Energy saving is one of the most important problems of modern civilization. Cleanrooms
consume a lot of energy, but common practice often specifies big power without proper
understanding what for. This presentation discusses ways to reduce energy consumption,
especially by means that are nearly obvious and can be realized immediately e.g. reducing
of air exchange rates and increasing recovery time. Examples based in real designs
made by company “Invar-project” are shown.
Cleanrooms consume a lot of energy in comparison with non-classified areas. But there
was a lack of proper attention to this problem until recent. In contrary, over-specifying of
requirements and over-designs can be observed. This negative picture is often based on
the normative documents or a wrong interpretation of them. Existing international standards
are too general and do not reflect specific features of cleanrooms. Detailed guidance how
to save energy in cleanrooms can be found in two national standards: British standard BS
8568:2013  and VDI 2083 standard . But the problem is so complicated and not
enough investigated that further efforts to reduce energy consumption are needed. Special
attention should be paid to reducing of air exchange rates and considering of real particles
generation in cleanrooms to determine really needed air exchange rates in operation.
Energy consumption grows quickly with increasing of cleanliness level. It is very difficult
to make general estimation, but practical examples show that power required for
cleanrooms can be on orders of magnitude bigger than for non-classified room, depending
of cleanliness class (Table 1). The air exchange rate depends also on e.g. heat load,
local exhausts and personnel present, which is different for different processes. But the trend is clear: energy consumption grows dramatically from one cleanliness class to another.
It can be different for one class depending on air exchange rate, recovery time and
other factors as shown below.
Methods of energy saving
Energy saving methods can either be general i.e. for all kinds of premises and buildings or specific i.e. for cleanrooms.
General methods focus the attention on:
minimizing heat gain/losses and providing heat isolation of buildings,
air recirculation with minimum outdoor air as possible on safety reasons,
placing plants in climate zones where high power in winter/summer is not needed,
use of high efficiency ventilators, AHUs and chillers,
use of equipment (HVAC units, filters, etc.) with reserve of power, taking in mind that equipment with bigger nominal power will consume less energy,
avoiding unnecessary narrow tolerances for temperature and humidity and / or
removal of heat load from process equipment rather by local in-built means then by HVAC systems and so on.
Specific methods concern cleanroom features and include:
reducing cleanroom squares to minimum,
avoiding over-specifying cleanliness class,
use of HEPA filters with reduced pressure drop (membrane filter 50 Pa instead of 250 Pa glass fiber filters),
sealing of leakages,
local protection when higher cleanliness class is maintained in a limited area as required by process,
minimizing number of personnel or using technologies without people (closed,
careful attention to operation, garments, hygiene, training, etc.,
reducing of air flow rate when not in operation,
avoiding over-specifying of air exchange rates and recovery time and /or
determining really needed air flow rates during testing and operation and adjusting
air exchange rate to minimum based on these data.
What air exchange rate is needed? Today most normative documents do not specify
exact numbers for air exchange rate, it is left to the designers. But some documents still
do specify these numbers. The most known requirement is 20 h-1. The development of
norms for air exchange rate is based on history.
Early age. In late 1950s USA set requirements for air exchange rate in surgical suites
to be a minimum of 12 h-1 in existing facilities and of 25 h-1 in new ones.
US Federal Standard 209. Probably the first mentioning of 20 h-1 was in early version
of old Federal Std. 209B. It reflected practice of 1960s. Later this norm was withdrawn
from Fed Std. 209, because it was understood that proper cleanliness level can be
achieved with less air exchange rate. The task of determining air exchange rate was left
for decision of designer.
FDA Aseptic Guide. FDA Aseptic Guide copied 20 h-1 in 1987 and nobody took care
to make correction even in 2004 when this Aseptic Guide was re-edited . Today FDA
requirement for 20 h-1 for supporting rooms (8 ISO in operation) is still in force: “For Class
100,000 (ISO 8) supporting rooms, air flow sufficient to achieve at least 20 air changes
per hour is typically acceptable. Significantly higher air change rates are normally needed
for Class 10,000 and Class 100 areas”. For ISO Class 7 this Guide requested even
greater air exchange rate than 20 h-1
GMP EC Guide. Early GMP EC Guide had this norm but it was cancelled in 1997. It is
worth to say that this step did not give much use, because conservative requirement for
recovery time 15–20 min remained (see be-low) .
ISPE Baseline. Many practitioners use ISPE Guide that recommends 20 h-1 for sterile process
(both aseptic and terminal sterilization): “Generally, air changes of at least 20 h-1 are expected
in Grade 7 and Grade 8 rooms” (ISO 7 and ISO 8 in operation respectively), item 5.5.3 .
Energy Saving in Cleanrooms
WHO report. The recent WHO report moved towards reduction of air ex-change rate:
“4.1.6 Air exchange rates are normally determined by the following considerations (could
normally vary between 6 and 20 air changes h-1)” . This is a step forward, but it can be
neglected be-cause the same report specifies strict values for recovery time: 20 min (item
4.1.10) and even 15 min (item 8.2.14, Table 3 of this Report). WHO Guide says about 6–
20 h-1 without explaining when and what to choose.
General. Old conservative value 20 h-1 is still strongly sitting in documents and embarrasses
minds (Table 3). There are no indications for non-sterile facilities, but some
designers prefer to use these 20 h-1, too. Sometimes 20 h-1 travel from “in operation”
requirement ISO Class 8 of FDA to zone D where ISO Class 8 is specified for “at rest”
condition only and for “in operation” no requirements exist at all!
Continue at: https://www.vtt.fi/inf/pdf/technology/2014/T168.pdf
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