Cleanroom AHU selection and external static pressure: a step-by-step calculation
How to build a cleanroom AHU static pressure budget with clean and loaded filters, select the coil, find the fan operating points, and size the motor, step by step.
What this means
A cleanroom air handler's fan is selected at its worst operating point: design airflow with every filter at its planned replacement pressure drop, not at the clean filter condition. Build the budget component by component, first with clean pressure drops inside the unit and in the external system, then with the added drop as each filter loads. In the worked 12,000 cfm unit, total static pressure rises from 3.50 inches of water column clean to 4.90 inches at filter replacement, fan shaft power from 10.2 to 14.2 horsepower, and a 15 horsepower motor with a variable frequency drive holds airflow constant across the filter life.
Equipment and model context
- A 12,000 cfm cleanroom air handling unit with prefilters, intermediate filters, cooling and reheat coils, and ducted terminal HEPA filters
- Every pressure drop, efficiency, and filter replacement point is a stated assumption to replace with manufacturer data
This builds a static pressure budget and fan selection for a ducted cleanroom air handler to show the method. Every value is an assumption; a real selection uses the filter, coil, and fan manufacturers' data at the design airflow. Fan filter unit ceilings are budgeted per unit and compared on the fan filter unit versus air handler page.
What this covers
- The difference between internal, external, and total static pressure in a cleanroom air handler.
- A complete static pressure budget for a 12,000 cfm unit with clean and loaded filters.
- Coil face area and wet pressure drop, and where the fan's heat lands.
- Fan operating points, brake horsepower, motor size, and input power at both filter conditions.
What changes the result
- The filter replacement pressure drop chosen for each stage, which sets the loaded operating point.
- Coil face velocity and wet condition, which set coil pressure drop.
- Terminal HEPA filters, duct length, and sound attenuators in the external system.
- Fan efficiency at the operating point and whether the motor sits in the air stream.
Why a cleanroom fan is selected at the loaded filter condition
A cleanroom's air change rate is a minimum it has to hold for the whole life of its filters, not only on certification day. Filters gain resistance as they collect particles, and a fan selected at the clean filter condition delivers design airflow once and less every week afterward, so the room drifts below its air change rate between filter changes without any alarm tied to the cause.
Select the fan at design airflow with every filter at its planned replacement pressure drop, then control the fan down to design airflow while the filters are new. The replacement pressure drop is a design decision recorded in the basis of design, and the cleanroom HVAC design guide places it among the items that basis has to state.
Internal, external, and total static pressure
Total static pressure is the resistance the fan works against across the whole air path. Internal static pressure is the part inside the air handler's casing: its filters, coils, and casing losses. External static pressure is everything outside the casing: supply duct, terminal filters, return path, dampers, and attenuators.
Catalog air handlers are rated by the external static pressure they can deliver at an airflow, with a stated set of internal components already counted. Check what that rating includes before using it, because a unit rated with clean filters, a dry coil, or no filters at all overstates what is left for the external system. For a custom cleanroom unit, build total static pressure from every component, as the budget below does. Coil pressure drop behaves differently from filter pressure drop, and the article on static pressure drop across a coil covers the coil side in depth.
Step 1: list every component in the air path
Walk the air from the return grilles to the supply terminals and write down every resistance in order: low-wall return grilles and return duct, mixing section with makeup air, prefilter, intermediate filter, cooling coil, reheat coil, fan inlet and casing losses, discharge, sound attenuator, supply duct and fittings, balancing dampers, and terminal HEPA filters.
Anything missing from the list is missing from the fan selection, and components added late in design are the easiest to miss: a sound attenuator, a smoke damper, or a second filter stage required by the owner's quality group. The table lists each component in the worked unit at 12,000 cfm.
Step 2: clean and replacement pressure drop for each filter stage
Take the clean pressure drop for each filter from its manufacturer at the actual face velocity, since filters are rated at a stated airflow and a filter run faster than its rating starts higher. Resistance through fibrous filter media rises close to in proportion with velocity, which is why filters are drawn as straight lines in the chart while coils and ducts are drawn as curves.
The replacement pressure drop is chosen, not measured. The worked unit changes the MERV 8 prefilter at 0.60 inches, the MERV 14 intermediate filter at 1.00, and the terminal HEPA filters at 1.00, double their clean values. A higher replacement point stretches filter life and raises fan power and motor size; a lower one does the opposite. For comparison, the DOE handbook notes that nuclear-grade HEPA filters qualified under ASME AG-1 have a clean resistance that does not exceed 1 inch of water gauge at rated airflow.
The filter stages add 1.40 inches between clean and replacement: 0.35 at the prefilter, 0.55 at the intermediate filter, and 0.50 at the terminal filters. That 1.40 inches is 40 percent of the clean total, and it is the margin the fan must have that a comfort cooling selection would not.
Step 3: select the coil for face velocity and wet pressure drop
At a 500 feet per minute face velocity, 12,000 cfm needs 24 square feet of coil face. The coil's leaving condition comes from the humidity design: if the unit dries recirculated and makeup air together, its leaving dew point is set as the cleanroom humidity control guide calculates, and the apparatus dew point calculation shows what coil depth that needs.
Use the wet pressure drop for a cooling coil that dehumidifies, since condensate on the fins raises resistance over the dry figure, and take it at the actual face velocity. The worked coil adds 0.65 inches wet. A lower face velocity cuts pressure drop and moisture carryover at the cost of a larger casing.
Decide where the fan's heat lands before fixing the coil leaving temperature. With the fan upstream of the coil, its heat enters the air before the coil removes it. With the fan downstream of the coil, it warms the supply: at the loaded condition this fan puts about 40,200 BTU per hour into 12,000 cfm, raising supply temperature by 40,200 ÷ (1.08 × 12,000) = 3.1°F, and the coil leaving temperature has to allow for that.
Step 4: find the operating points on the fan curve
The budget gives two points at 12,000 cfm: 3.50 inches with clean filters and 4.90 inches at replacement. Plot both on the candidate fan's curves. The loaded point sets the maximum speed and must sit in a stable region of the fan curve, away from the peak where airflow can hunt; the clean point is where the fan runs at the start of each filter cycle, at a lower speed on its drive.
Check any reduced-fan condition too. If the unit has two fans for redundancy, the National Institutes of Health bulletin on fan arrays asks designers to verify that the remaining fans operate in a stable region with one fan out, and the redundancy design page shows how far along its curve a surviving fan travels.
A fan without a drive, selected for the loaded point, would supply too much air with clean filters and need a throttling damper to bring airflow down, spending the difference as damper loss for most of each filter cycle. A variable frequency drive controlled on measured airflow avoids that loss and keeps the room at design airflow as the filters load.
Step 5: brake horsepower and motor size
Brake horsepower equals cfm × static pressure in inches of water column ÷ (6,356 × fan static efficiency). With an assumed static efficiency of 0.65, the clean point needs 12,000 × 3.50 ÷ (6,356 × 0.65) = 10.2 horsepower and the loaded point 12,000 × 4.90 ÷ (6,356 × 0.65) = 14.2 horsepower.
Size the motor for the loaded point within its nameplate rating, which gives a 15 horsepower motor here. The service factor is margin for voltage and ambient variation, not for a condition the fan reaches every filter cycle. Check the motor also at the highest airflow the fan could reach at full speed with a filter bank removed during a change, since that run-out point can load the motor more than the design point.
For a fan array, the NIH bulletin limits each fan's motor to 30 horsepower, gives each fan its own drive, and keeps direct-drive motors on drives at or below 90 hertz, with motor size based on the operating frequency.
Step 6: input power and fan heat across the filter life
With an assumed motor efficiency of 93 percent and drive efficiency of 97 percent, input power is 10.2 × 0.746 ÷ 0.93 ÷ 0.97 = 8.4 kW with clean filters and 14.2 × 0.746 ÷ 0.93 ÷ 0.97 = 11.8 kW at replacement. The fan draws 40 percent more power at the end of each filter cycle than at the start, and the energy between the two is what changing filters earlier or later trades against filter cost.
When the motor sits in the air stream, all of that input becomes heat in the supply air: 28,700 BTU per hour clean and 40,200 BTU per hour loaded. Both figures belong in the room load, as the cleanroom heat load calculation shows for fan filter units, and the coil is sized for the loaded figure.
| Component | Clean (in. w.c.) | At filter replacement (in. w.c.) |
|---|---|---|
| Prefilter, MERV 8, inside the unit | 0.25 | 0.60 |
| Intermediate filter, MERV 14, inside the unit | 0.45 | 1.00 |
| Cooling coil, wet, at 500 fpm face velocity | 0.65 | 0.65 unchanged |
| Reheat coil | 0.15 | 0.15 unchanged |
| Casing, inlet, and discharge losses | 0.25 | 0.25 unchanged |
| Inside the unit, subtotal | 1.75 | 2.65 |
| Supply duct and fittings | 0.60 | 0.60 unchanged |
| Terminal HEPA filters | 0.50 | 1.00 |
| Low-wall returns and return duct | 0.35 | 0.35 unchanged |
| Balancing dampers and sound attenuator | 0.30 | 0.30 unchanged |
| External static pressure | 1.75 | 2.25 |
| Total static pressure at 12,000 cfm | 3.50 | 4.90 |
Total static pressure against airflow for the worked system, built from coils, casing, and ducts that follow the square of airflow and filters whose resistance rises in proportion to airflow.
- Clean filters
- Filters at replacement
- At the design 12,000 cfm, total static pressure rises by 1.40 inches of water column, 40 percent, between new filters and filters at replacement.
- The loaded curve sits further above the clean curve at high airflow than at low airflow, because filter resistance grows in proportion to airflow while coils and ducts follow its square.
- A fan held at one speed from the clean point would lose airflow as the filters load; with a drive, speed rises to stay on 12,000 cfm, and the motor is sized at the loaded point.
Questions people ask about this
How high can external static pressure run on a cleanroom AHU?
There is no standard value; it is the sum of that system's external components. Ducted terminal HEPA filters, long supply runs, sound attenuators, and low-wall returns all add to it, and the worked 12,000 cfm example reaches 1.75 inches of water column clean and 2.25 inches with terminal filters at replacement, before the unit's own filters and coils are counted.
When should cleanroom AHU filters be replaced?
At the pressure drop chosen as the replacement point in the basis of design, which the fan and motor were selected to reach. Replacing earlier saves fan energy and uses more filters, while running past the design point pushes the fan beyond its selection, and airflow falls once the drive reaches full speed.
Should the fan sit upstream or downstream of the cooling coil?
Either works if the heat is counted where it lands. A fan upstream of the coil adds its heat before the coil removes it, while a fan downstream of the coil warms the supply by its heat, about 3°F in the worked example at the loaded condition, which the coil leaving temperature has to allow for.
Can the motor service factor cover the loaded filter condition?
No. Service factor is margin for voltage variation, ambient conditions, and occasional overload, not for a condition the fan reaches every filter cycle. Size the motor so the loaded operating point sits within its nameplate rating, with motor and drive efficiency counted.
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