908.8 Cooling towers
Open-circuit and closed-circuit cooling towers and evaporative condensers comply with 908.8.1 and 908.8.2.
908.8.1 Control of cycles of concentration
The equipment must have controls that automate system bleed. The basis can be:
Conductivity.
Fraction of metered makeup volume.
Metered bleed volume.
Recirculating pump run time.
Bleed time.
The requirement is automation: bleed is not left to manual operator action alone.
908.8.2 Drift eliminators
Cooling towers and evaporative condensers must have drift eliminators with a maximum drift rate of 0.005 percent of the circulated water flow rate, as established in the equipment’s design specifications.
Why it matters
The two requirements solve different problems. Automated bleed manages cycles of concentration through a defined control strategy. Drift eliminators limit the water carried out with the discharge air. Both should be verifiable from the equipment selection and the controls documents before submittal review.
Where to show it
As applicable: the cooling-tower schedule (with the drift rate), controls drawings, the sequence of operation naming the bleed method, equipment specifications, manufacturer product data, and the water-treatment and bleed-control details.
Do
Name the bleed-control method.
Show how bleed is automated and what instrument drives it.
Verify the selected equipment includes compliant drift eliminators.
Get the 0.005 percent maximum drift rate in writing in the equipment data.
Coordinate the controls before final equipment selection.
Do not
Leave bleed control as an undefined field-setting item.
Treat manual blowdown as the automation requirement.
Accept “drift eliminators included” on a submittal without a rate.
Omit controls information the reviewer needs.
Field tip
The tower schedule should say more than capacity and fan horsepower. Add the bleed-control basis and the drift rate, and the schedule becomes a QA/QC tool instead of an equipment list. Next: IMC 908 Cooling Towers Recap.
Code basis
2024 IMC 908.8–908.8.2 model-code education from the owner-provided Daily Code Talk; the 0.005 percent drift wording matches published adopted text based on the IMC. Confirm the adopted edition and local amendments. Not a water-treatment program design.