Calandria Evaporator Design Calculations
Cortney Ruic
This document discusses evaporator equipment design and types of evaporators used in food processing. It describes the key parts of an evaporator including the heat exchanger and means of separating vapors. Selection criteria for evaporators include operating capacity, degree of concentration required, and heat sensitivity of the product. Common evaporator types are described such as open pan, short tube, long tube, climbing film, falling film, forced circulation, plate, expanding flow, and scraped surface evaporators. The effect of evaporation on foods such as loss of aroma compounds and darkening of color is also summarized. Finally, the document provides a brief overview of distillation and how it separates more volatile components from residues in food processing.Read less
The object of evaporation may be to concentrate a solution containing the desired product or to recover the solvent. Sometimes both may be accomplished. Evaporator design consists of three principal elements: heat transfer, vapor-liquid separation, and efficient utilization of energy.
calandria evaporator design calculations
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Sugar industries using formula to calculate heat transfer coefficient of a evaporator is= ( Evaporation coefficient x Latent heat of steam or vapour)/ Temperature difference. I am not sure about this calculation. According to chemical engineering Overall Heat transfer coefficient is a function of flow geometry, fluid properties and material composition of evaporator. Heat transfer taken place by conduction , convection and radiation.
I want to know that you have done any calculation by considering above mentioned factors to calculate Overall heat transfer coefficient of a evaporator.
The flow rate through the circulation loop of an evaporator determines the heat transfer coefficient in the calandria and also the quality (weight fraction vapour) of the two-phase mixture leaving the calandria, and is a key parameter in the design. In order to determine the circulation, it is necessary to be able to estimate the pressure drop through the circulation loop. This is especially important in a natural circulation evaporator, where the circulation rate must be determined by trial and error so that the head available from the differences between the densities in the riser and downcomer equals the loss due to friction and acceleration around the circulation loop. Computer programs are normally used for this. With assisted and forced circulation it is also necessary to be able to estimate pressure drop so that the head required from the impeller can be estimated.
The estimation of pressure drop in the regions of single-phase flow presents no major problems, and the methods can be obtained from Section 144 and Section 146. The pressure drop in the boiling zone is more important because it is usually greater and always much more difficult to estimate, there being big discrepancies between the various published and proprietary correlations. It is first necessary to be able to estimate the density of a two-phase mixture, for the estimation both of the pressure drop due to acceleration and of the pressure rise or drop due to gravity. Second, the pressure loss due to friction must be estimated. Correlations for two-phase density and friction are given in Section 154.
Two phase fluid flows are liable to be unstable. There are several different forms of instability, and it is important to ensure that the flow in an evaporator will be steady. More information on the subject may be found in Hewitt et al. (1994). Unsteady flow in an evaporator can upset other items of equipment on the plant, and it has a very serious effect in a crystallizing evaporator on the size of the crystals.
Concentration of a liquid by evaporation under vacuum was introduced in 1913. The process was based on a British patent by E.C. Howard, which covered a steam-heated, double-bottomed vacuum pan with condenser and vacuum pump.
Distribution in a plate-type, falling-film evaporator can be arranged with two pipes running through the plate pack. For each product plate (in Figure 6.5.4), there is a spray nozzle in each product pipe, spraying the product in a thin, even film over the plate surface. In this case, the product enters at evaporation temperature to avoid instant flash evaporation during the distribution phase.
The water component of the thin product film evaporates rapidly as the product passes over the heating surface. A vapour cyclone separator (2) is fitted at the outlet of the evaporator. This separates the vapour from the concentrated liquid.
As evaporation proceeds, the volume of liquid decreases and the volume of vapour increases. If the vapour volume exceeds the available space, the velocity of the vapour will rise, resulting in a higher pressure drop. This will require a higher temperature difference between the heating steam and the product. To avoid this, the available space for vapour must be increased as vapour volume increases.
To achieve optimum evaporation conditions, the product film needs to have approximately the same thickness over the length of the heating surface. Since the volume of available liquid steadily decreases as the product runs down the heating surface, the perimeter of the heating surface must be decreased to keep the film thickness constant. Both of these conditions are fulfilled by the plate design of the falling-film cassette evaporator shown in Figure 6.5.4. This unique solution makes it possible to evaporate using very small temperature differences at low temperatures.
The residence time in a falling-film evaporator is short compared to other types. The combination of temperature and time in the evaporator determines the thermal impact on the product. Using a falling-film evaporator with a low temperature profile (low evaporation temperatures, small temperature differences, and low heat load) is a considerable advantage for the concentration of dairy products which are sensitive to heat treatment.
This is the evaporator type most often used in the dairy industry. The key to success with falling-film evaporators is to obtain uniform distribution of the product over the heating surfaces. Vertically arranged tubes are used for the most part, where the product flows downwards on the inner surface of the tubes and the heating steam condenses on the outer surface of the tubes.
The length of the tubes may vary up to 20 m. The length of the tubes is selected in order to promote good circulation of the heating steam around the tubes. The tubes are encased and could be insulated.
The overall heating surface is divided into a number of sections and the milk flows only once through each of these. Uniform distribution of the product over the heating surface is very important for economical operation of an evaporator. Gaps in the distribution lead to local overheating. This causes the product to stick, thereby impairing the transfer of heat into the product and impeding cleaning. This reduces production uptime.
Uniform product spreading in the head section (Figure 6.5.8) of the evaporator is required for good distribution, as is correct calculation of the sections. This is achieved by means of a horizontal spreader plate beneath the cover of the heater. Holes drilled concentrically around the downpipes lead the product into the tubes as a uniform film. Slightly superheating the product when feeding it into the spreader section makes it expand and thus ensures immediate partial evaporation and good distribution. The vapour forces the product to the inner surface of the evaporator tubes, where it flows away as a thin film.
Multiple-effect evaporators are usually used. The theory is that if two evaporators are connected in series, the second effect can operate at a higher vacuum (and therefore at a lower temperature) than the first. The vapour evolved from the product in the first effect can be used as the heating medium for the next effect, which operates at a lower boiling temperature due to the higher vacuum. One kilogram of water can be evaporated from a product with a primary steam input of 0.6 kg, even allowing for heat losses.
It is also possible to connect several evaporator effects in series to further improve steam economy. However, this makes the equipment more expensive and involves a higher temperature in the first effect. The total volume of product in the evaporator system increases with the number of effects connected in series. This is a drawback in the treatment of heat-sensitive products. However, evaporators with four to seven effects and additional finishers have been used in the dairy industry for a long time now in order to save energy.
The milk is pumped from a balance tank (1) to the pasteurizer (2), where it is pasteurized and heated to a temperature slightly above the boiling point of the first evaporator effect. The milk then continues to the first effect (4) of the evaporator, which is under a vacuum corresponding to a boiling temperature of 60 C. The water evaporates and the milk is concentrated as the thin film of milk flows downwards in the tubes.
The concentrate is separated from the vapour in the bottom part of the calandria and the vapour separator(5) and pumped to the second effect (6). The vacuum is lower in this effect, corresponding to a boiling temperature of 50 C. After further evaporation in the second effect, the concentrate is again separated from the vapour in the bottom part of the calandria and the vapour separator (5) and pumped out of the system for further treatment (10).
The injection of high-pressure steam into the thermocompressor (7) increases the pressure of the vapour from the first effect. The live steam/vapour mixture is then used to heat the first effect (4).
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