Question

Bag filters used in spray dryer have powder separation efficiency of :

• A. 69 to 69.5 %
• B. 79 to 79.5 %
• C. 89 to 89.5 %
• D. 99 to 99.5 %

Hint:

Cyclones are usually the "primary" separator, and bag filters or wet scrubbers act as the "secondary" separator to catch the last $1\%$ of fines.

Answer 1

The Correct Option is D

Step 1: Understanding the Concept:
In a spray drying system, the drying air carries fine powder particles ("fines") out of the drying chamber.
These particles must be recovered for economic reasons and to comply with environmental air quality regulations.
Various separation devices are used, including cyclones and bag filters.

Step 2: Detailed Explanation:
Bag filters (also known as fabric collectors or baghouses) are highly efficient mechanical separation devices.
They consist of long fabric bags through which the dust-laden air is passed. The fabric acts as a sieve, trapping even the smallest particles.
Compared to cyclones, which typically have an efficiency of 85-95% (and struggle with very fine particles), bag filters offer exceptional performance.
A well-maintained bag filter system in a dairy plant can achieve a powder separation efficiency of 99% to 99.5%.
This ensures that almost no milk powder is wasted and the air exhausted into the atmosphere is clean.
Options 1, 2, and 3 (69%, 79%, 89%) are far too low for a commercial spray drying operation, as such low efficiencies would result in massive financial losses and significant environmental pollution.
The downside of bag filters compared to cyclones is that they are harder to clean (CIP - Cleaning In Place) and present a higher risk of microbial contamination if not managed correctly.

Step 3: Final Answer:
The separation efficiency is 99 to 99.5 %.