Ductless Jet ventilation system

Jet fan systems have been developed to ventilate underground cark parks for both CO removal during normal conditions & extracting smoke during emergency situations. A single system can handle both these requirements more effectively & economically, i.e., jet fan ductless car park ventilation systems.

The complete system consists of Jet fans, main axial fans to extract bad quality air & return fresh air. Normally fresh air fans are designed for basements where there are not enough natural cut-outs such as ventilation shafts or ramps. In addition to fans, a unique control system is designed for specific projects, where the signals from CO/heat/smoke sensors operate the system at different speeds according to the need of the situation as well as save energy.

To design an efficient system, we need to involve the designers quite well in advance for suggesting the openings in the basement for an efficient fresh air intake & proper CO/ smoke removal. Fans are normally designed on the basis of air exchange rates for both normal & emergency applications. The local codes or building regulations play a vital role in designing the air flows, control systems & zones in the basement.

The jet fans work at different operating conditions depending on the signals from the CO sensors giving possibilities to save energy “working on-demand” ventilation principle. Jet fans can be controlled individually or together as a group of fire zones. During fire operations, a complete system including main fresh air extractor fans and jet fans runs at full speed for cleaning the basement from smoke efficiently and quickly.

Advantages of Jet Fans

The jet fan system has its own advantages over conventional ducted systems.

• As the system doesn’t need ducts, it allows the structure to have smaller heights, appx 0.5m to 0.75m lesser than normal basements.
• A ductless system is easy to install in a shorter time compared to conventional, expensive & complex ducted systems. It means saving on installation time & cost.
• Jet fan systems are easy to maintain & an ideal solution for renovations for car parks.
• It is possible to operate systems in parts/zones as per signals given from CO/smoke sensors.
• As air is free to move in specified directions given by the jet fans, air mixing is proper in all the layers & provides better air quality in the basements compared to a conventional ducted system.
• Jet fan systems allow CO & smoke extraction removal as one system with maximum level of automation & high efficiency. CO sensors ensure that only fans in the polluted zones run. Smoke sensors decide that the system should start in full swing once the signal is given to the controlling device.
• Elimination of ducts in systems gives less resistance to the main fans resulting in lesser static pressure, such as lower motor KW of the fans.
• Jet fans are very flexible to position in a car park.
• The low height of the jet fans allows the car park to be neat & clean. As a result, it helps to provide better possibilities to monitor the basement with help of CCTVs.
• These industrial fan systems also prevent expensive structural damages in emergency fire conditions by providing proper mixing of fresh air & quick removal of smoke from the basement. This limits the heat exposure to ceilings/slabs.

System Requirements

The jet fans system design needs jet fans based on the area of the basement. Main fresh air fans (where cutouts are not sufficient) & extracts fans for both CO & smoke removal. The capacities of the main extract fans are based on the air changes. International codes refer to 4-6 ACH for CO removal & 8-10 ACH for smoke extraction.

The Control system is the heart of the system & needs to be designed carefully. The control system decides the response time for the speed of the jet fans after getting signals from CO sensors. The complete system is, therefore, able to optimize the energy at different parking loads of the day. This means the control system will start/stop the fans individually or zone wise pending the demand of the cark park.

When connected to the fire detection or in high CO levels, the controlling device needs to switch on the fans at full speed. This happens after getting the signal from the heat/smoke sensor or CO sensor. In addition to switching jet fans on high speed on the fire floor, the control system is also responsible to switch off fans on other levels. It may be the upper or lower levels than the fire floors. The Control system will also regulate the main supply/extract fans on-demand or signal from the respective sensors in addition to jet fans, quickly & effectively.

Building Codes

International Mechanical Code – Section 403.5 Public garages

Mechanical ventilation systems for public garages are not required to operate continuously. Here, the system is arranged to operate automatically upon detection of a concentration of carbon monoxide of 25 ppm by approved detection devices.

Uniform Building Code – Section 705

In parking garages, automatic CO sensing devices may be employed to modulate the ventilation system. This is done in order to maintain a maximum average CO of 50 ppm during an eight-hour period. Also, the maximum average concentration should not be greater than 200 ppm for a period not exceeding one hour.

State, municipal and other building codes

Most state and local municipal building codes recognize and recommend using carbon monoxide monitors in enclosed parking garages. If the building code in your area does not have any provisions for CO monitoring, do not forget that carbon monoxide is still a very dangerous gas. It can be harmful to personal health and safety. Therefore, the use of common sense by design engineers is still the best “building code” to use.

CO Alarm Level and Sequence of Operation

First Alarm level

1. Set at low concentration. (See Table for recommended low alarm level)
2. A gas monitoring system shall be able to actuate exhaust fans and outside air intake devices to bring the carbon monoxide level down to an acceptable level.
3. Optional: if the parking structure is equipped with either two-speed fans or with a second set of fans, only the first speed of fans or the first set of fans should be actuated.

Second Alarm level

1. Set at high concentration. See Table for recommended high alarm level).
2. Ventilation equipment previously actuated at low level in the parking garage shall remain operational.
3. Optional: if the parking structure is equipped with either two-speed fans or with a second set of fans, they should be activated at full speed or all fans should be activated.
4. Activation of strobe light (red) and horn might be required.
5. People might be required to evacuate the parking garage.