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Because high pressure steam is very valuable, exhaust steam is often used for juice heating or, if possible, preferably bled vapour from the evaporators. It is thus necessary to have a heat exchanger between vapour and juice; this is provided by the juice heaters.
The juice heater (below) consists of an assembly of tubes; the juice circulates through the tubes, and the vapour outside them. Suitable headers force the juice to pass a certain number of times from bottom to top and from top to bottom of the heater by restricting the juice each time to a few of the tubes.
The basic calculation of the juice heater is to calculate the amount of heat transferred using the overall heat transfer co-efficient (OHTC), the log mean temperature difference (LMTD) and the heating surface area.
Q = h· A· ΔTlog
|Perk, ISSCT 1962, p601||Exhaust steam||46-231||BTU/ft2/°F/h|
|Webre in Hugot, 2nd Ed p452||250-300||BTU/ft2/°F/h|
|Tromp p359||Quoting Cuban conditions||100-170||BTU/ft2/°F/h|
|Oliver Lyle EUS Table XLVII||Calorifiers - low velocity||150||BTU/ft2/°F/h|
|Calorifiers - high velocity||300||BTU/ft2/°F/h|
|Bubnik et al 1995, table 445/2||water heated by saturated steam||1.300||kW/m2K|
|raw juice heated by vapour||0.600||kW/m2K|
1 BTU/ft2/°F/h = 5.678 W/m2K
The log mean temperature diffrence is calculated according to the following formula
The temperature diffrences,
The amount of heat transferred is calculated by
Q = m· (h1 - h0)
It is important to keep the juice velocity in the tubes in the range 1.5 m/s to 2 m/s; if the juice velocity is too low the OHTC is low and the heater is prone to scaling of the tubes, if the juice velocity in the tubes is too high there will be a high pressure drop across the heater resulting in a higher pumping load on the juice pumps.