The evaporators in which the movement of the liquid takes places as a result of convection current set up by the heating process are called evaporating pans or steam jacketed kettle.
Evaporating pans or steam jacketed kettle comes under the classification of the natural circulation evaporators.
It consists of a hemispherical pan surrounded by a steam jacket. Hemispherical shape provides a large surface area for the evaporation. The evaporation pan may be fixed and the emptying of the pan can be done from the product outlet.
In other cases the evaporators are mounted in such a way that they can be tilted to remove the product. The evaporating pans are heated by the steam which passes through a steam jacket.
The mechanism involved in this evaporation process is conduction and convection so that the heat is transferred by this mechanism to the extract. Steam which gives out heat is supplied to a jacketed kettle or evaporating pan in which the aqueous extract is placed. The temperature raises and the escaping tendency of the solvent molecules in to the vapor increases and enhances the vaporization of the solvent molecules.
- The construction of the steam jacketed kettle or evaporating pan consists of hemispherical structure consisting of an inner pan called kettle and enveloped with an outer pan called jacket. The two pans are joined to enclose a space through which steam is passed.
- Several metals are used as a materials for the construction of the kettle.
- Copper is an excellent material is good for the kettle, because of its good conductivity.
- Some of the acidic materials when evaporated gets some copper to dissolve so such preparations tinned copper is used.
- Iron is used for the construction of the jacketed because it has minimum conductivity.
- To prevent rusting of the jacket the iron is tinned or enameled on the inner surface.
- An inlet for the steam and an outlet or vent for the non condensed gases are provided near the top of the jacket.
- Condensate leaves the jacket through the outlet provided at the bottom. The kettle is provided with one outlet for the product discharge at its bottom.
- Aqueous extract to be evaporated is placed in the kettle and Steam which gives out heat to the content is supplied through the inlet condensate leaves through the outlet.
- For smaller volumes the contents must be stirred manually and mechanically for larger volumes. The rate of evaporation is fast in the initial stages.
- The room where evaporation is carried must have good ventilation to remove the vapor by this apparatus to avoid fog formation of condensed vapor.
- To vapor and prevents condensation in the room and also to accelerates the rate of evaporation fans are fitted.
- The kettle may be fixed or made to tilt. A kettle of capacity up to about 90 liters may be made to tilt. The bottom out let is used to collect the concentrated products.
- Evaporating pan or steam jacketed kettle is suitable for concentrating aqueous liquids.
- Evaporating pan or steam jacketed kettle is suitable for concentrating thermo stable liquors such as liquorices extracts.
- Evaporating pan or steam jacketed kettle is constructed both for small scale and large scale batch operations.
- Evaporating pan or steam jacketed kettle is a simple in construction and easy to operates, clean and maintain.
- Its cost of installation and maintenance is low.
- For the construction of the evaporating pan materials such as copper, stainless steel and aluminum so on wide variety of materials can be used.
- Stirring of the contents or materials and removal of the products is easy.
- Due to natural circulation of the product the coefficient of the heat transfer is poor and solids get deposited which cause the decomposition of the product.
- The heating surface is limited and decreases proportionally to increase in size of the pan.
- It is not suitable for the concentration of thermo labile materials because the liquid is heated all through out in an open atmosphere. Further there is no provision to evaporate under a reduced pressure.
- The evaporating pan cannot be used for the evaporation of thermo labile pharmaceutical materials which are dissolved in an organic solvents, such as ethyl alcohol, as there is no provision to recollect the costly organic solvents.
- The evaporating pan is an open an and the vapor can pass direct to the atmosphere which may be lead to saturation of the atmosphere with the vapors. This in turn may cause discomfort to the worker. It may also slow down the process of evaporation.
- Mehta RM, Pharmaceutics-I: heat processes - evaporation (1996), Page no: 163 - 164.
- C.v.s subrahmanyam et al., pharmaceutical engineering principles and practices - evaporation, 2001, page no. 341 - 343.