What are Bag Filters in HVAC?

In this post we’re going to look at bag filters in more detail and provide a short guide covering the following:

• What are bag filters?
• What different types of bag filters are there?
• How to choose the right bag filter?
• Where can I buy replacement bag filters online?

So let’s get started…

What are bag filters?

Bag filters, also known as pocket filters, are utilised in HVAC applications as prefilters in HEPA installations as well as high efficiency final filters in commercial, industrial, medical, and institutional applications.

Bag filters are the most popular air filters used in HVAC systems as high efficiency filters in commercial, industrial, medical, and institutional settings to enhance indoor air quality and comfort.

For these applications, the filters in the supply air are used as the first and second filter stages; either as complete filtration solutions or as prefilters for cleanroom process applications. To safeguard the air handling equipment, the filters are also utilised in the exhaust air or in recirculation systems.

Bag filters outperform most other filters in terms of dust holding capacity and lifespan.

What different types of bag filters are there?

There are several different types of bag filters which can be grouped by 3 main characteristics.

1. Cleaning methods
2. Filter direction
3. Shape of the bag

1) Cleaning methods

Bag filters can be categorised into three groups based on the various techniques used to remove dust: mechanical shakers, reverse air, and pulse jet.

• Mechanical shakers
  The most basic cleaning technique uses shakers, which can be either mechanical or manual. The length of the filter bag is typically shorter, the acceleration attenuation is faster, and the filter wind speed is likewise smaller. In general, the mechanical shaker bag’s vibration dispersion in the axial direction is uneven.
• Reverse air
  In order to achieve dust cleaning, reverse air bag filters use valves to switch airflow and cause the filter bag to contract and bulge while being backblown. On the other hand, due to the change in airflow direction, the filter bag creates expansion and contraction vibration, which is also beneficial to the shedding of the dust. The viscous stripping force created by the dust cleaning air flow in the opposite direction directly peels off the dust layer.
• Pulse jet
  The surrounding air is forced to spray into the filter bag in a very short period of time when the compressed air is discharged at a fast rate through the nozzle, causing the filter bag to rapidly expand and shrink. The pulse jet bag filter has steadily emerged as one of the most crucial dust removal tools due to the various benefits of pulse jet cleaning methods.

Filter direction

Filter direction can be divided into two categories:

• Internal Bag Filter
  The dusty air moves from the inside of the filter bag to the outside, depositing dust on the inner surface of the filter bag.
• External Bag Filter
  The dusty air flows from the outside of the filter bag to the inside, and the dust is deposited on the outer surface of the filter bag,

3) Shape of the bag

Bag filters come in four different varieties:

1. Diamond bag filters – Smaller than the ordinary round bag, the filter area can be increased in the same box, and it is only suitable for external filtration type. Double-layer bag filters – The filter area is increased on the basis of the round bag, and the long bag is folded into a double layer, which can nearly double the area.
   Flat bag filters – Bag width 35-50mm, area 1-4㎡, larger filtration area per unit volume, external filter type, with frame.
   Circular bag filters – The diameter of the external bag filter is φ120mm-160mm, and the diameter of the internal bag filter is φ200mm-300mm or larger. It is the most widely used form of filter bag.
2. Double-layer bag filters – The filter area is increased on the basis of the round bag, and the long bag is folded into a double layer, which can nearly double the area.
3. Flat bag filters – Bag width 35-50mm, area 1-4㎡, larger filtration area per unit volume, external filter type, with frame.
4. Circular bag filters – The diameter of the external bag filter is φ120mm-160mm, and the diameter of the internal bag filter is φ200mm-300mm or larger. It is the most widely used form of filter bag.

How to choose the right bag filter?

Choosing the correct bag filter depends on different production conditions.

Here are 7 things you need to consider when searching for the right bag filters for your specific needs.

1) Dusty Gas Temperature

The primary criterion for selecting bag filters is the temperature of the dusty gas. The filter bag will burn out and distort if the gas temperature is too high. In order to prevent dew from forming, high temperature gas must be cooled to below the temperature that the filter material can withstand. The gas temperature must be kept above 20°C ( dew point).

2) Inlet Dust Concentration

The concentration of inlet dust is determined by the production process, so it is an important factor next to the treatment air volume

3) Outlet Dust Concentration

This refers to the emission concentration of the bag filter. Outlet dust concentration is mainly determined by the local environmental protection emission requirements and also user requirements.

4) Treatment Air Volume

Treatment air volume is the volume of the gas that can be purified by the deduster in a unit time. This is one of the main considerations when it comes to selecting the right bag filter for your needs.

5) Pressure loss

This is the pressure drop, or resistance, of gas from the inlet to the outlet of the deduster.

6) Operating Pressure

The static pressure value of the microfan before and after the bag filter, as well as its installation position, are used to calculate the working pressure of the bag filter, which is also the bag filter’s design pressure value.

7) Filtration Speed (air-cloth ratio)

The rate at which the filter gas moves through the filter media, also known as the air volume through the filter media to the area of the filter media, is known as the filtration speed. The most important consideration when choosing a dust collector is the filtering speed, which should be made in accordance with the characteristics of the smoke or dust, the application, the size, viscosity, temperature, and water content of the gas, as well as the concentration and type of filter bag material.

The pressure loss will be greater, the effectiveness of dust removal will be lower, and the dust filter bag will become blocked and destroyed earlier if the filtration speed is set too high. However, raising the filtration speed can decrease the amount of filtration space needed by the bag filter, allowing for the use of a smaller filter to handle the same amount of gas.

Where can I buy replacement bag filters online?

You can buy online from our website. We are one of the UK’s leading air filter suppliers, selling Bag Filters and Rigid Bag Filters from our secure online shop.

If you have any questions about selecting the correct bag filter for your needs, then don’t hesitate to give the team a call on 01747 897479.