Air pollution control: identify the wrong statement about visible haze at chimney exit, bag filters at very high temperature, and electrostatic precipitators for sub-micronic dust removal.

Introduction / Context:
Engineers select particulate control equipment by balancing efficiency, temperature tolerance, pressure drop, maintenance, and operating cost. This question asks you to spot the incorrect statement about common devices and operating cues.

Given Data / Assumptions:

• Devices considered: bag filter (fabric filter) and electrostatic precipitator (ESP).
• Operating cue: visible haze at chimney top.
• Particle size of interest: sub-micronic regime.

Concept / Approach:
Fabric filters are highly efficient for fine particles but they impose a moderate to high pressure drop and typically cannot operate at very high gas temperatures without cooling or high-temperature fabrics. ESPs achieve very high efficiency for fine particles with comparatively lower pressure drop. Visible haze or smoke indicates incomplete combustion and higher particulate carryover, not ideal combustion.

Step-by-Step Solution:

Verification / Alternative check:
Typical pressure drop across bag filters is several centimeters of water gauge and temperatures may require gas cooling or special fabrics, confirming that the statement claiming very small pressure drop at very high temperature is incorrect.

Why Other Options Are Wrong:
Option (a) is also suspicious, but not as technically definitive as (b) in all contexts; some transient plumes may occur while still having acceptable combustion. Option (c) is correct for many industrial cases. Option (d) claims that none are wrong, which is not true.

Common Pitfalls:
Assuming all fabric filters can tolerate furnace exit temperatures; ignoring pressure-drop penalties; conflating occasional visible water vapor plumes with smoke due to incomplete combustion.

Final Answer:
A bag filter has very small pressure drop and is highly efficient for removing sub-micronic dust from very hot flue gas.