Proper cabinet selection

With respect to the problem illustrated above in figures 1 & 2, according to the user, their goals were not met. The user is correct in that they had straight forward expectations of a constant 1% RH. This type of expectation is not uncommon, nor is the frustration and disappointment that typically follows when the RH meters regularly display increased levels of humidity, and never seems to stabilize at the set point.. However the problem may lie with a less than adequate determination of the load which would be placed on the cabinets.

In order to effectively maintain the desired RH Level consideration must be given to the ‘load’ that a cabinet will be required to handle. To fully evaluate the ‘load’ the following questions should be examined;

Will the cabinet be opened frequently? How many times per day? How many times per hour?
Is the inventory ‘conditioned’ (Dry), or recently exposed to high RH Levels (Saturated)

How long will the inventory remain in the cabinet?

What other packaging materials will be stored in the cabinet?
Will the cabinet be located in an inventory / stores area or on the production floor?

How much inventory will you actually be storing at any given time?

How is the inventory to be presented? (boxes, bags, reels, trays, tubes, mounted on P&P Feeders, etc…)
Installation location

When selecting from among the various manufacturers for any dry cabinet application it’s important to evaluate just how air tight the cabinet itself is. Most manufacturers will offer data that illustrates how quickly the cabinet environment will degrade during a simulated power outage. This data will give a good indication of the cabinet’s ability to contribute in limiting the load placed on the desiccant dryer.

Equally important as the air tightness of the cabinet is its ability to ‘recover’. Recovery time can be defined as the time required for the cabinet to return to its set point after an introduction of moisture into the cabinet. There is a variety of approaches to minimizing recovery time, some manufacturers employ a variety of methods in order to assist the cabinet in regaining their set point RH levels, among these approaches are the following;

Varying degrees of forced air circulation triggered by the opening of the cabinet doors.

Employing one, two or even three dryer units in a single cabinet.

Adding heating elements to cabinet design.

A constant slow trickle of N2. (appx 1/10 standard N2 cabinet volume)

N2 cabinet purge triggered by a micro switch on cabinet door.

The addition of any one of the options above or any combination thereof will have an effect on the recovery times that the cabinet will be capable of providing. Of course each of the options comes with an accompanying cost, in some cases a one time charge, in other cases adding to running costs each month. Some of the options may be added in the field as an upgrade, others will require factory installation and should be requested at the time of cabinet ordering.

The questions discussed above pertaining to load will also have an impact on how easily a cabinet can recover. For example consider a dry cabinet that is located in an inventory / store room area, and is 50% loaded with inventory which has been in dry storage for the past 120 days; therefore well conditioned / dry. If two of it’s 1/6 size doors are opened for 45 seconds to have items removed, the recovery time will be minimal. If that same cabinet was sitting on the production floor empty and RH levels were stable at it’s set point and it had two 1/6 sized doors opened for the same 45 seconds to introduce a pick and place feeder base complete with mated feeders and components in paper tape that had been on the production floor for the previous 8 hours (50% RH, 73 Deg F), the recovery time would be considerably longer compared to the first example, and most likely unacceptable.

Typically cabinets located in the inventory or stores room area are less frequently accessed, their contents is more likely to have been in dry storage for prolonged periods and is often already conditioned. The additional load on the cabinet from a small amount of moisture being introduced (as per the examples above) will be light and the recovery time may be inconsequential since the inventory is already well conditioned and the cabinet may not be accessed for another 8 hrs. or more. In a busy production environment however, it is not uncommon to see the dry cabinet accessed every 20 minutes or more often to retrieve or return inventory. If a cabinet’s recovery time is 20 minutes then it’s clear that it would be ineffective against the load presented by the frequent opening of the doors to perform its primary function; protecting parts from hazardous moisture levels. The most common response offered by cabinet manufacturers to reduce recovery times is the addition of a second desiccant dryer unit. This solution reduces the recovery time by effectively doubling the desiccant activity available in the cabinet.

Another benefit that comes from having two desiccant dryer units per cabinet is that the control system will prevent both dryers from recycling their desiccant at the same time. Thereby ensuring that one dryer will always be actively desiccating the inside of the cabinet at any given time. When considering cabinets with only one dryer it’s important to remember that there will be no drying inside the cabinet during the recycling phase (which can be as long as 70 minutes every 6 hours). If a cabinet with one dryer has a door accessed during the recycling phase the increased moisture levels inside the cabinet would remain until the dryer unit came back ‘online’. It’s worth noting that some desiccant dryers functioning with closed loop humidity sensors will end recycling and return dryers to operation as soon as possible once any increase in cabinet humidity is detected.

Some manufacturers have also opted to offer models with three desiccant dryers per cabinet, tripling desiccant activity. These dryers all operate in tandem to ensure that a minimum of two dryers are actively collecting moisture from the cabinet interior at any one time.

Other options offered to minimize recovery times are variations of N2 purge systems. At first this may seem like a contradiction to the concept of a desiccant dry cabinet. However the addition of slow trickle N2 or a solenoid valve system triggered by the door closing can be practical enhancements for installation sites where N2 is readily available and traffic is expected to be unusually high. Recovery times are substantially minimized with the combination of N2 purge and desiccant dryers. Total volume of N2 consumed is reported to be on average 1/10 that of a similarly sized N2 cabinet.

Even more elaborate dry cabinets are available in the market; some offering heated shelving. These systems although considerably more expensive than a basic dry cabinet, offer excellent recovery times and the added benefit of providing a low temp bake (up to 60 Deg C.); preparing parts going into production.

Other performance enhancing features that can optionally be considered and may add to the effectiveness of a cabinet installation are forced air circulation and HEPA filtration systems capable of class 100 clean room standards.

There are also a number of other practical options offered by various cabinet manufacturers which may prove valuable for some applications. These are;

Data collection devices may be a good addition. In all cases cabinets are shipped with some form of readout to provide information on the cabinet’s RH level. It may be a dial type analog meter, an independent digital meter, or a built in digital display providing data from the cabinets own sensors. In all cases however, standard equipment included provides no more than a constantly updated readout, and tolerances on these meters can be as loose as +/- 5% RH. Further, the user has no idea of what the conditions were at any other point in the past; no ability to audit the data when a problem occurs. Manufacturers offer a variety of data collection solutions as options, they include (but are not limited to);

Portable digital dataloggers which are independent of the systems within the cabinet. Typically providing data download capability.
Traditional paper and ink style chart recorders
Integrated data collection systems, reporting over a network, capable of being serialized and daisy chained for multi cabinet installation monitoring.

Additional options such as audible alarms and light towers for notification during unexpected increases in humidity or when a cabinet door is left open unintentionally are available from some suppliers. As are various devices for making basic configuration changes inside of the cabinet.

Some suppliers have a wide selection of standard configurations and sizes ranging from desktop to 32” deep cabinets (and larger) equipped with a selection of pick and place feeder bases for the user to configure from. Other manufacturers provide a more basic offering, however there is a collection of standard sizes that seems to be common among all suppliers.

Serious investigation should also go into the verification of certification by North American electrical standards agencies. As a result of where the majority of desiccant dry cabinets are manufactured, many fail to carry the appropriate recognizable approvals such as UL, CSA, or CSA-US. There is at least one known case of a cabinet without proper regulatory approvals being purchased and installed in North America. After installation the new owner discovered that the unit was not approved for use and an attempted in house certification by the local agency, the cabinet failed as a result of sub-standard electrical components. The electrical components in question were replaced / rebuilt by the manufacturer but not without substantial aggravation and loss of time. Also worth noting is that currently only a small number of dry cabinet manufacturers have North American sales / support offices or established sales agreements to promote and support their products locally. The majority of the suppliers provide service and support directly from the Asian offices.

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