CLIMATE CONTROL, ACTIVE A N D P A S S I V E

The EgyptiuB GaZZenès ut the MetropoLtutz Mcsecm of Art In June 1983 the Department of Egyp- tian Art at the New York Metropolitan Museum of Art opened the third and final series of new galleries, concluding an eleven-year project to reinstall the entire Egyptian collection. * All the estimated 40,000 objects are now on view in thirty-two climate-controlled galleries, occupying nearly 7,000 square metres of exhibition space. Of the many prob- lems facing the curatorial, conservation, architectural and engineering personnel in undertaking a project of this scale few were more formidable than the problem of climate control and show-case design.

The new galleries were installed in three phases. In each phase a different approach to climate control was em- ployed. The purpose of this article is to describe these three different ap- proaches. I shall also try to shed a little light on some of the reasons for the gradual evolution away from air-con- ditioned cases towards the use of sealed cases within air-conditioned gallery spaces. Finally, I shall describe. the pro- cedure used for testing these sealed cases.2

Phase I. Difficult to maintah

The first of eleven galleries opened in 1976. In planning for this first phase of the installation, the Curator, Christine Lilyquist, decided to attempt to display all the objects - following a general recommendation by the architect, Kevin Roche - and to place them in chronolo- gical order. This meant displaying to- gether objects with quite different environmental requirements - for ex- ample artefacts in ivory, bronze, linen and stone in the same case. It also meant that very large cases would be required to accommodate objects of greatly varying scale and to facilitate access to them.

The resulting design scheme called for

large (380-7,300 m3) walk-in cases fronted by 2.3 x 2.3 m tempered glass lights butted one against the other with a 6 mm air space between each panel. In order to make these large display cases meet the museum staffs recommenda- tion of 72 "F (22 OC), 50 per cent relative humidity (RH), our consulting engineers designed a separate air-conditioning system for the cases themselves. This was designed to provide a closely controlled (72 * 1 OF, 50 t- 5 % RH) temperature and humidity-treated supply of extra- clean air to the cases. This air was to filter out through the joints between the glass panels into the public areas because of higher air pressure in the cases. The public areas were air-conditioned to com- fort conditions by an existing system, and return air from this system was the source of supply air for the show-case air-condi- tioning.

Low-voltage incandescent lighting fix- tures were mounted in compartments in- side the ceiling of the majority of the cases, providing 75 watts per linear foot of casework. This lighting, together with the transmission load from the surround- ing spaces, constituted case-cooling load.

Large load savings occurred when the lighting was turned off at night. Separate zoning was required for virtually each case, with temperature and humidity sensors within the cases varying the in- coming air temperature and relative humidity and compensating for the case loads. This feature made it possible to modify the conditions inside the cases to suit special requirements of certain sen- sitive objects such as unstable bronzes or ivories.

1. See the article by Christine Lilyquist in Jfuxeum, No. 142, 1984.

2. I would like to thank John Alticri of John Altieri, P.E., Consulting Engineers, Norwalk, Connecticut, for providing much of the technical information contained in this article.

Bill Barrette

A senior restorer with the Metropolitan Museum of Art, has worked in the Egyptian Department since 1975, participating in all three phases of its re-in- stallation. Initially responsible for the conservation of the papyrus collection, he also worked on the treatment of polychrome wooden coffins and, beginning in 1977, was charged with the monitor- ing and assessment of climatic conditions in the newly constructed air-conditioned show-cases. These studies played a decisive role in the evolution of the department's approach to the problem of climate control, eventually leading to the use of sealed cases within air-conditioned galleries.

82 Bill Barrette

The volume of air required to maintain pressurization and proper temperatures and humidities resulted in approximately fifeen to thirty air exchanges per hour.

Filters were provided in the inlet and outlet to each zone. These filters were 99.97 per cent HEPA filters normally used in controlling radioactive exhausts, screening out particulates to 0.3 of a micron in size. The effectiveness of these filters was somewhat lowered in several cases where the new case-supply duct- work was connected to existing riser ducts which were lined with accumulated dirt. The result was that loose duct lining and particulate matter could enter the air stream after filtration occurred, creating a maintenance and potential conservation problem in these cases.

A system of continuously operating 35.6-ton-capacity dual reciprocating compressor machines provided 40 "F (4.5 "C) chilled water to coils located in each zone. A pneumatic control system maintained a saturated discharge tem- perature off the cooling coils in summer to maintain fixed humidity, with re- heat coils maintaining temperature. In winter, humidity was provided by steam grid humidifiers with humidistats con- trolling steam flow.

These steam grid humidifiers turned out to be the most troublesome feature of this system. On numerous occasions a humidifier would stick in the full-open position and quickly fill the case with air at 100 per cent RH, resulting in condensation on the objects and the in-

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25 In the Phase III system the sealed casework and the hygroscopic materials within help to maintain stability even with fairly wide fluctuations in gallery temperature and relative humidity. The top chart records gallery space outside cases, the bottom chart records conditions inside a sealed case.

terior surfaces of the casework. Some damage to objects resulted from these in- cidents. Conservators and curators soon found themselves in the uncomfortable position of having to protect the art from the system which had been installed to protect it in the first place!

It was at this time that another con- servator and myself were assigned to monitor conditions inside all the cases using recording hygrothermographs. This process was invaluable in determin- ing the seriousness of system malfunc- tions, and provided a useful diagnostic tool which the engineers could eventually use in prescribing remedial work and in planning the design of the air-condi- tioning systems that were to follow for Phases II and III. We now have some thirty-six hygrothermographs in place throughout the wing, and while they present a considerable maintenance chore they remain the most reliable way of knowing with any certainty that condi- tions inside the cases are not presenting a danger to the objects.

After about two years of operation it became obvious that substantial remedial work would be necessary to correct the shortcomings of this system. A number of modifications were made to provide for more stable operation and for better notification in the case of mechanical fail- ures. These devices included a back-up shut-off solenoid valve in the feed of the steam humidifiers controlled by high- limit humidistats located in each case. In the event of the RH level exceeding 60

per cent inside a case the solenoid valve would then shut off the steam supply for the entire case system and sound an alarm in the engineers' office. Other modifica- tions made at this time were the addition of a back-up chiller pump and a booster fan to improve air distribution in some of the cases and the addition of fibreglass in- sulation to the backs of cases that were situated against exterior walls. In general these measures were effective; however, this system will always require careful monitoring and extra maintenance to assure its stable operation.

Phase II. Better cZìmate controd, bat more probdems

Phase II consists of nine galleries opened in November 1978. The design of the show-case remains the same, but condi- tioned air is first passed into the case and then into the gallery through the 6-mm air space between tempered glass panels butted one against the other. This approach allowed us to improve the climate control design of Phase I. This design provides for dual chilling equip- ment with a complete system of elec- tric / electronic controls. Fogging devices (cold water particles broken up by com- pressed air sprayed into the cooling coil) maintain a futed dew-point on all air sup- plied to cases. The case system and sur- rounding gallery system are conditioned to a futed-space dew-point of 52 "F (11 "C) combined with a fixed-space temperature of 72 "F (22 OC) resultingin

. ". , . I . .

The Egyptian Galleries a t the Metropolitan Museum of Art 83

26 Phase III: General view of the show-cases which are now sealed and no air space is left between adjacent glass panels.

a constant relative humidity of 50 per cent. For each system a dew-point master controller (hygrometer) raises or lowers cooling-coil discharge dew-point to match space conditions as sensed.

Should the gallery become heavily oc- cupied, the rise in moisture is sensed as an increasing dew-point. The dew-point control acts to modulate the chilled water valve open, lowering supply air tempe- rature (dehumidification). The various case and zone reheat coils modulate to maintain temperature.

Should there be a reverse - a fall in sensed dew-point - the dew-point con- trol acts to modulate the chilled water valve closed. In this manner, little or no dehumidification occurs, and warmer moisture-carrying air is supplied.

Reheat is provided using silicon con- trolled rectifier (SCR) controlled electric heaters. The SCR provides closely con- trollable, infinitely variable power to the heaters.

The Phase II system achieves a much more stable operation in comparison with Phase I. System problems were reduced due to a number of factors, including single control of moisture content inside all the cases; electronic rather than pneumatic controls; electric rather than water reheat; less dust in cases because the system used no outside air; better distribution of air to cases and, most significantly, in the event of mechanical failure, the foggers did not fill with ex- cessive humidity.

In spite of the obvious improvement of this rather elegant system over its pre- decessor, the first eighteen months of

operation were plagued by a host of mechanical and control problems. Some of these problems included impurities in the water supply to foggers which caused them to clog frequently and re- sulted in unacceptable RH fluctuations; difficulty in servicing the SCRs and some of the other sophisticated electronic control devices; difficulty in maintaining temperatures during peak load periods due to lack of adequate chilled water capacity; presence of dust in cases due to ongoing construction in the area. These problems were eventually resolved, and the system is now capable of providing 72 "F (22 "C)/50 per cent RH conditions in a reasonably stable manner.

However, at the time we had to make a decision regarding the design of the Phase III system, the Phase II system was not operating satisfactorily. Together with the ongoing problems of the Phase I system, these problems helped to create a consensus between the Egyptian, Ob- jects Conservation and Textiles Conser- vation departments against another at- tempt at air-conditioned cases.

By this time the museum staff also had enough experience with the various air- conditioning problems to make the fol- lowing basic assumptions about the longterm effects of air-conditioned casework.

First, even under the best of circum- stances, no air-conditioning system would be capable of providing constant temperature and relative humidity day in, day out, 365 days a year. Mechanical fail- ures were inevitable, and deviations from mean temperature and RH - lasting

2 7 ( 4 Phases I and II: (1) pretreated air supplied directly into show-cases; (2) higher air pressure in show-cases causes air to exfiltrate through joints between glass panels into gallery space; (3) air-conditioning for gallery space; ( 4 ) gallery.

27(b) Phase III: (1) sealed showcases containing buffering material; (2) air-conditioning for gallery; (3) gallery.

84 Bill Banette

anything from a few minutes to a few days - could be expected.

Second, the Textile Conservation de- partment observed that the constantly moving air physically disturbed the outer layers of the linens and expressed the con- cern that air constantly pumped into the cases was creating an oxygen-rich en- vironment which could increase the rate of oxidation of the organic materials being housed therein. Also, the presence of particulate matter in the cases along with staining of the wall linen at air out- lets suggested the possibility of pollu- tants being introduced with the air, steam, or both.

Third, the increasing complexity of the systems and controls being installed re- quired more maintenance, and in some cases more expertise than the museum’s Buildings Department could provide at that time. Even now the manpower need- ed just to keep the various controls, sen- sors and alarm devices properly calibrated exceeds what the museum’s staff can pro- vide, with the result that outside contrac- tors are hired to supplement their efforts.

It was against this background that the decision to eliminate air conditioning from the Phase III cases was made.

P h s e III

It was decided that temperature and relative humidity in the entire gallery space - in which sealed cases are con- structed - would be controlled by an upgraded HVAC (heating, ventilation, and air-conditioning) system. To imple- ment this, two new chillers and cooling towers were installed not only to run Phase III but also to add supplementary cooling capacity for Phases I and II. In ad- dition, two new fans and an enthalpy control system with an economizer cycle were installed. The new system is rated at 200 tons of refrigeration to cool 2,140 m3 of air per minute to a sufficiently dry state so that moisture can be added con- tinuously to maintain a constant relative humidity of 50 per cent. There are eight- een zones of temperature and humidity control, some located in the return air ducts above light attics of casework in order to respond quickly to varying lighting loads.

In spite of these and other im- provements, it was anticipated that there would be some daily variation in RH and occasional mechanical failures. The

casework was therefore designed to be sealed so that the RH in the cases would not be immediately affected in the event of short-term fluctuations in the gallery space (Fig. 26).

It was felt that in order to be effective, the cases should at least be able to slow down air exchange over a period of several days in order to maintain a stable RH en- vironment during periods when the system was ‘down’. This would also make it possible for the cases to be retrofitted with silica gel as an RH buffer should an auxiliary humidity control be required.

The success of this approach hinged solely on whether or not sealed cases of the size required (up to 180 m3) could be constructed to meet the conservation staff criterion of 1.5 air exchanges per day. The idea of sealed cases had been brought up by the conservation staff previously, mainly in discussions concerning the climate-control design of Phase II. The concept was ultimately rejected as being impractical if not impossible if curatorial access to the cases was to be a factor. This time, however, it was decided to assess the feasibility of sealed casework by having the architect design. a full scale prototype case which would be rigor- ously tested by conservation staff prior to making final design decisions regarding Phase III.

A prototype case was constructed in which all the movable elements were carefully sealed. Access to the case con- sisted of pivot and sliding doors which, when closed, were sealed top and bottom by a movable neoprene gasket which was ratcheted into place. The problem of dissipating heat load from the lighting fixtures was solved by positioning them in a special chamber above the case and directing return air through these chambers thereby removing the heat. To test accurately whether the prototype case met the design requirements, a method was developed by Steve Weintraub of the museum’s Objects Conservation depart- ment.

Test procedure

Briefly described, the testing procedure consisted of filling the cases with carbon dioxide or helium (COz, because of its weight, worked better for testing the lower parts of the case and, conversely, helium worked best for testing the leakage along ceiling joints) and then

measuring the rate of the tracer gas loss by a Gow-Mac gas analyser fitted with a strip chart recorder. Further, in order to test the integrity of the seals, and to detect specific points of leakage, a gas-leak detector with a probe attachment was used, making it possible to improve seal integrity where needed until the case con- formed to design criteria.

On the basis of this prototype and this testing method, it was possible to avoid costly or irreparable mistakes during the final design construction and installation stages of Phase III. The contractor who in- stalled the cases worked closely with con- servation staff and was able to modify and improve seals in response to test results. This resulted in even the largest of the finished cases being acceptable in their ability to maintain RH stability over a considerable period of time.

It should be noted that this work was carried out well in advance of the actual installation of objects; almost a full year was allowed for the testing of both the air-conditioning system and the cases before the installation of sensitive material.

ConcZusion

On the basis of a year’s observation of the Phase III system it is possible to state that air of constant temperature and humidity surrounding well-sealed and buffered cases does indeed result in quite stable relative humidities inside the cases, even during periods of instability in the gallery system. The mass of the casework and the hygroscopic material within them help to maintain this stability even with fairly wide fluctuations in gallery temperature and relative humidity (Fig. 27(b)).

It can be noted at the same time that with in-case air-conditioning systems, such as Phases I and II, operating costs can be reduced when loading and operating of central air-conditioning systems are reduced (i.e. in case of an emergency or for economic considera- tions, the case system can be run in- dependently of the gallery system with great savings in energy). However, the greater stability afforded by using tightly sealed cases in conjunction with con- tinuously operated central air-condi- tioning, and the increased safety of the objects displayed in this manner, lead me to strongly recommend this alterna- tive.