Holding tube
Holding tube
The holding tube is the key component of an inline thermal
process. It has to ensure a specific microbiological destruction
and/or a chemical reaction in a product and has to maintain the
temperature and time in a continuous thermal process.
Critical issues
Certain critical points have to be controlled, in order to have
a correct thermal treatment.
Back pressure
The backpressure is applied to avoid gas bobbles coming from
non-condensed steam or from air content. Non-condensed steam can
lead to believe in a constant sterilization temperature and the
calculated holding time can be wrong.
Recommended holding tube pressure is 1 2 bars above the boiling
point:
Fouling
Fouling in a holding tube, can shorten the holding time,
increase the pressure loss in the tube, indicate wrong temperatures
due to coated sensors and give brown particles in the product. To
overcome this problem certain points can be applied: Product
adaptations (pre-denaturation, pH adjustment, ingredients choice)
Higher holding tube pressure = thinner layer Teflon sleeve coating
of holding tube High flow velocity (>4m/s) Longer running time
with bigger tube diameter (More CIP problems) Direct heating
generally less fouling than indirect heating
Deaeration
Product with a lot of cooperated air, will give increased
fouling in the holding tube. Nestl recommends turbulent conditions
with >15000 Re for any position of the holding tube.
Heat loss
In a UHT process with lower temperatures and longer holding
tubes, the heat loss can be significant. In order to detect this
problem, temperature sensors at the beginning and at the end of the
holding tube are recommended. Proper insulation of the pipe will
prevent any heat loss.
