Batch ovens

Batch ovens Forced convection Paste, granules, extrude cake Pigment dyestuffs, pharmaceuticals, fibres... 

Principles as described above for the physical process validation of batch ovens apply also in the validation of tunnel sterilizers however, in addition to the variables affecting batch oven validation, tunnel sterilizers have an extra variable-belt speed. This variable can be held constant by maintaining the same belt speed throughout the validation process and not changing it after validation has been completed. 

Tunnel sterilizers must demonstrate mechanical repeatability in the same manner as batch ovens. Air velocity, air particulates, temperature consistency, and reliability of all the tunnel controls (heat zone temperatures, belt speed, and blower functions) must be proved during the physical validation studies. 

A suggested step-by-step sequence in the microbial validation of a dry-heat process for sterilizing and depyrogenating large-volume glass containers by a convection batch oven is presented. Procedures for the validation of a tunnel sterilization process have been reported by Wegel [25] and Akers et al. [26],... 

Extruded rod can be used either for packing or converted into tape by calendering. Rod for packings is sometimes used with the lubricant still in it. If the lubricant is to be removed, this may be done in a simple in-line oven immediately after the extruder, or in a separate batch oven. 

Fig. 7.6. (a) Standard screen-printing equipment, (b) Master plates leaving the in-line drying furnace after screen-printing, (c) Master plates after sintering in a batch oven... 

Sterilization by dry heat is only infrequently used, preference being given to the use of steam (due to its higher speed) or dry-heat depyrogenation (affording an added measure of safety using the same equipment). Where it is employed temperatures in the range of 170-180°C are employed, and a batch oven is customarily used. 

Spray dried products are typically temperature sensitive, therefore, air temperature should be controlled and as low as possible. Design of the atomizing device should ensure that product will not adhere to vessel walls. Surface drying and depyrogenation can be done in a continuous operated tunnel or batch oven. The former method is preferred since it minimizes the potential of particulate contamination during loading. 

FIGURE 3.18 Solution of a batch oven dryer model—solid dry mass is 90 kg, internal heater power is 20 kW and air ventilation rate is 0.1 kg/s (dry basis) external air humidity is 2 g/kg and temperature 20°C (a) moisture content X (solid line) and air humidity Y (broken... 

Batch ovens and low-temperature batch furnaces (400-1400 F, 200-760 C) are in a range where convection capability may exceed radiation capability. (See fig. 2.10 in chap. 2.) Convection is used for effective heating in this temperature range where radiation is weak or has a shadow problem because it travels only in straight lines. 

The reader should review section 3.8.1 on batch ovens and low-temperature batch furnaces because many of the ideas discussed there also apply to continuous dryers, ovens, and furnaces. Dryers and drying ovens usually release large quantities of water vapor or of solvents, the accumulation of which can have at least two bad effects (1) an explosion hazard with flammable solvents and (2) a reduced rate of drying (mass transfer) with either water or solvent drying. Tables B.3 and B.4 of reference 51 give heat requirements for drying. 

The fibre is produced by blending together the raw materials (sand, kaolin, limestone, and colemanite) and feeding the mix into a batch oven heated to about 16()()°C. The liquid glass Hows into channels and the fibres are drawn through electrically heated bushings, each of which can produce thousands of filaments of 10-24 pm in diameter. 

Solid particles of uniform size of 10 mm diameter move from the top of the bed at a rate of 100 kgs/min counter-current to a stream of gas flowing at a rate of 400 m /min. The bed is of 1 m diameter. The solid fraction in the bed is 0.7. The partial pressure of A in the feed is 0.3 atm. The reactor is operating at 1 atm pressure and 200°C temperature. The solid density is 4700 kg/m and the molecular weight of B is 120. Calculate the height of the bed required for 70% conversion of B. The results of a batch oven experiment conducted in a constant gas environment showed that 5 h was required for complete conversion of 5 mm diameter particles. SCM holds good and the reaction is rate controlling. 

Most ovens found in smaller operations are batch models. In one respect, batch ovens are more hazardous than continuous ovens because there will be a peak in the rate of evolution of volatile material. In a continuous process, there is a nearly constant, substantially lower rate of evolution. Much lower rates of ventilation may suffice for dilution of this vapor. In a batch process without complex control or close monitoring, the ventilation rate during much of the cycle will usually be much higher than ideally required. This is wasteful of ventilating gas and thermal energy, and it makes the task of safe removal and recovery of solvent vapors more difficult. 

Oven types used for tack drying inclnde batch ovens, tunnel ovens, and infrared (IR) ovens. Batch ovens can be as small as a bench-top unit where panels may be inserted individually or as large as a walk-in oven that may be loaded with many panels in racks or carts. The wet panels are loaded through a door or doors that are gasketed or sealed to the operating area so that the vapors emitted are not exhansted into the work environment. 

Total tack drying time in a batch oven is the sum of the ramp-up time plus the desired time at temperature. Ramp-up time of the panels is dependent upon the initial temperatnre of the oven, the heating capadty of the oven, the amount of makeup or purge air, and the loading (thermal mass) of the panels, racks, and carts placed in the oven at start-up. 

---