Assessment of types of climate control according to a handbook of the American Society of Heating, Refrigerating and Air-Conditioning Engineers ASHRAE (2023)
Background
The types of climate control are based on specifications for temperature and relative humidity provided in Table 13A in chapter 24 ‘Museums, Galleries, Libraries, and Archives’ in the handbook of the American Society of Heating, Refrigeration and Air Conditioning Engineers (2023). The specifications comprise four components: long-term outer limits, annual averages, seasonal adjustments from annual averages, and short-term fluctuations. Five types of climate control AA, A1, A2, B, C, and D are distinguished and the specifications for each type are listed in the table below.
| Type of control | Long-term outer limits | Annual averages | Seasonal adjustments from annual average | Short term fluctuations |
|---|---|---|---|---|
|
AA Precision control, no seasonal changes to relative humidity |
≥35% rh ≤65% rh ≥10°C ≤25°C |
For permanent collections: historic annual average of relative humidity and temperature.
In public display areas, human comfort temperatures can apply |
No change to relative humidity Increase by 5°C Decrease by 5°C |
±5% rh, ±2°C |
|
A1 Precision control, seasonal changes in temperature and relative humidity |
Increase by 10% rh Decrease by 10% rh Increase by 5°C Decrease by 5°C |
±5% rh, ±2°C |
||
|
A2 Precision control, seasonal changes in temperature only |
No change to rh. Increase by 5°C Decrease by 10°C |
±10% rh ±2°C |
||
|
B Precision control, seasonal changes in temperature and relative humidity |
≥ 30% rh ≤ 70% rh ≤ 30°C |
For permanent collections: historic annual average of relative humidity and temperature. |
Increase by 10% rh. Decrease by 10% rh. Increase by 10 K Decrease by up to 20 K |
±10% rh, ±5°C |
|
C Prevent relative humidity extremes (damp or desiccation) and prevent high temperature extremes |
≥ 25% rh ≤ 75% rh ≤ 40°C |
Within 25% to 75% rh year-round.
Temperature usually below 25°C |
Not continually above 65% rh for longer than X days. | |
|
D Prevent very high relative humidity (dampness) |
≤ 75% rh | Relative humidity reliably below 75% rh | Not continually above 65% rh for longer than X days. |
Further, the specifications stipulate that rate of seasonal adjustments should not exceed the short-term fluctuation limits each 30 or 7 days for relative humidity and temperature, respectively.
Due to editorial errors, ambiguous formulations and redundancies of some criteria, the group of experts decided to adopt modified criteria for the ASHRAE classes, which are presented in the table below.
The components of the specifications listed in the table are calculated from the temperature and relative humidity data by the HERIe software as described below.
Annual averages (AA)
The annual average is determined as the arithmetic mean of the temperature or relative humidity records in the uploaded data set (AAT or AARH), that is to say, the sum of all records divided by their number. To obtain a correct value, data collected over at least one calendar year must be used. If long-term records are available, only whole multiples of the calendar year should be used.
Seasonal cycle and adjustments from annual average (SA)
The seasonal cycle is obtained by calculating, for each RH record,
the moving average MARH which is the arithmetic mean of all the RH records taken in 30 days before the time at which the average is computed.
In turn, for each temperature record, the moving average MAT is calculated from records taken in a previous 7-day period.
To enable the calculation of MARH for the first 30 days of the uploaded data, it assumed that RH during a previous 30-day period is equal to the average RH
in these initial 30 days. Similarly, it is assumed that temperature during a previous 7-day period is equal to the average temperature in the initial 7 days.
The moving average smooths out the short–term fluctuations and highlights longer-term trends in the data.
The seasonal adjustments are calculated as two maximum deviations of MARH or MAT ‒ positive and negative ‒ from the respective AA levels:
SARH positive = maximum MARH– AARH or SAT positive = maximum MAT – AAT
SARH negative = minimum MARH – AARH or SAT negative = minimum MAT – AAT
Short term fluctuations (STF)
For types of control AA and A2, a short-term fluctuation is calculated as a difference between a current temperature or RH reading and AAT or AARH:
STFT = T – AAT
STFRH = RH – AARH
For types of control A1 and B, a short-term fluctuation is calculated as a difference between a current temperature or RH
reading and MAT or MARH calculated for that reading:
STFT = T – MAT
STFRH = RH – MARH
Rate of seasonal adjustment RSA
The rate of seasonal adjustment is defined as a change of this parameter – positive or negative ‒ in each previous 30 or 7 days for relative humidity and temperature, respectively.
For a record number k ‒ SARH(k) ‒ sampled at time t(k), RSARH = absolute value │SARH(k) - SARH(i) │ where i denotes any time interval t(k) – t(i) < 30 days, previous to t(k)
For a record number k ‒ SAT(k) ‒ sampled at time t(k), RSAT = absolute value │SAT(k) – SAT(i) │ where i denotes any time interval t(k) – t(i) < 7 days, previous to t(k)
Temperature usually below 25oC
The chemical degradation module is used to calculate the relative lifetime of unstable paper materials in two microclimate conditions:
- uploaded environmental data (RLdata)
- constant conditions of 25oC and 50% RH (RL25,50)
The condition is deemed to be fulfilled if RLdata > RL25,50 that is to say variations in the conditions do not induce greater risk to the collection than the constant conditions, in particular the temperature of 25oC specified as the target upper limit.
Not continually above 65% for longer than X days
The relationship between time to mould tmould in days and RH shown in Fig. 3 in the ASHRAE Chapter is described by the following equation:
tmould = 1.9 + 6.9∙1014∙exp(RH/2.3) + 4∙108∙exp(RH/4.6)
Times expressed in days during which MARH was at 66, 67 …. 98, 99% are calculated as X66, X67 … X98, X99.
The mould risk index in period k in which RH level exceeded 65% (MRIk) is defined as the sum of the ratios of X/tmould ,
that is to say, as the ratio of the total time over the threshold to the accumulated time to mould in this period:
MRIk = ∑X/tmould = X66/tmould,66 + X67/tmould,67 + … + X98/tmould,98
+ X99/tmould,99
Using the WUFI® Bio software, it was calculated that the time required for mould spores to reduce their moisture content from the level at 65% RH to the level at 60% RH (the assumed condition preventing any growth of mould) is two weeks.
Therefore, if the wet period k is followed by the dry period with RH at or below 60% RH that is longer than two weeks and if MRIk is less than 1, that is to say, the total time over the threshold in period k has not reached the accumulated time to mould, MRKk is set to zero.
At the end, the mould risk index MRI for the uploaded data is calculated:
MRI = ∑ MRIk
If all MRIk are less than 1, then MRI is equal to the maximum MRIk.
The condition that RH is ‘not continually above 65% for longer than X days’ is deemed to be fulfilled if MRI is less than 1.
The specifications for temperature and relative humidity are expressed in the following way in the HERIe algorithms with the use of the parameters defined above: