T - D value of the BI at reference temperature

LR- Log reduction of BI population achieved during the cycle

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Article Reference:  PDA Technical Report No.1 “Validation of moist heat sterilization process. Cycle design, development, qualification and ongoing control”