Falling Film Evaporators in the Food Industry

Falling film evaporators are especially popular in the food industry where many substances are heat sensitive.A thin film of the product to be concentrated trickles down inside the wall of the heat exchanging tubes.   Steam condenses on the outside of the tubes supplying the required energy to the inside of the tubes.  As the water from the process stream evaporates (inside the tubes) from the solids dissolved, the product become more and more concentrated.  The evaporators are designed such that a given flow of material can be concentrated to needed solids concentration by the time the stream exits the evaporator.  The two-phase product stream is usually run through a vapor-liquid separator after exiting the evaporator.  This separator allows the vapors to be drawn off the top and the concentrated liquid to exit the bottom (most falling film evaporators are run under vacuum conditions on the process side).

Understanding the Heat Transfer

The simple heat transfer balance for falling film evaporators is:

Eq. (1)

where:

Figure 1: Typical Falling Film 

Evaporator Configuration

Q = heat duty
U = overall heat transfer coefficient
A = heat transfer area
T_S = temperature of condensing steam
T₁ = boiling point of process liquid

The overall heat transfer coefficient consist of the steam side condensing coefficient (usually about 5700 W/m² K), a metal wall with small resistance (depending on steam pressure, wall thickness), scale resistance on the process side, and a liquid film coefficient on the process side which will be extremely dependent on the viscosity of the process liquid over the concentration range.
   

The steam side coefficient can be estimated as above or it can be calculated by the following equation for laminar flow:

Eq. (2)

 and

Eq. (3)

for turbulent flow.  For the equations above,

ρ_L = liquid density (kg/m³)
ρ_V = vapor density (kg/m³)
g = 9.8066 m/s²
L = vertical height of tubes (M)
μ_L = liquid viscosity (Pa s)
μ_V = vapor viscosity (Pa s)
k_L = liquid thermal conductivity (W/m K)
ΔT = T_sat-T_wall (K)
λ = latent heat (J/kg)

All physical properties should be evaluated at the film temperature, T_f = (T_sat - T_wall)/2 except for the latent heat which is evaluated at the saturation temperature.  The resistance due to scale formation cannot be predicted and will probably have to be estimated or compensated for by added a fouling coefficient or by added 5-10% to the calculated heat transfer area (sometimes industry data is available for similar fluids to help estimate this value).  

For the process fluid, the heat transfer coefficient can be calculated with the following expression: 

Eq. (4)

where:

b = 128 (SI units) or 39 (Imperial units)
N_Pr = Prandtl number = 
D = tube diameter
N_Re = Reynolds number = 
ρ = density (subscript "L" for liquids, "V" for vapor)
μ = viscosity (subscript "L" for liquids, "V" for vapor)

Calculating pressure drops in falling film evaporators has been investigated since the late 1940's.  A universal equation is really not agreed upon.  Typically, a constant dependent on the percentage of vapor exiting the evaporator is used in a pressure drop relationship.  If your process fluid shares physical properties close to water, you may be able to accurately predict the pressure drop by using graphs and relations found in Perry's Chemical Engineers' Handbook.  

Evaporating fruit and vegetable juices presents a special challenge for chemical engineers.  Juices are heat sensitive and their viscosities increase significantly as they are concentrated.  Small solids in the juices tend to cling to the heat transfer surface thus causing spoilage and burning

Juice evaporations are usually performed in a vacuum to reduce boiling temperatures (due to heat sensitivity).  High flow circulation rates help avoid build-ups on the tube walls.

For some juices (e.g. orange), it is unavoidalbe that the flavor changes as concentration increases.  Some of the volatile, flavor-containing components are lost during evaporation.  In this case, some of the raw juice is mixed with the concentrate to replace the lost flavors.

Considering that the components of juices have close boiling points, a standard, single evaporator is seldom sufficient.  Either a multi-effect evaporation system must be used (lower capital cost, higher energy costs) or a vapor recompression evaporator (higher capital cost, lower energy costs) is employed.  In a multi-effect system, the pressure is incrementally lowered in each stage, thus pushing the boiling point lower gradually.  This permits more control over the vapor products to be discarded from the system (mainly water) and the vapors to be condensed back into the system (volatile juice components).

The vapor recompression evaporator was designed for maximum efficiency.   These units generally operate at low optimum temperature differences of 5-10 °C.   This requires a larger heat transfer area than multi-effect evaporators, thus the larger capital costs.  However, the energy savings, generally make vapor recompression the evaporator of choice in the food industry.

Figure 2: Typical Vapor Recompression Evaporator Arrangement

References:

Geankoplis, Christie J., Transport Processes and Unit Operations, 3rd Ed., Prentice Hall, 1993, ISBN 0139304398, pages 263-267

Perry, Robert H., et al, Perry's Chemical Engineers' Handbook, 6th Ed., McGraw-Hill, 1984, ISBN 0070494797, pages 10-34 through 10-38

**Special thanks to Rossana Milie from the Department of Chemical Engineering, University of Pisa, Italy for supplying the idea for this article