Cold Link Africa Online

 online directorycla   Twitter   Contact   search
Log in Register

Login to your account

Username *
Password *
Remember Me

Create an account

Fields marked with an asterisk (*) are required.
Name *
Username *
Password *
Verify password *
Email *
Verify email *

ASD in cold store and freezer environments (Part 2)

By Kevin Botha of Xtralis

We continue from the September/October issue, looking at possible pitfalls of installing an Aspirating Smoke Detector (ASD) — a technology originally developed to provide early warning of incipient fires in telecom and data facilities.

In Part 1, we looked at how ASD has evolved to provide effective protection in cold stores and freezers. It is generally accepted that ASD provides the earliest possible warning of fire, but what are the possible pitfalls and what should stakeholders consider with respect to product and installation to ensure that an ASD system provides reliable and trouble-free protection for their cold store or freezer facility?

Product

A first consideration should be product quality and in the fire detection industry, we are fortunate that products are rigidly tested to meet national and international standards. EN54-20 is a product standard specifically developed for ASD systems, which has been adopted across all EU countries as well as South Africa under SANS 50054-20. While the approval provides confidence that an independent third party has thoroughly tested the product, there is no guarantee that it is suitable for use in harsh environments such as cold stores and freezers, and stakeholders should explore in some detail the product’s technical capabilities as well as its history of successful installations.

For example, EN54-20 defines the number of sampling orifices that an ASD is capable of supporting, and it is important to verify that the selected product can provide enough sampling point holes to cover the area to be protected adequately. Manufacturers’ product data sheets should clearly state this information as well as the product’s associated class compliance. However, there is no requirement under the EN54-20 standard for manufacturers to publish this data and some make the information difficult to find.

The ASD should provide a wide sensitivity range with multiple staged alarms. That will provide the user with an early warning Class A or B alarm for early intervention — in layperson’s terms a ‘please investigate there is something wrong’ condition — as well as a later standard sensitivity Class C alarm, aka ‘there is a confirmed fire, please activate fire extinguishing protocols’. The ASD should be able to provide the staged alarms from a single detection device to keep costs down. There is little advantage in installing two low-cost ASD units — one to provide an early warning alarm and another to provide a confirmed fire alarm — when a single, slightly more expensive ASD can do both. As can be seen, manufacturers’ published EN54-20 performance data becomes quite important in helping with the choice of suitable product. Lower cost products that often don’t have the data readily available should be treated with caution.

A third, but vitally important, consideration is lifetime performance. Like all optical smoke detection systems, ASDs are prone to the effects of contamination from constantly passing air across optical components in the detection chamber. Advanced ASD systems use filters to remove more than 99% of contaminants from part of the air sample to provide a clean air barrier across sensitive optical components. This ensures that the detector’s design performance is maintained over the product’s lifetime, that service life is increased, and that lower ongoing maintenance costs are achieved for the user.

Less advanced ASDs employ lower cost software algorithms to alter the calibration of the detector artificially as it ages and becomes contaminated; an approach that is always a compromise because it inevitably affects the system’s detection performance and may even change the intended class compliance. To clarify why this is so important: ASDs are designed, installed, tested and commissioned to provide optimal detection performance, which is just sensitive enough to provide the earliest possible warning, but at the same time avoids the probability of unwanted or false alarms. That is a fine balance, which is relatively easily achieved through a structured specification, design and commissioning process with guidance from manufacturers and associated application standards documents. With that in mind, there is little point in going through the process of testing and optimising a low-cost ASD system if its detection performance drifts from original design and intent. There is also a resulting risk that fires may be detected later than was intended, with subsequent associated loss.

Design and application

Similar to other smoke detection technologies, for example point type detectors, the quantity and positioning of ASD detection points or sample holes is governed by prescriptive standards such as SANS-10139. However, the principle of transporting air samples along a network of sampling pipes means that ASDs are inherently flexible when it comes to design. It is easy to meet prescriptive design and installation standards while at the same time exceeding those requirements to provide improved performance for the user. A performance based approach, rather than a simple ‘deemed to comply’ solution is therefore easily and cost effectively achieved. That approach adds measurable value to the detection system; something normally not easily achieved with fire detection.

For instance, sampling holes can be positioned in locations where fire is most easily and quickly detected. Very often that is not where a ‘deemed to comply’ design tells us, but rather where air movement due to chilling equipment transports the smoke.

Sampling pipe can be positioned inside the protected area below the ceiling, a technique that minimises the number of penetrations through walls and ceilings. Alternatively, sample pipes can be positioned outside of the chilled area with penetrations at each sampling location. While the latter method seems more complex, it has proven to be the most effective at reducing the risk of condensation and icing in sub 0°C freezers. Innovative design of newer ‘through ceiling’ refrigerated storage sample kits has all but eliminated the possibility of frozen sample holes by removing the sample point from the cold area — instead, placing it outside the freezer.

The sampling kit shown in Figure 1 branches off the main pipe, and with its unique design the sampling hole is transposed to an upstream location (that is, at ambient temperature) while maintaining an open-ended pipe inside the refrigerated area. This arrangement also allows pipe network maintenance to be carried out entirely above the refrigerated area at ambient conditions rather than inside the refrigerated area.

In freezer applications where sampling pipes are installed outside the chilled area, the pipes should be lagged or insulated to avoid condensation and possible freezing on the exterior walls of the pipe. Normally a single layer of 9mm thick insulation is adequate. Sampling pipe penetrations, through ceiling or wall insulation panels, must be properly sealed with urethane foam and/or flexible mastic.

Locating ASD sampling holes at specified intervals at the ceiling level is in many cases all that is needed to be code compliant. However, sampling pipes can often easily be extended to provide detection at lower elevation, for example within storage racks. That not only provides a higher density of sampling locations at a relatively low cost, but also helps to ensure that cold, diluted smoke from a small fire can quickly be detected.

The ASD units need to be installed in locations where they are unlikely to experience sub-zero temperatures. For sub-zero freezers it stands to reason that the units will always be installed outside the protected area, typically in a service corridor or ceiling void. High-end ASD systems will tolerate sampled air temperatures as low as -20°C, but do check specifications to assess operating limits. If sampled air is likely to arrive at the ASD unit at a temperature lower than product specification allows, it is relatively easy to warm the sampled air before it enters the detector, either through ambient temperature transfer or in some cases by installing dedicated trace heating on the sample pipes. Manufacturers of ASD systems should be able to provide advice, taking into consideration temperature and pipe material characteristics.

Exhaust air from the ASD unit must be piped back into the protected area. That creates a closed loop system, which prevents pressure differences from affecting airflow monitoring and avoids warm air migrating into the freezer if the unit is shut down.

ASD systems are often used to activate pre-action sprinklers. Early warning alarms from the ASD may be used for early, manual intervention, while the later, confirmed fire alarm can be used for activation. Alternatively, if more than two ASD units are installed, then ‘coincidence detection’ can be deployed where both detectors need to reach pre-determined alarm thresholds before extinguishing is activated.

A properly engineered ASD design using a product that does not alter its calibration and performance over time, accompanied by a pipe network design using established methodologies, should result in a system that provides cost effective, reliable detection at all times, with significantly reduced probability of false alarms.

Figure 1: Cold storage sampling kit for warehouse.

 

 

 

Figure 2: Sampling point line drawing.

 

Pin It
Home REFRIGERATION Warehousing and Storage ASD in cold store and freezer environments (Part 2)

Talk to Us

Monday - Friday, 8 AM - 4 PM 

This email address is being protected from spambots. You need JavaScript enabled to view it.  00 27 11 579 4940