Ask the Expert: I am building out a WFI system where I will need approximately 6 ambient use points. Three of which will need to pull 2000-3000 liters for media/buffer production. What would be your suggestion as to handle this?
By: Brian White
Director, Process Engineering
IPS- Integrated Project Services, LLC
As a general rule, Bioprocess facilities have much greater need for Ambient WFI (AWFI) than for Hot (HWFI). Consequently, these facilities have some of the greatest justifications for considering some sort of ambient WFI system.
From a capital cost standpoint each POU cooler is going to add roughly $50K to the cost of your capital project. On a current project we are adding a couple of $35,000 package coolers to deliver 3.5-gpm of ambient WFI for formulation and cleaning applications. The heat exchanger may only be 30% of that cost, but the valves, instrumentation and controls add up quickly. Include the cost of installation and utility piping to and from use-points in the plant and you can quickly get to $50K before you’ve added the soft cost to qualify them. Simply putting in sub-loops to reduce the number of exchangers can produce significant capital cost savings and potentially reduce system complexity.
Operations costs for those exchangers will also be significant if you are cooling hot WFI on demand for every application. The table below looks at the high level energy cost associated with cooling a 1000-liter batch of WFI from 80°C to 25°C for a generic application (media, buffer, etc.). If you increase the flow, the instantaneous demand goes up dramatically and you quickly find yourself adding capital for additional cooling capacity to support these users.
| Batch Volume | 264 | gallons | 1000 | Liters |
| Flow | 3.5 | gpm | 13 | lpm |
| Time | 75 | minutes | 75 | minutes |
| Temperature In | 176 | Tin °F | 80 | Tin °C |
| Temperature Out | 77 | Tout °F | 25 | Tout °C |
| Heat Load Instantaneous | 0.24 | Ton chilled | ||
| Heat Load Batch | 0.30 | Ton chilled |
From a capital cost stand point one can quickly justify an ambient loop, employing a chase-the-tail configuration. The utility loads will still be roughly the same, but system complexity and corresponding system cost will be reduced. Alternately, you could consider an ambient storage and distribution loop.
Several years ago I was involved in a project for a bioprocess facility that installed and qualified an ambient WFI system with 2,500-gallon reservoir and 100-gpm distribution loop. The system would operate all week tempered to 25°C. Then on the weekend, the system would heat up to 80°C for sanitization, and then be cooled back down before return to service. Heat-up took less than an hour, but cooldown could take up to 6-hours. The biggest advantage to this approach is the sanitization loads all hit the plant utilities during off-peak time. Capital costs were reduced, not only by not having to install point-of-use coolers throughout the plant, but also by eliminating chiller tonnage and boiler horsepower to support the heating and cooling loads associated with maintaining a hot loop.
We’ve had other clients employ Ozone sanitization for WFI storage and distribution, and thereby, maintaining a persistent sanitization of the contents of the storage reservoir and enjoying the significant energy savings of an ambient loop.
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