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Explosion-Proof Fan-Filter Unit
For Class I or II, Division 1 Hazardous Locations

Explosion proof fan filter
cULus logo

No. 6601-24-UESS,
shown mounted on Terra cleanroom ceiling grid
Terra's Explosion-Proof WhisperFlow™ Fan-Filter Unit is UL-listed for safe operation in Class I or Class II, Division 1 as specified by the National Electrical Code (NEC). Its direct-drive, 0.25HP fan-cooled motor is designed for continuous-duty operation. Selection of this unit includes installation of filled rigid conduit and wiring through the enclosure.
  • Non-sparking motor, mounted in a cast-iron, explosion-proof housing, and particle filter provide uniform flow of filtered air at an average speed of 90 ft/min (0.458 m/sec)
  • Select 99.99% efficient HEPA filter (rated @ 0.3 µm-diameter particles) or 99.999% efficient ULPA filter (rated at 0.12 µm-diameter particles)
  • Average air flow: 650+ CFM (1095 m3/hr) under filter load (2' x 4' 610 mm x 1219 mm, model; see chart below)
  • Price includes installation of wiring in filled rigid conduit, and modification of Power Distribution Module(s) and Control Panel switches to meet NEC requirements for Class I/Class II, Division 1 Hazardous Locations

  FEATURES & BENEFITS | Explosion-Proof Fan/Filter Units
Alt Text
Pre-Filter Grounding Clip
A grounding wire connects the body of the FFU to a metal wireframe embedded in the pre-filter.
Non-Sparking, Direct-Drive Motor
Explosion-proof motors use a non-sparking polypropylene fan and cast-iron brackets attached to the heavy duty rolled steel frame.
Filter Screen Grounding Clip
Another grounding wire clips to the mesh screen that protects the face of the HEPA filter.

Explosion-Proof Outlet Box
The reinforced outlet box is built to withstand and contain an explosive reaction within the conduit system.

Sealed Wiring Conduit
All FFU wiring is sealed within steel conduit that is joined together by UL-listed explosion-proof fittings.

Explosion-Proof WhisperFlow Fan/Filter Unit: Airflow and Electrical Specifications (120V)
Model Nominal Dimensions*
Inches (mm)
Avg. CFM
Airflow ft/min (m/s) Run Amps (Watts)
@ 60Hz
2 x 4 23.72 x 47.63 x 19.86
(603 x 1210 x 504)
2 x 3 23.72 x 35.63 x 19.86
(603 x 905 x 504)
2 x 2 23.72 x 23.63 x 19.86
(603 x 600 x 504)
* With pre-filter installed.
Filter shipped uninstalled to prevent shifting in the mounting gel that could cause leaks. Also available: Explosion-Proof, Room-Side Replaceable fan/filter units.
Explosion-Proof WhisperFlow Fan/Filter Unit: Ordering Information
Nominal Dimensions*
Inches (mm)
Power Weight
lbs. (kg)
Standard Flow
With ULPA Filter
Reverse Flow
With ULPA Filter
Cat. #  Price Cat. #  Price
23.72 x 47.63 x 19.86
(603 x 1210 x 504)
120/220VAC, 60 Hz 116 (53) 6601-24-HESS $ 3,102 6601-24-UEVSS $ 3,997
23.72 x 35.63 x 19.86
(603 x 905 x 504)
120/220VAC, 60 Hz 107 (49) 6601-23-HESS $ 3,005 6601-23-UEVSS $ 3,997
23.72 x 23.63 x 19.86
(603 x 600 x 504)
120/220VAC, 60 Hz 100 (45) 6601-22-HESS $ 2,900 6601-22-UEVSS $ 3,997
* All models include 99.999% efficient ULPA filter (rated with 0.12 micron particles) and all-stainless steel housing, diffuser and filter frame.
Explosion proof fan filter Explosion proof fan filter
Reverse-flow configuration ventilates and removes particles from an enclosure to allow safe release into a cleanroom. Slotted polypropylene support frame allows easy replacement of prefilter.
  NEC Type: Explosion-proof, Class I Group C&D, Class II Group F&G, Division 1, Fan-cooled
  Electrical Type: Capacitor start, induction run, single-speed
  Design: 0.25 HP, 1140 RPM, Direct-drive
  Frame Construction: Cast Iron motor frame; 304 SS housing, diffuser and filter frame
Filter: 99.999% efficient @ 0.12 micron particles (ULPA):
Housing: 304 Stainless Steel
Impeller: Forward-curved, nonsparking centrifugal type; factory balanced

Terra Universal Class I/Class II, Division 1 Enclosures
To ensure safe operation, specify a Terra explosion-proof enclosure for operations that entail the presence of combustible fumes, vapors or dust, such as explosion-proof modular cleanrooms, laminar-flow workstations or laminar-flow hoods.

FFU Challenge Port

Air filters keep your environment clean, but validating their integrity can be cumbersome and knowing when to replace them is a challenge. Room designs, applications and staffing make each cleanroom unique. Therefore, it’s important to get baseline measurements of your equipment when first installed, and then set up a schedule for regular monitoring.

Tubing fits into both the orange challenge port valve and the Magnehelic® gauge to easily record FFU pressure differential.
Upgrade your fan/filter unit (FFU) to include a Terra-installed Challenge Port that both simplifies leak testing and helps establish a filter replacement schedule. The port is 3/8"-diameter, nickel-plated brass adapter fitting with a white polypropylene plug.

Learn more about Terra's FFU Challenge Port.

Price includes installation on specified FFUs.

FFU Challenge Port
Cat. # Price
6601-99 $ 67

File Type File Name Publication Date
Manual (PDF) Explosion-Proof Fan Filter Unit (FFU): Doc 1800-99
Explosion-proof Fan/Filter Unit (FFU) Quick Start User Guide
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