Test chamber, fine particles

Design variations include different chamber configurations for use in coating tablets, coarse granules, or fine particles (Fig. 6.2-85). For scale-up, the outer vessel diameter and the number (rather than the size) of inner tubes increase. Each tube features its own gas distributor and spray nozzle and is essentially identical with that in the test equipment that was used during development work [B.97]. 

If the initial feasibility evaluation is successful, it is reasonable to commit additional materials for a spray-drying trial. A laboratory dryer at least 500 mm in diameter is recommended for such tests. Bench-scale spray dryers are available but are limited in their ability to provide adequate atomization or sufficient process air flow for the successful production of dried particles. The laboratory unit, however, eom-bined with very fine atomization (two-fluid or rotary) will often produce aceeptable product for further testing. A series of tests can be performed at different inlet-outlet temperature combinations using small quantities of material and these samples ean be tested for chemieal stability to evaluate thermal effects from process air contact. The relationship between outlet temperature and final product moisture can also be established for this seale. While samples produced in a laboratory dryer are suitable for evaluating the effeet of spray drying on the product, they are not suitable for use in downstream proeessing because the fine particle distribution produced as a result of the small drying chamber dimensions may not be representative of the final spray-dried produet. 

In view of the discussion above, testing for the effects of fine particles should be a priority. Work to develop test methods is just beginning. Only in the past few years have test procedures and chambers been developed for evaluating the effects of fine particulates (Sinclair, 1988 Comizzoli, Fran-kenthal et al., 1993). These methods will be discussed below. It should be noted that salt spray tests are usually too harsh for simulating fine particle effects in electronics. Another useful test, in conjunction with standard laboratory tests, is to deploy a prototype of the product in the harshest environment that can be found and to monitor and evaluate the performance of the materials for any early signs of unexpected behavior. High airflow velocity and elevated humidity will frequently provide even further acceleration. 

Sandroff and Burnett (1992) recently suggested a test for simulating the electrical leakage effects on circuit boards of ionic contamination derived from particles. The test is intended to simulate the effects of fine particle deposition, though it uses an atomized spray of an aqueous solution to deposit the contamination. Following drying, the test devices, usually circuit boards, are placed in a humidity chamber where their operation is monitored. 

---