Push-pull ventilation system for open surface tank saves air flow

 

Three sections in this blog:

1. Push-pull ventilation system and its advantages
2. Disadvantages of general ventilation and side exhaust systems
3. Methods to design a push-pull ventilation system

Push-pull ventilation system and its advantages

Open surface tanks used in industrial processes often need ventilating to remove harmful pollutants (vapors, fumes or aerosols) from the working environment. One method of removing pollutants is the so-called side push-pull ventilation system, in which a jet of air is blown (or pushed) from one side of the tank and collected (or pulled) by an exhaust hood on the opposite parallel side. This system is particularly useful for large tanks where access requirements preclude the use of an overhead canopy, and the size of the tank makes side (or rim) exhaust systems prohibitively expensive.

Most agree that a push-pull ventilation system can yield air savings of up to 50% compared to a side suction only hood.

An illustration of a push-pull system for an open surface tank

Disadvantages of general ventilation and side exhaust systems 

General ventilation is frequently too costly and inefficient to be suitable for open surface tanks. Local ventilation allows the designer to minimize the air flow requirements while achieving greater control of the pollutant fumes. Side (or rim) exhaust systems are a possibility for small tanks, but become prohibitively expensive on larger tanks. The American Conference of Governmental Industrial Hygienists (ACGIH) (1995) recommends that such ventilation systems are unsuitable for tanks in excess of about 2 ft long.

Methods to design a push-pull ventilation system

One objective in designing the push-pull ventilation system is to induce a velocity over the whole of the tank which is sufficiently large to overcome any cross-draught and deflect the pollutant flow towards the exhaust hood. 

1. The ACGIH (1995) gives guidelines for the minimum capture velocity, Vcap, which must be induced by a Push-pull ventilation system The value of Vcap depends on the industrial process and the local conditions.
   For an open surface tank in an undisturbed location, Vcap should be in the range 50 fpm – 200 fpm, and for typical industrial situations with ambient cross draughts, the required value of the minimum capture velocity increases to 295 fpm < Vcap < 394 fpm. The system must be designed so that the induced velocity must be greater than some specified 'minimum capture velocity' Vcap for all x in order to deflect the pollutant towards the exhaust hood.
2. A different method other than the one recommended by ACGIH has been developed to design a push-pull system, which takes rising gas velocity at exit of open surface tank into consideration, Vg as shown in the above illustration. It can save more air flows (both push and pull) and consequently reduce the size of a system and the energy cost. 

Considering an application or even the feasibility of push-pull system? Contact Airvate now at info@airvate.com.