O-12
Some insights into flow instability during wort boiling. Flow instability during wort boiling is defined as the unstable flow, or
surging, of wort through the kettle calandria during preheating and/or boiling.
In extreme cases, the violent nature of surging can result in shattered kettle
sight glasses, blowing open of man-hole covers and wort being ejected into the
brewhouse. In less severe cases, the extended residence time of the wort within
the calandria could exert elevated levels of thermal stress, resulting in loss
of foam potential, excessive fobbing and inconsistent processing. Despite this
significance, little is reported on the nature and causes of wort boiler
instability and the study reported here attempts to partially redress this. The
study focused on two approaches: i) a bench-scale examination of the boiling
behaviour of wort together with the determination of any inter-relationship to
its physical properties; and ii) an examination of surging within a pilot-scale
internal thermosyphon wort kettle (90 hl) to determine the influence of
operational and design parameters such as fill height, steam pressure and the
presence of relief slots in the calandria body. Temperature probes were fitted
within the kettle body to determine the impact of surging on the homogeneity of
heating action. Results from the bench scale investigation suggested that the
physical properties (principally surface tension and wettability) of wort were
key parameters in surging as, unlike wort, water was found not to surge. To
assess this, water was used in the pilot-scale boiler and was found to surge
extremely violently, which suggested that boiler geometry and operational
parameters were as influential as physical properties. In the absence of relief
slots, surging was found to be extremely violent and commenced at a bulk
temperature 20°C higher than when relief slots were present. In both cases the
surging period was found to be in the range 1 to 4 seconds. Violent surging
(with no relief slots present) was found to generally be accompanied by a higher
degree of temperature homogeneity in the kettle body during preheat than when
the slots were present. Clearly these and other influential surging parameters
need to be identified further in order to ensure that future boiler designs have
minimal or no surging behaviour, as operational changes may be less-effective
and more constraining from a brewing perspective.
Dr. Bob Stafford is currently Engineering Development Consultant for South
African Breweries based in SAB's Corporate Technical Centre, Sandton,
Johannesburg. He joined SAB at the beginning of 2001. However, prior to that he
lectured in Chemical Engineering at Heriot-Watt University in Scotland where he
was heavily involved with the development of the distance learning p.g.dip/M.Sc.
in Malting & Brewing. In the preceding years his work concentrated on two-phase
flow and heat transfer in process plant conducted within academia and the
scientific civil service. He currently holds a Visiting Lectureship at
Heriot-Watt University.
Danielle Bekker, Rosanna Martin, Manoj Gosai, and ROBERT A. STAFFORD. The South
African Breweries Ltd., PO Box 782178, Sandton, 2146, South Africa.