Refrigerated Storage Design Guide
VESDA
®
Figure 1 – Top view of a grid layout for VESDA sample holes.
In some freezers, depending on the effectiveness and frequency of defrosting management, ice will
build up around the entrances. Under these circumstances, VESDA sample holes in the immediate
proximity may become blocked by ice. This is normal and can be managed by specifying an
appropriate system airflow fault threshold. The effects of blocked sample holes could be
compensated for as follows:
•
By placing the sample hole closest to the entrances as far away as is legally permitted by local
codes and standards.
•
By partial in-rack sampling near the entrances. Ice build-up is usually at the ceiling above the
doorways, seldom within the racks.
•
By placing VESDA pipes away from the path of chiller air supplies.
Note:
Sample hole diameter MUST be larger than 3 mm.
Referring to the grid layout shown above (Figure 1), there are two alternative approaches to pipe
network configuration design for optimal air sampling performance:
1.
The pipes can be run along the ceiling inside the area to be protected as shown (Figure 2).
This technique minimizes the number of pipe penetrations through the walls or ceiling and
can be applied in most practical situations. Sample holes MUST be drilled after installation in
this case.
2.
The pipes can be positioned outside the area to be protected with smaller diameter capillary
tubes (16 mm (3/4 inches) outer diameter) fed through the ceiling insulation into the
protected area as shown (Figure 3). This method requires one ceiling penetration per
capillary tube and more pipe connections. All penetrations must be adequately sealed to
prevent the formation of ice resulting from the entry of humid air or condensation. This
approach is only appropriate where the refrigerated storage facility has a ceiling void. The
capillary sampling tubes should be as short as possible; less than 4 m (13 ft) being
recommended. They MUST also be sealed during installation.
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