By Ron Burns
With HVAC contractors becoming larger players in the smoke and heat exhaust ventilation systems (SHEVS) segment of the market, there are inherent risks related to different skill sets between the two air-moving disciplines.
Taking a look at the fundamental differences will bring the associated risks to the fore and create some awareness of the unique challenges faced by the HVAC contractor when entering this market. The objective is not to scare away the HVAC contractor, but to equip the contractor with some additional knowledge to compete in the market confidently, achieving the objectives without compromising the system functionality.
The following extract from EN 12101:5 directly speaks to the HVAC systems, with good reason:
7.7 Heating, ventilation and air-conditioning (HVAC)
An HVAC system (or air-conditioning and mechanical ventilation system) is designed to achieve different objectives in comparison to a SHEVS. Not only are the quantities of gases being moved usually smaller, they are generally moved in different directions. For example, it is common for HVAC systems to introduce replacement air at high level in a room, and to exhaust used air at low level; the opposite to that recommended for SHEVS. Even when an HVAC system has been shut off, air ducts can provide pathways for the unwanted movement of smoke unless measures have been taken to prevent this from happening.
HVAC systems can be incorporated in whole or part, in a SHEVS. Where this is done it is necessary to isolate those parts not incorporate, and to ensure that the parts which are incorporated meet the same performance standards as the rest of the SHEVS. Dampers which can only be reset manually can make the regular functional testing of a SHEVS extremely difficult. Consequently, it is necessary for smoke dampers to be capable of being both opened and closed by powered mechanisms.
Where the internal air temperature in a building, e.g. in an atrium, is lower than the external by an amount such that the internal hot buoyant smoke layer is itself cooler than the outside air, opening a natural SHEVS causes the smoke to exhaust downwards. This could adversely affect the means of escape.
[FD CEN/TR 12101-10: 2006-11; p. 53]
As we unpack the specific challenges addressed in this portion of the code, we will get a better understanding of the comments made in this commentary. It is not impossible to combine the HVAC and SHEVS functionality in common equipment; however, certain requirements in the SHEVS require specific attention. Over the past few years, the air-conditioning contracting companies have been taking a larger role in the SHEVS market. This brings advantages to the client, as the HVAC contractors are able to use their buying power and present cost savings; yet, the skill sets are fundamentally different when appreciating the operating conditions. Similarly, when comparing the difference between the external cladding of chilled water piping and steam piping. The HVAC contractor needs to ensure code compliance; the higher temperatures can, if not taken in careful consideration, render the SHEVS system vulnerable to failure once smoke temperatures exceed 80°C.
In a building where the HVAC system is completely separate from the SHEVS, it is vital to isolate the HVAC system on receipt of a fire signal.
In a building where the HVAC system is completely separate from the SHEVS system, it is vital to isolate the HVAC system on receipt of a fire signal. Generally, an HVAC system supplies air to the building through diffusers at ceiling level. In an office environment, the ceilings are generally around 2.8m above finished floor level. The requirement of isolating the HVAC system is to prevent the cooling of the smoke generated from the fire. In the early stages of the fire, the smoke contains very little heat energy and is termed ‘cool’ below 50°C. The smoke is buoyant in comparison to the 23°C return air and can easily find its way into the return air path. The smoke will then pass over an air handling unit cooling coil, between 5°C and 7°C, and will be distributed through the supply air duct network, subsequently being distributed into the building.
Smoke is dangerous, but not for the reasons we immediately think of. Our default is temperature; however, the temperature of the smoke is generally insignificant when it comes to threatening human life. Cool smoke has the same rate of asphyxiating people as hot smoke. Hot smoke is difficult to extract from an equipment handling perspective. Smoke which finds its way into the return air path is extremely dangerous. This cool smoke, which is heavy, will easily be dragged back into the seat of the fire. This creates an incomplete process of combustion, which generates more smoke than a clean burning fire. This fire feeding off cool smoke, rapidly generates larger volumes of smoke than the designer calculated. Isolating the HVAC air handling unit from the smoke is paramount to a successful extraction of smoke and should be achieved as quickly as possible without compromising the health of the system. Automatically closing dampers without allowing the system to equalise in pressure should be avoided and pressure relief should be designed into the system.
Typically, a high-rise office block will have an extraction rate of 16.6m3/s with a smoke plume temperature of 180°C, provided the building is sprinkler protected. This scenario is based on a building being located at 1 660m above sea level. Immediately, the HVAC contractor will appreciate the high extraction rate and the high temperature for a simple 2 600m2 office floor plan. Should the building not be fitted with a sprinkler system, the internal conditions are vastly different. The extraction rate will increase to 47.5m3/s with the smoke plume temperature averaging 488°C; a clear layer of 2.5m cannot be achieved within the temperature limitations of a safe SHEVS. Using the existing HVAC duct network to exhaust 16.6m3/s will appear daunting enough; raising the extraction rate to 47.5m3/s becomes challenging to say the least. Cooling the 488°C smoke temperature to 292°C requires an extraction volume of 60.8m3/s, bordering on the impossible for a standard HVAC system providing comfort cooling to a 2 600m2 single-level office floor. Ignoring the elevated temperature of the building without sprinklers will result in SHEVS equipment failure.
With the above insight, we need to think through the following questions — exhausting the gentle condition of 16.6m3/s at 180°C comes with a set of unique challenges. Firstly, the intake level of the smoke into the exhaust system must be above the clear layer of 2.5m. Secondly, when selecting the intake position of the extraction points, the designer needs to consider the ‘plug-holing’ effect. The smoke will find its own natural level based on plug-holing irrespective of the volume extracted. Simply extracting a larger volume does not negate plug-holing and the smoke will find it equilibrium level quickly. These calculations are important and need to be done — the results applied to the intake centre line of the smoke extraction inlets and the plug-holing effect neutralised.
Other considerations now become important: Can the duct withstand the elevated temperature of the smoke, the gaskets? How is the smoke entering the duct network? Through the air-conditioning diffusion equipment? Can the flexible connections withstand the temperature? The argument of the flexible duct perishing carries no weight. Although the flexible duct will perish, the question is, when? What level will the smoke layer be when the flexible perishes? What temperature is the designer surmising or is there research data on this? Do all inlets perish simultaneously? There are just too many questions here, too many unaccounted variables.
The single most valuable component of this discussion is the isolation of the air handling unit. Smoke detection systems of any new development is fraught with false alarms during the first few months and will definitely result in a few callouts resulting from failures in associated systems. These inconveniences are well worth the frustration should a fire occur. The HVAC contractor will be proud should all occupants be safely evacuated from the building during the fire. That in itself is job satisfaction seldom surpassed.