Key Takeaways
- Medical device environments usually need a dew point specification, not a relative humidity one, because the underlying requirements are absolute moisture limits.
- Refrigerant equipment cannot practically reach the dew points these applications require; desiccant equipment can.
- Desiccant systems have two air streams — process and reactivation — and the reactivation side drives most of the layout and energy consequences.
- A dehumidifier controls moisture. It does not make a room cleanroom-compliant, and it is not a substitute for a contamination control strategy.
Why Moisture Matters Here
Moisture affects medical device manufacturing and storage through several distinct mechanisms, and knowing which one applies determines what the specification should be.
Hygroscopic polymers absorb water from air. Materials commonly used in device manufacture — including polyamides, polycarbonate and PET — take up moisture in proportion to the surrounding air's humidity. When such a material is melt-processed with absorbed moisture present, the water can cause hydrolytic degradation of the polymer chain and produce visual and mechanical defects. This is why resin drying before processing is standard practice, and why the humidity of the space in which material is stored and handled between drying and processing matters.
Adhesives and cure chemistry. Many bonding processes are humidity-sensitive, either because the cure mechanism involves moisture or because surface moisture interferes with adhesion. The window is often specified as a humidity range, and drifting outside it produces bonds that pass visual inspection and fail later.
Sterile barrier sealing. Heat-sealing of packaging materials is affected by moisture in the substrate and at the seal interface. Seal integrity is a critical quality attribute for sterile barrier systems, and moisture-related seal defects can be subtle.
Electronics and electrical assemblies. Moisture films support electrochemical migration between closely spaced conductors, and moisture absorbed into plastic packages can cause damage during reflow soldering.
Powders and lyophilised products. Hygroscopic powders cake, change flow behaviour, and in some cases change in ways that affect the product.
Corrosion of metal components during storage, particularly where components are held for extended periods.
Condensation on any surface cooler than the air's dew point, which introduces free water into an environment where it is not acceptable.
Specify Dew Point, Not Relative Humidity
Most of the mechanisms above depend on the absolute amount of water present, not on how close the air is to saturation at its current temperature.
Relative humidity is temperature-dependent: the same RH value corresponds to very different absolute moisture contents at different temperatures. A specification of "below 30 % RH" permits substantially more water at 25 °C than at 18 °C. Where the underlying requirement is an absolute moisture limit — which it usually is for polymer moisture uptake, adhesive chemistry and corrosion — a dew point specification describes it directly and does not change meaning when the room temperature moves.
Dew point is also the correct variable where condensation on tooling, product or packaging is the concern, for the reasons set out in condensation risk and dew point.
Where an existing specification, customer requirement or standard is written in relative humidity terms, it is worth establishing the dew point it implies at the room's actual temperature range, so that the real requirement is visible.
Why Desiccant Rather Than Refrigerant
Refrigerant dehumidification works by cooling air below its dew point and condensing water on a coil. Two limits follow.
Capacity falls as air gets cooler and drier. As entering-air dew point approaches the coil temperature, the driving force for condensation shrinks. The explanation is in what a daily water removal rating really means.
Coil frosting. Below roughly 15–18 °C entering-air temperature the coil surface can fall below freezing, and the machine must spend part of its cycle defrosting rather than dehumidifying.
Together these mean refrigerant equipment cannot practically reach the low dew points many medical device processes require.
Desiccant equipment removes water by adsorption onto a solid medium rather than by condensation, so it is far less sensitive to air temperature and can reach much lower dew points. Published experimental work indicates molecular sieve media are appropriate where dehumidification to very low dew points is required — figures in the region of −40 to −60 °C appear in the literature — while silica gel media are favoured where greater moisture removal is needed at high inlet relative humidity, and molecular sieve where a low dew point is required at low inlet relative humidity.
Media selection therefore follows from the target: the two are not interchangeable, and specifying the target dew point is what allows the medium to be chosen.
Define the Air System, Not Just the Machine
A desiccant dehumidifier has two air streams, and the second one is what most projects underestimate.
Process air passes through the desiccant medium, gives up its moisture, and leaves dry. It also leaves warm, because adsorption releases heat. Where the room has a temperature specification, that heat has to be accounted for — often requiring downstream cooling, which then becomes part of the system rather than an afterthought.
Reactivation air is heated and passed through the medium in the opposite direction to drive off the adsorbed water, leaving hot and very humid. This stream must be exhausted, and where it is exhausted matters:
- It cannot discharge into the conditioned space, which would return the moisture just removed.
- Its route must respect any pressure and zoning arrangements the facility maintains.
- It carries significant heat, which may be recoverable or may be a load on adjacent spaces.
Published work on silica gel wheels places typical regeneration temperatures in the region of 120–180 °C, and the heat source — electric, gas, steam or hot water — is a significant design and operating cost decision. Regeneration energy does not appear in a water removal figure and has to be requested separately when comparing equipment.
Other air system decisions:
Recirculation versus once-through. Recirculating room air is more energy-efficient; a once-through arrangement handles higher outdoor air requirements. Most installations are a mixture, and the ratio follows from the ventilation requirement.
Make-up and ventilation air. Outdoor air is usually the dominant moisture load in a low dew point room, and treating it before it reaches the space is generally more effective than diluting it afterwards.
Location. Equipment inside a controlled area affects that area's cleanliness and heat balance. Equipment outside it requires ducting and adds pressure loss. The reactivation exhaust route often decides this.
Room pressure. Low dew point rooms are usually held positive to adjacent spaces so that moist air does not migrate in. Equipment airflows have to be consistent with maintaining that relationship.
What a Dehumidifier Does Not Do
This is worth stating plainly, because it is a recurring misunderstanding.
A dehumidifier controls moisture content. It does not make a room a cleanroom.
Cleanroom classification concerns airborne particle concentration and is achieved through filtration, air change rates, airflow pattern, pressure regime, construction, gowning and operational discipline — the framework set out in the ISO 14644 series. Humidity control is one environmental parameter among several and does not confer classification.
Where a controlled environment requires both, they are designed together: the dehumidification equipment must be compatible with the contamination control strategy, and its filtration, materials, location and airflows have to be consistent with the room's classification. But the dehumidifier is a component within that strategy, not a substitute for it.
Similarly, equipment supplied for a medical device environment does not by itself satisfy the manufacturer's quality system obligations. Requirements under a quality management system such as ISO 13485, and the qualification and validation activities associated with controlled environments, are the device manufacturer's responsibility and are determined by their quality system and applicable regulatory requirements.
Hygiene, Materials and Access
Materials and finish appropriate to the environment and to the cleaning agents in use.
Cleanable construction without ledges or crevices, extending to the installation — brackets, supports and duct penetrations.
Filtration matched to the room's requirement, with filter changes possible without compromising the space.
Access for inspection and cleaning, secured at layout stage. Equipment that cannot be inspected cannot be verified.
Vibration and noise, where the space houses sensitive processes or measurement.
Condensate and drainage, where present — in a low dew point desiccant system there is typically no process condensate, which is one of the practical advantages, but any pre-cooling coil in the system will produce it.
Controls, Monitoring and Records
Control on dew point where that is the requirement, which needs equipment and instrumentation capable of measuring and controlling it. A conventional relative humidity sensor at low humidity has meaningful uncertainty, and dew point instrumentation appropriate to the range should be specified.
Sensor location at the point where the requirement applies, not in the return duct.
Continuous logging of temperature and dew point together, retained. In a regulated environment this is usually required rather than optional.
Alarms on excursion and on equipment failure, with failure to reach setpoint treated as distinct from failure to run.
Calibration. Instruments drift, and in a controlled environment the calibration interval and traceability are part of the quality system rather than a maintenance preference.
Baseline records at commissioning — achieved dew point, process and reactivation conditions, airflows, power — which is what later performance questions are assessed against, as described in commercial dehumidifier maintenance.
Information Needed for Selection
- The controlled parameter and its value: target dew point, with tolerance, and the temperature range over which it applies.
- Which process or material drives the requirement, and why.
- Room dimensions, construction, and whether it is sealed.
- Ventilation and make-up air requirement, and its basis.
- Pressure relationships with adjacent spaces.
- Internal heat and moisture sources, including people and process.
- Site outdoor design conditions, summer and winter.
- Any cleanroom classification requirement and the contamination control strategy it belongs to.
- Available reactivation heat source, and an acceptable exhaust route.
- Equipment location options, ducting constraints and access provision.
- Materials, filtration and cleanability requirements.
- Monitoring, alarm, calibration and record-keeping requirements.
- Qualification approach the facility intends to follow, and what documentation the equipment supplier is expected to provide.
The last point should be agreed early. What documentation accompanies the equipment, and in what form, is much easier to arrange before manufacture than afterwards.
Discussing a Project
Yakeclimate designs and manufactures industrial dehumidification equipment for complex climate applications, with a focus on agriculture and energy projects.
We co-develop application-specific dehumidification equipment around the operating conditions, interfaces, and integration requirements of the wider project or system.
Cleanroom design and classification, contamination control strategy, quality system requirements and validation activities remain with the device manufacturer and their specialists. Our scope is the dehumidification equipment, its performance at the required dew point, and its interfaces with the air, control and monitoring arrangements those systems establish.
Review desiccant rotor dehumidifiers, explore industrial dehumidifiers, or contact Yakeclimate to review the operating conditions for your facility.
FAQ
Frequently Asked Questions
Should a medical device environment be specified in relative humidity or dew point?
Dew point, in most cases. The mechanisms that matter — moisture uptake by hygroscopic polymers, humidity-sensitive adhesive chemistry, corrosion, condensation — depend on the absolute amount of water present, not on how close the air is to saturation at its current temperature. A relative humidity specification permits different absolute moisture contents as room temperature varies. Where an existing specification is written in RH terms, establishing the dew point it implies across the room's temperature range makes the real requirement visible.
Why is desiccant equipment used rather than refrigerant?
Because refrigerant equipment cannot practically reach the dew points these applications require. It works by condensing water on a cold coil, and as entering air becomes cooler and drier the driving force for condensation shrinks; below roughly 15–18 °C entering-air temperature the coil can also begin frosting, costing further capacity to defrost cycles. Desiccant equipment adsorbs water onto a solid medium instead, so it is far less temperature-sensitive and reaches much lower dew points.
Which desiccant medium should be specified?
It follows from the target. Published experimental work indicates molecular sieve media suit applications requiring very low dew points — figures in the region of −40 to −60 °C appear in the literature — and are favoured where a low dew point is needed at low inlet relative humidity. Silica gel media are favoured where greater moisture removal is needed at high inlet relative humidity. The two are not interchangeable, which is why stating the target dew point is what allows the medium to be selected.
What does the reactivation air stream require?
A heat source and an exhaust route, both of which usually constrain where the equipment can be placed. Reactivation air is heated — published work places typical silica gel regeneration in the region of 120–180 °C — and leaves hot and very humid. It cannot discharge into the conditioned space, its route must respect the facility's pressure and zoning arrangements, and it carries significant heat that may be recoverable. Regeneration energy is a substantial operating cost and does not appear in the equipment's water removal figure, so it should be requested separately.
Does a desiccant dehumidifier make a room cleanroom-compliant?
No. Cleanroom classification concerns airborne particle concentration and is achieved through filtration, air change rates, airflow pattern, pressure regime, construction, gowning and operational discipline — the framework in the ISO 14644 series. Humidity control is one environmental parameter among several and does not confer classification. Where both are required they are designed together, with the dehumidification equipment's filtration, materials, location and airflows made consistent with the contamination control strategy.
Can the dehumidifier be installed outside the controlled room?
Often yes, and it is frequently preferable — it keeps the equipment's heat, filtration burden and service activity outside the controlled area. It requires ducted supply and return, with the pressure loss accounted for in the equipment selection, and the room's pressure relationship with adjacent spaces maintained. In practice the reactivation exhaust route often decides the location, since it has to discharge somewhere acceptable and cannot return moisture to the space.
What documentation should be agreed with the supplier?
Establish early what accompanies the equipment and in what form: performance data at the specified condition, test records, material and finish specifications, instrument details and calibration information, installation and maintenance documentation, and anything the facility's qualification approach requires. This is considerably easier to arrange before manufacture than afterwards. What qualification and validation activities are needed is determined by the device manufacturer's quality system and applicable regulatory requirements.
References
- Parametric studies of silica gel and molecular sieve desiccant wheels: experimental and modeling approaches
- Performance evaluation of a rotary dehumidifier with molecular sieve desiccant using coupled regeneration mode
- ISO 14644 series, Cleanrooms and associated controlled environments
- ISO 13485, Medical devices — Quality management systems
