Evaporation 481 The input data that is needed to complete the heat exchanger specifi- cation sheet for an evaporation system can be grouped together in three categones Process variables: material balance and flow rates, oper- ating pressure, operating temperature, heating medium temperature, and flow rate Physical property data: specific gravities, viscosity-tem perature relationships, molecular weights, and thermody- Mechanical design variables: pressure drop limitations corrosion allowances, materials of construction, fouling factors, code considerations(ASME, TEMA, etc. 3.0 LIQUID CHARACTERISTICS The properties of the liquid feed and the concentrate are important factors to consider in the engineering and design of an evaporation system The liquid characteristics can greatly influence, for example, the choice of metallurgy, mechanical design, geometry, and type of evaporator. 4 Some of the most important general properties of liquids which can affect evapo- rator design and performances are Concentration--Most dilute aqueous solutions have physical proper ties that are approximately the sameas water. As the concentration increases the solution properties may change rapidly. liquid viscosity will increas dramatically as the concentration approaches saturation and crystals begin to form. If the concentration is increased further, the crystals must be remove to prevent plugging or fouling of the heat transfer surface. The boiling point of a solution may rise considerably as the concentration progresses Foaming-Some materials, particularly certain organic substances may foam when vapor is generated. Stable foams may be carried out with the vapor and, thus, cause excessive entrainment. Foaming may be caused by dissolved gases in the liquor, by an air leak below the liquid level, and by the presence of surface-active agents or finely divided particles in the liquor Foams may be suppressed by antifoaming agents, by operating at low liquid levels, by mechanical methods, or by hydraulic methods Temperature Sensittvity-Many fine chemicals, food products, and pharmaceuticals can be degraded when exposed to only moderate tempera- tures for relatively brief time periods. When processing or handling heat sensitive compounds, special techniques may be needed to regulate the temperature/time relationship in the evaporation systerEvaporation 481 The input data that is needed to complete the heat exchanger specifi￾cation sheet for an evaporation system can be grouped together in three categories: Process variables: material balance and flow rates, oper￾ating pressure, operating temperature, heating medium temperature, and flow rate. Physicalproperty data: specific gravities, viscosity-tem￾perature relationships, molecular weights, and thermody￾namic properties. Mechanical design variables: pressure drop limitations, corrosion allowances, materials of construction, fouling factors, code considerations (ASME, TEMA, etc.). 3.0 LIQUID CHARACTERISTICS The properties of the liquid feed and the concentrate are important factors to consider in the engineering and design of an evaporation system. The liquid characteristics can greatly influence, for example, the choice of metallurgy, mechanical design, geometry, and type of evaporat~r.[~] Some of the most important general properties of liquids which can affect evapo￾rator design and performances are: Concentration-Most dilute aqueous solutions have physical proper￾ties that are approximately the same as water. As the concentration increases, the solution properties may change rapidly. Liquid viscosity will increase dramatically as the concentration approaches saturation and crystals begin to form. Ifthe concentration is increased further, the crystals must be removed to prevent plugging or fouling of the heat transfer surface. The boiling point of a solution may rise considerably as the concentration progresses. Foaming-Some materials, particularly certain organic substances, may foam when vapor is generated. Stable foams may be carried out with the vapor and, thus, cause excessive entrainment. Foaming may be caused by dissolved gases in the liquor, by an air leak below the liquid level, and by the presence of surface-active agents or finely divided particles in the liquor. Foams may be suppressed by antifoaming agents, by operating at low liquid levels, by mechanical methods, or by hydraulic methods. Temperature Sensitivity-Many fine chemicals, food products, and pharmaceuticals can be degraded when exposed to only moderate tempera￾tures for relatively brief time periods. When processing or handling heat sensitive compounds, special techniques may be needed to regulate the temperaturehime relationship in the evaporation system