Cooling bath is used in cold water, crushed ice, mixture of eutectic salt and ice, dry ice (solid carbon-dioxide) or even in liquid nitrogen too.
In pharmacy circumstances, consistent and slow cooling of melted ointment materials is normally performed with manual mixing and in patendula with pestle, in order to create the appropriate gel structure. At preparation of higher amount of ointment, cooling and mixing have to be regulated and carried out in dedicated devices (frequently in duplicators). At the preparation of heated solutions, syrups, decoctions, infusions, cooling bath or perfusion of cooling medium are also used.
Laboratory refrigerators are able to condensate hot steams, which are operated by perfusion
of cooling medium. These refrigerators have different capacity of heat, which depends on their internal surface (e.g. Liebig cooler, cooling coil and ball). Continuous heat reduction is needed to maintain the cooling evenly, which is achieved by perfusion of cooling liquid, frequently by water. Even more amount of steams can be condensed by these cooler, than in case of air-cooled condenser, or retort.
Devices providing continuous heat reduction are termed to refrigerators.
Temperature, which is lower than environmental can be developed and permanently maintained. Energy investment is needed for this process, while during this process, heat has to be reduced from a closed space what has to be transferred in another place.
These devices are called as heat pumps.
Based on their operation, the most frequent used cooling devices can be:
vapor-compression-type and,
absorption-type refrigerators.
Vapor-compression-type of refrigerators is applied in many places for example as refrigerators in pharmacy, to store medicines, or in air conditioning systems. The refrigerators circulate the cooling medium (e.g. ammonia) in a closed space. During the process of cooling, the cooling medium absorbs heat from space, which should be cooled, then transfers and emits. Steam developed after evaporation of cooling medium is led to a compressor, which condenses the steams to an appropriate pressure. The high-pressure steam is revamped to overheated steam by compressor, then goes to condenser. The condenser is a special heat exchanger, where the steam is cooled and condensed by cooling air or cooling water, meanwhile the cooling medium emits the heat absorbed from system to water or air.
The condensed, liquid cooling medium is led through a controllable throttle, where pressure of liquid cooling medium decreases suddenly by adiabatic change. One part of cooling medium evaporates and cools down to the temperature of space, which should be cooled. Due to evaporation it absorbs the heat from its environment, as well as cools its environment too. The absorbed heat from cooled space is transferred to circulating in evaporator. From here the steam goes into compressor again and the entire process repeats.
Chapter 8: Heat reduction
Fig. 8.1.
Operation of vapor-compression-type of refrigerators
The absorption-type refrigerators do not contain any moving parts, thus there is no noise, vibration and no internal oil contamination, with which has to be regarded in case of vapor-compression type. Their three major structural elements (condenser, throttle, evaporator) are the same as in the last mentioned type, but compressor are substituted by a new structural element. The essence of cooling process is the absorption and desorption process between gas and liquid. Absorption has to carried out in cold medium, while desorption in warm medium. If ammonia is used as a cooling medium, then water is the most suitable solvent. Firstly liquid ammonia goes into the evaporator (similarly to vapor-compressor type), where forms into steam, during which cools down its environment. Cooling medium coming from evaporator moves into dissolution container, in which dissolves in the proper chosen, and matched solvent, namely absorbs. The heat released during dissolution is absorbed by coil heat exchanger. The created solution is transferred by pump through the heat exchanger into boiler, where pressure is much higher than before. The released ammonia steam is transferred into condenser, and cooled with water, while ammonia liquefies. The pressure in condenser is determined by the temperature of cooling water. The liquefied ammonia is transferred through a throttle, while its pressure is decreasing. In evaporator, ammonia becomes steam again.
Fig. 8.2.
Operation of absorption-type refrigerators
One part of pharmaceutical materials and preparation is stored in room temperature (15–25°C), and the other has to be stored only in cooled area, when their it is justified due to their physical, chemical, microbiological stability, or heat sensitivity.
Material, or medicine in small amount, which should be kept in cool place has to be stored in refrigerators. According to standards insulin preparations and eye drops from FoNo have to be kept in refrigerators (2-8 °C).
Chapter 8: Heat reduction
Fig. 8.3.
Pharmacy refrigerator
Oculogutta rifampicini FoNoVII. preparation has to be stored in freezer, which means under -15 C°. Its expiry date besides this storage is 1 month, but after unfolding until only 5 day can be used. In the meantime it has to be stored on 2-8 °C.
In pharmacy, medicines in high amount are stored in cooling rooms, in the case of storage of higher stock in factories and wholesalers products are kept in cooling halls.
Cooling rooms and halls which are planned on stock amount and achievable, sustainable temperature, are rooms equipped with devices with suitable capacity and surrounded by appropriate insulated walls. Their internal temperature is adjustable and controllable. Medicines should be placed on internal shelving systems.
Mucuses (Mucilago hydroxyaethylcellulosi, Mucilago methylcellulosi), syrups (Sirupus aurantii, Sirupus laxans, Sirupus liquiritiae, Sirupus simplex), ointments (Unguentum emolliens) from Hungarian Pharmacopoeia (Ph. Hg. VII) and nutrition are stored in cool place, which means 8-15 °C.
Questions
1) What are the required properties of cooling media?
2) What are the media most frequently used for heat reduction, and what temperatures can they achieve?
3) What are the main types of the most commonly used cooling devices?
4) What are the main attributes of vapor compression type cooling devices?
5) What are the main attributes of absorption type cooling devices?
6) What is the required storage temperature of different kinds of pharmaceutical prep-arations?
References
Banker G.,S.,. Rhodes Ch., T.: Modern Pharmaceutics, Marcell Dekker, Inc., New York, USA, 2002.
Langley A.,C., Belcher D.: Applied Pharmaceutical Practice, University Press, Cam-bridge, Great Britain, 2009.
Augsburger L.,L., Hoag S.,W.: Pharmaceutical dosage forms: Tabletts, Informa Healthcare, Inc. New York, USA, 2008.
Chowhana Z.,T., Linna E.E.: Mixing of pharmaceutical solids. I. Effect of particle size on mixing in cylindrical shear and V-shaped tumbling mixers, Powder Technology,.24, 2, 237-244, 1979.
Aulton E.,A.:The Design and Manufacture of Medicines, Elsevier Ltd, New York,USA,2007.
Recommended websites
http://www.ubter.in/Curriculum/Chemical/Document/sem5.pdf http://www.nzifst.org.nz/unitoperations/httrapps.htm