Refrigerated Storage Design Guide
VESDA
®
4.5
Crystallization Inside Sample Pipes
In order to minimise the possibility of ice plug formation inside the sample pipe, 90° elbow junctions
between pipe sections should be avoided. Where it is essential to change the orientation of the
pipe from horizontal to vertical, or vice versa, large radius bends should be used.
The chiller air supply is several degrees lower than the air in the rest of the chiller so, to prevent ice
from forming and blocking the sample holes, sample pipes MUST NOT be installed in the direct
path of the chiller’s supply air vent. Doing so will lead to crystallisation when the warmer ambient
air in the protected area enters the cool pipe. If for practical reasons, this cannot be avoided, the
pipe MUST be insulated as shown below (Figure 10).
Figure 10 – Example of pipe insulation to combat ice
formation in pipes directly in the path of the chiller air
supply (the top is a good design, the bottom is not).
Important Note:
Pipes MUST NOT run from a high temperature area into a lower temperature
area. Separate VESDA detectors should be installed in the chiller, cooler, ceiling void, and office
areas.
5. Installation
Considerations
5.1
Sampled Air Warming
For optimum detection and ease of maintenance access, the VESDA detector MUST be installed in
a location where it is unlikely to experience sub-zero temperatures. Low temperature sampled air
can be easily warmed up, before it reaches the detector, by extending the sample pipe length
beyond its point of exit from the cooler protected area. The warmer air outside the pipe near the
detector will heat the cold sampled air within it. Usually, only a short pipe extension is necessary.
The charts below (Figure 11 to Figure 13) provide conservative estimates of the pipe extensions
required, to raise the temperature of the sampled air to an acceptable value, for a number of
commonly used pipe materials and range of flow rates. All calculations assume an external
ambient temperature of 20
°
C (68
°
F) and a sampled air temperature of 4
°
C (39
°
F).
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