Humidification

Carole DeFord <carole_deford [at] cc__cranbrook__edu> writes

>I am looking for experiences where your organization has added
>humidity to large showcases such as 260 cubic feet of poorly sealed
>older wall cases which are currently monitored at about 20% humidity
>and need to be brought up to 48% RH range. How did you retrofit the
>case and what type of equipment did you use to do this.

The case retrofits that are necessary will vary depending on their
leakage rates. Most museum display cases are in the range of
one-half an air change per day (acd) to 20 acd.  A one cubic metre
(35 cu. ft.) glass case with a cube shape and no gaps greater than
0.3 mm (0.01 in.) is near 1 acd. A case with long gaps of 1.5 mm to
2 mm on the top and bottom will have a leakage rate of 20 acd or
more. For more information see the article by Stefan Michalski in
Studies in Conservation 39 (1994) p.p. 169-186.

With improved sealing and appropriate coverings (e.g., for wood
components) it may be possible to reduce case leakage to 1 acd. Then
passive humidity control with buffering material is a viable option.
Controlling seasonal humidity fluctuations in a case with 1 acd
leakage rate requires about 20kg (44 lbs.) of silica gel. This works
out to a silica gel cost of about $200 per cubic metre ($6 per cu.
ft.). Reducing case leakage and retrofitting a case to contain
silica gel (e.g., adding a false base) will add to this cost.

Mechanical humidity control is another option. Two different systems
have been used for this purpose. One is a humidity control module
developed at the Canadian Conservation Institute (CCI) which
supplies conditioned air to display cases through a network
small-diameter pipes. The second is a Micro Climate Generator which
is placed near a case and employs a recirculating air stream. The
Micro Climate Generator used to be available from Microclimate
Technology in Toronto Ontario. I am not sure about the current
status of this company or the availability of their products.

The CCI module is intended for use in museums with large numbers of
cases that are located in an area of uniform temperature.  Its
operation differs from conventional approaches to humidity control;
air is supplied to the cases at a rate that is just sufficient to
compensate for natural leakage, and there is no return air system.
In a typical installation, conditioned air from the module is
distributed through an air supply system made from ABS pipe which is
inexpensive and is easy to retrofit into an exhibit area. A short
length of small diameter tubing is used to control the air supply to
each case depending on its volume and leakage rate. This tubing is
only a few millimeters in diameter so the air supply point inside
the case is easy to conceal.

One module can control 200 cubic metres (7000 cu. ft.) of case
volume having a leakage rate of 1 acd.  If  260 cubic foot case
volume mentioned in the original query was very leaky (20 acd) then
it is possible to control the entire case volume with one module
without improving the case seals.  However reasonably well sealed
cases have a number of advantages even if a control module is used,
e.g., pest exclusion, longer response time in the event of
mechanical problems etc. The remaining module capacity would also be
available for additional storage or display cabinets.

The CCI module is described in detail in the reference mentioned
below. It was originally designed to be built with common workshop
tools and some (considerable for some parts) labor. At least one
company now sells a complete working module: Kennedy Trimnell, 109
North Kenilworth, Oak Park, Illinois.

A subsequent variation of the original module design has
incorporated some commercial devices (dehumidifiers and heat
exchangers). We used this approach to design a system for supplying
low-humidity air to an entire exhibit hall of display cases
containing metal artifacts at a large museum in Canada. The
low-humidity module was constructed at cost of $3000 for parts in
1986. The unit  provided many years of trouble-free operation with
low energy consumption and minimal maintenance.

Further development of a module incorporating some commercial
components is now in progress at the Canadian Conservation
Institute. One of the features that may be included in the new
prototype is dual air streams; one providing low RH for metals or
acidic paper materials and the other providing a variable humidity
setpoint.  Recent advances in microcontrollers have made a number of
other improvements possible. Highly accurate, maintenance-free
chilled-mirror humidity sensors can now be incorporated into the
module at reasonable cost in addition to other functions such as
monitoring, diagnostic capabilities and automatic remote fault
annunciation.

Reference:

Michalski, Stefan, "A Control Module for Relative Humidity in
Display Cases", Preprints of the Contributions to the IIC
(International Institute for Conservation of Historic and Artistic
Works) Washington Congress 3-9 September 1982, p.p. 28-31.

Paul Marcon
Preventive Conservation Services
Canadian Conservation Institute
Ottawa, Ontario, Canada

                                  ***
                  Conservation DistList Instance 11:72
                Distributed: Tuesday, February 24, 1998
                       Message Id: cdl-11-72-002
                                  ***