Filter Housing Heaters: Electrical Versus Steam

1 ManufacturingVent filters are critical to the safe and sterile operationof chemical reaction and storage vessels in thebiopharmaceutical industry. Normally, a vent Filter isoperational during the introduction of sterile air ornitrogen during and after vessel sterilisation, as well asduring other routine operations such as venting the gasin a vessel while it is being filled. The upstream side ofthe vent Filter constitutes one boundary of the sterileoperation envelope of the reactor system. Clean in place (CIP) procedures are commonly usedin the biopharmaceutical industry to clean andsterilise reaction vessels, and most CIP routinesemploy Steam sterilisation of the vessel as a finalstep.

2 While saturated Steam behaves as agas and can pass cleanly through the filtermembranes, if the Filter surfaces are coolenough to permit water condensation,the condensate will plug the Filter . Forthis reason, sterile vent filters and theirhousings must be heated to temperaturesthat prevent Steam condensation. Sincethey constitute a boundary in the sterileoperation envelope, vent Filter housingsand ancillary components must besterilisable up to and including the ventfilter element to prevent contaminationfrom reaching the bioreactor.

3 The vent Filter is contained in a stainlesssteel Housing that is hermetically sealed toa biopharmaceutical reaction vessel orstorage tank. Sterilisation grade filters aretypically constructed of hydrophobicPTFE or PVDF membranes. In thepresence of condensable water vapour, these filtermembranes will foul or plug. This can result in as muchas a 40 psig pressure build-up on the upstream side of thefilter Housing , with obvious negative consequences forthe biopharmaceutical process and system safety. ThePTFE or PVDF Filter elements have temperature/dutycycle limits of 135 C for 30 minutes, and thus can beheated to sufficiently high temperatures to eliminatecondensation.

4 This article reviews the issues surroundingthe different approaches to heating these units anddetails a new heater system that addresses a number ofthe outstanding issues with this VENT Filter HOUSINGSVent Filter housings in the biopharmaceutical industrycan be heated in one of three ways Electrical trace tape, Steam jackets or Electrical heater jackets. Electrical tracetape has the advantage of low relative cost; it is not a veryrobust solution, however, and there are inherentinstallation issues that frequently lead to the presence ofcold spots on complex topologies such as vent filterhousings.

5 Typically, trace tape cannot be wound overeither itself or another heating tape, since this will causelocal overheating and tape failure. This limitation canmake it extremely difficult to provide uniform heat to allof the corners and crevices in a vent Filter installation,resulting in cold or cool spots. Because of this problem, Electrical trace tape is rarely used in thebiopharmaceutical industry. Steam jacketing is a more common means of providingheat to vent Filter housings. This method has distinctadvantages over heat tape tracing in that it is able toprovide more constant and uniform heat to the surface ofFilter Housing Heaters: Electrical Versus SteamThe thermal characteristics of vent Filter housings must be tightly controlled toprevent condensation on the Filter element that may compromise the vessel itis protecting.

6 While Steam -jacketing is the most common means of achievingthis, Electrical heater jackets offer a number of advantages that makes themthe preferred system for use in biopharmaceutical manufacturing Paul Trunzo at MKS InstrumentsPaul Trunzo is the Biopharm Product Manager for MKS Instruments, working out of the HPS Products offices in Boulder(Colorado, US). He has over 18 years of experience in sanitary products and biopharmaceutical equipment andprocesses. Mr Trunzo holds a four-year degree in Mechanical Engineering, as well as a four-year degree in in Pharmaceutical Technology100 Figure 1:Differentconfigurationsof electric heater jackets fitted on vent Filter housingsconfigured for in-line useCourtesy MKSI nstruments IncIPT 21 2006 15/12/06 15:29 Page 100the Filter Housing .

7 Also, biopharmaceutical plantstypically have Steam available throughout the plant site. Safety interlocks based on the steampressure/temperature relationship are critical in Steam -jacketed systems since a significant risk of over-temperature operation exists should the pressure get toohigh. Temperatures beyond that for which the filtermembrane is rated will actually cause the membrane tomelt and flow, resulting in contamination of the reactionvessel that is extremely difficult to remove.

8 It is alsodifficult to vary the temperature in Steam -jacketedsystems, and this presents a significant impediment tothe implementation of feedback control of the heatingsystem. Steam -jacketing systems tend to require relativelycomplex piping which can make it difficult to heat thesurfaces of ancillary components associated with the ventfilter Housing but within the sterile envelope (that is,valves and sensors). The insulation, traps, valves,regulators and Steam piping interconnection required toimplement Steam -jacketed heating represent a significantaddition to the cost of the vent Filter Housing additional high pressure Steam lines going to thevent Housing also represent a safety risk to workers, eitherfrom accidental contact or from the possibility of third technology for heating vent Filter assemblies usesconformal Electrical heater jackets.

9 These jackets aremoulded so as to conform to the topography of the ventfilter Housing . Because of this intimate contact, theyprovide a uniform and consistent heat to the assemblysurfaces. Electric heater jackets are relatively simple toinstall, and factory acceptance testing is less complex thanwith trace tape or Steam jacketing. The Electrical heaterjackets available to date have had certain implementationissues in that the available models have required a separatecontroller and have not been waterproof. Also, over-temperature conditions can occur with conventionalheating jackets, leading to Filter element VENT Filter Housing HEATERSR ecently, an advanced Electrical heater jacket for ventfilter housings has become available that addresses thelimitations of earlier systems outlined above.

10 Figure 1(page 100) shows this system, the MKS Series 46 FilterHousing Heater. The jacket is configured to becompatible with industry standard vent Filter housingsthat range in size from small single round units ( x4 ) to larger multi-round housings (8 x 20 ). The heaterjacket is composed of a heating element with anembedded thermocouple encased in 1/2 thick siliconefoam insulation. The heater jacket is installed by using asimple snap-on arrangement that also allows for easyremoval for system washdown. The integrated insulationmakes this heater safer than Steam or trace tape, with thesurface temperature of the insulating layer being typically60 C or less.