# Juice Heaters

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.

Vertical Juice Heater (Cail)

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

where

• Q
= heat transfered [kW]
• h
= OHTC [kW/m2K]
• A
= heat transfer area [m2]
• ΔTlog
= LMTD

Values of OHTC from various sources
Source Duty Value Unit
Perk, ISSCT 1962, p601 Exhaust steam 46-231 BTU/ft2/°F/h
Vapour 1 43-221 BTU/ft2/°F/h
Vapour 2 41-129 BTU/ft2/°F/h
Vapour 3 26-125 BTU/ft2/°F/h
Webre in Hugot, 2nd Ed p452   250-300 BTU/ft2/°F/h
Tromp p359   120 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,

ΔT1
and
ΔT2
are most easily defined by example.
In the diagram below
ΔT1
= 120°C - 30°C and
ΔT2
= 120°C - 100°C

The amount of heat transferred is calculated by

Q = m· (h1 - h0)

where

• Q
= heat transfered [kW]
• m
= mass flow rate [kg/s]
• h1
= enthalpy of exiting juice [kJ/kg]
• h0
= enthalpy of entering juice [kJ/kg]

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.