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Quotes - Engineering - SalesWFI Condensers
Condensing Pure Steam to Compendial Water for Injection
On Water for Injection duty the condensate is the product. We supply WFI and pure steam condensers with double tubesheet construction, 316L mirror-polished and passivated wetted surfaces, and the documentation package your qualification effort needs.
A WFI condenser occupies an unusual position: it is the last piece of equipment in the generation train, and its outlet is the product. Pure steam entering the condenser has already been separated from entrained droplets and is dry and pyrogen-free. What leaves as condensate has to meet compendial requirements for Water for Injection under the applicable pharmacopeia. Anything the condenser contributes — a trace of cooling water through a failed joint, metal ions from an unsuitable alloy, bioburden from a pocket that did not drain — lands directly in the product.
How WFI is generated. Multiple-effect distillation is the most widely used route. Feedwater is evaporated in the first column to produce pure steam, which is condensed and re-distilled through successive columns at descending pressure, so each effect recovers the latent heat of the one before it. Adding columns improves steam and cooling water economy without changing product quality or capacity. Vapor compression distillation and dedicated pure steam generators serve the same end by different means. In all of them, heating and condensing duty is handled by double tubesheet exchangers, and condensation is commonly arranged as a thin falling film.
The last piece of equipment in the generation train, and its outlet is the product. Pure steam entering it has already been separated from entrained droplets and is dry and pyrogen-free.
Why double tubesheets are effectively standard here. In a conventional exchanger a single tube-to-tubesheet joint separates cooling water from what will become WFI. If that joint opens, contamination is continuous and silent, and the first indication may be an out-of-specification water result days later — with every batch made since the last passing sample now in question. A double tubesheet puts two joints in series with a vented gap between them, so a leak in either reports to that gap and drains where it can be seen. Still builders apply this throughout: first effect, interstage exchangers and the condenser itself.
Drainability and hold-up. Residual water at temperature is where bioburden and endotoxin develop, and endotoxin is not removed by the fact that the system was clean at installation. Sloped shells, low-point drains, minimized dead legs and sanitary connections are what make a unit genuinely self-draining. Many stills mount the condenser high so the WFI flows to the storage tank under gravity, which removes a pump and a set of seals from the product path.
Materials and finish. 316L wetted surfaces, mechanically polished to a specified maximum roughness and passivated, are the usual baseline, with full material traceability. Surface finish is not cosmetic here: roughness governs how readily a surface can be cleaned and how easily a biofilm can establish. Connections are sanitary clamp or extended tube weld ends, never threaded.
Documentation. A WFI condenser that meets its thermal duty but arrives without material test reports, weld maps and coupons, passivation certificates, hydrotest records and slope verification is not a usable unit, because it cannot be qualified. We scope the documentation package at quotation alongside the mechanical design.
For solvent and reactor duty see API process condensers; for general hygienic process condensing see pharmaceutical condensers. Clean steam duty in food plants is covered under clean steam condensers.
Water for Injection questions
The design detail that makes the double tubesheet worth having is the gap, which has to be open to atmosphere at a drain visible to an operator and must not be able to fill and hide a leak. A gap of around 1 to 2 in is typical, with the tubes expanded into both sheets and the outer product-side sheet welded to the tube for a crevice-free joint.
The question then becomes what the plant does when the drain weeps. On WFI service the correct response is to isolate the still, since the leak path from cooling water into product has been blocked but a product leak into the gap is a loss of integrity as well. A conductivity sensor on the drain turns the check from a walk-round into an alarm, which is usually worth the small cost on a still that runs unattended overnight.
Pure steam is steam who's condensate is tested to the compendial WFI requirements for conductivity, total organic carbon and endotoxin, while clean steam is a looser term for chemical-free steam used where that testing is not required. The practical tests tell them apart: pure steam for sterilization is usually also checked for dryness fraction, superheat and non-condensable gas content, since wet steam does not sterilize evenly and gas pockets shield surfaces.
Clean steam in a food plant is normally judged on its feedwater and its generator rather than on its condensate. For the condenser the difference is in documentation and in sampling: a pure steam condenser often carries a sample cooler on its condensate line so the quality unit can draw a test sample at the point of use, which a clean steam condenser rarely needs.
Yes on most multiple-effect stills, with the limit being that steam drawn off for sterilization is steam not available to the later effects, so WFI output falls while pure steam is being drawn. On a four-effect still rated for 1,000 gal/hr, drawing 500 lb/hr of pure steam from the first effect reduces water make noticeably, and the condenser at the end of the train sees a lighter load during that period.
The first-effect pressure also has to be high enough to deliver pure steam at the sterilization pressure the plant needs, typically 30 to 45 psig, which sets the plant steam pressure required to drive the still. Where sterilization demand is continuous or large relative to water demand, a separate pure steam generator is usually a better arrangement than loading the still with both duties.
The finish on a WFI condenser is specified as a maximum Ra, usually 20 microinch mechanically polished or 15 microinch electropolished. It is measured with a profilometer at recorded points because a visual mirror finish does not prove either number. Electropolishing does more than smooth the surface: it removes the smeared and worked layer that mechanical polishing leaves and enriches the surface in chromium, which improves the passive film and makes the surface less prone to rouging in hot WFI service.
On the tube interiors, which are the condensing surface, the finish is bought into the tubing from the mill and then protected, since there is no way to re-polish the bore after bundle assembly. Finish on the cooling water side is irrelevant to product and is left as mill finish to save cost.
High enough that the condensate line drops continuously to the tank inlet with no pocket. In practice that means the condenser outlet several feet above the tank top nozzle and a line slope of at least 1/8 in per ft throughout. The condensate is close to its boiling point as it leaves, since WFI is normally stored hot at 176 F or above to control bioburden, so any pump below the condenser is at risk of cavitation and any pocket in the line will vapor-lock.
A gravity line also needs a vent at the high point, and that vent has to be a sterile filtered connection, not an open pipe. Where the still is on a lower floor than the tank, the usual arrangement is a small break tank at the still with a sanitary pump, and that tank then becomes part of the qualified system.
The one item specific to this duty is the still: its number of effects, the pure steam pressure and temperature leaving the last effect, and the required WFI outlet temperature. The condenser is the last stage of a train, and its inlet condition is set by the equipment ahead of it. We also need to know whether the condensate is going to a hot storage tank or a cooler, because a subcooling requirement changes the surface by a wide margin.
On the water side, the cooling water quality decides the shell-side fouling allowance and whether a closed loop is wise. The applicable pharmacopeia and the documentation list for your qualification are the other two items that cannot be assumed.
