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The importance of knowing the dew-point of your airflow to avoid condensation inside of a baghouse

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Introduction

Most baghouse users are confused why their baghouses suffer moisture condensation problems in winter. This blog post tabulates the dew-point temperatures of the airflows with different Relative Humidity (RH) at three different airflow temperatures of 68 oF, 120 oF and 250 oF to show why moisture problems in baghouses or cartridge collectors occur  in winter.

Definitions of dew-point and Relative Humidity

Dew-point is defined as the temperature at which condensation begins when the air is cooled at constant pressure.

Relative Humidity (RH) expresses the moisture content of air as a percent of what it can hold when the air is saturated.

The temperature of the airflow from a process will likely remain fairly constant and warm year round. However, a temperature differential (approximately 15oF or more) between the dust collection point and the dust collector may indicate a potential condensation risk.

Condensation in a baghouse causes problems

When water vapor in airflow condenses to a liquid state in baghouses, it can result in blinded bag filters, rapid dust buildup on the baghouse’s inside walls, bridging across the hopper discharge opening, and metal corrosion inside the baghouse.

If the ambient air temperature surrounding a baghouse is much cooler than dust laden airflow temperature, the baghouse’s operating temperature might be close to or even below the gas dew point temperature.

In some areas of North America, winter temperatures can drop even below 0 oF. If a dust collector is located outside in those conditions, the temperature of the compressed air, if not heated, in the pulse-jet cleaning system will tend to closely mirror the ambient temperature. Especially for medium pressure-medium volume cleaning systems, the PD blower units are normally installed just beside the baghouses. The temperature of the compressed air in this kind of cleaning system mirrors the ambient temperature too.

When the temperatures anywhere on the walls or inside of the baghouse drop below the dew-point temperature of the airflow, moisture condensation is inevitable. 

Airflow Dew-point calculations

Airvate did some calculations to decide the dew-points of airflows with different Relative Humidity (RH) at three process temperatures: 68o F, 120o F, and 250o F.

The airflow starts from a hood inlet, at which the pressure is 0” wg. Airflow temperature and its Relative Humidity are measured here. The RH values are listed in the first column of the tables, while the dew-points calculated correspondingly are in the second column.

 Through ductwork, the airflow flows to a baghouse. There are two cases here:

Case 1, the baghouse is running under negative pressure, the static pressure at its inlet is assumed to be -6” wg, and the fan is located behind the baghouse.

Case 2, under positive pressure, the static pressure at its inlet is assumed to be 12”wg. The fan is located in front of the baghouse.

For case 1, it is assumed that the temperature of the airflow doesn’t change from the hood inlet to the inlet of the baghouse, while the pressure changes from 0” wg to -6”wg. The RH and Dewpoint values at this point were calculated and are listed in the third and fourth columns respectively.

For case 2, it is assumed the total pressure caused by the fan is 16”wg. Since the fan does work on the airflow, the airflow temperature is increased by about 7 oF. At the inlet of the baghouse, the temperature of the airflow is changed from the temperature at the hood inlet to that plus 7 oF, while the pressure changes from 0” wg to 12”wg.  RH and Dewpoint values at this point were calculated and are listed in the fifth and sixth columns respectively.

How to read the tables

For example, the measured values of temperature and RH of the airflow at the hood inlet are 68oF and 50% respectively. The calculated dewpoint of the airflow is 48.7oF based on the measured conditions.

After the airflow travels to the inlet of a baghouse, airflow conditions change.

In case 1, airflow temperature stays constant, while the airflow pressure changes from 0”wg to -6”wg, so the dewpoint and RH of the airflow also change correspondingly, and the calculated values are 48.3 oF and 49.3% respectively.

In case 2, airflow temperature was increased from 68oF to 75oF (68oF + 7oF) , while the airflow pressure changes from 0”wg to 12”wg, so the dewpoint and RH of the airflow also change correspondingly, and the calculated values are 49.5oF and 40.3% respectively.

An engineer cares about the conditions of the airflow that will enter the baghouse.

Airflow dew-point calculation at Temperature 68F

Airfow dew-point calculation at Temperature 120F



Airflow dew-point calculation at Temperature 250F


Per the calculation results, these conclusions can be drawn:

  1. Knowing the dew point of your airflow is necessary to avoid condensation in the dust collection system.
  2. When the baghouse dust collection system runs under negative pressure, it makes the condensation harder to occur, since the dewpoint becomes slightly lower, though negligible (compare column 2 and column 4), while under positive pressure, condensation occurs slightly more frequently, since the dewpoint becomes a little bit higher (compare column 2 and column 6).
  3. When the temperature of process airflow is low, like the example temperature of 68oF in the calculation, keeping the operating temperature of the baghouse 20oF or more above the dew point temperature of the airflow is good enough to avoid condensation in the baghouse. However, when the temperature of process airflow is high, like the example temperature of 250oF in the calculation, the operating temperature of the baghouse should always be 50oF or more higher than the dew point temperature of the airflow.

Condensation example in a baghouse

Here is one example of moisture condensation inside of a baghouse; the pictures were taken in the month of January:

Picture 1 was taken at the walk-in door on the clean side: liquid water is almost everywhere, and the bags were wet too.

Picture 1

Picture 2 was taken at the access door on the hopper: collected dust inside was wet and visible ice was around the door rim.
Picture 2

Picture 3 was taken inside of the clean side, above the tube sheet. The tubesheet is rusting.
Picture 3

Picture 4 shows the blinded bags that had to be pulled out and replaced with new ones.

Picture 4

Some suggestions about applying a baghouse filtering dust laden airflow with moisture:

  1. Preventative action to keep condensation from developing can take the form of insulation of the housing or additional heating elements on the exterior of the hoppers.
  2. Some environments even require heating of the compressed air used in pulse cleaning to prevent the collector from passing through a dew point. This is because of the chilling effect from expanding compressed air released during each pulse.


Other sources of moisture entering a baghouse

Other than moisture in the airflow, other common sources of condensation and moisture in a baghouse are leaking gaskets around the doors and airlocks or upset conditions in the process. 

Humidity can also be introduced in the dust collector in the compressed air.  This is why it is important to dry compressed air.

Thermal calculation services by Airvate

     1.  Solve condensation problem in a baghouse or other dust collectors
2. Dew point temperature and other psychrometric property (saturated vapor pressure, partial vapor pressure, relative humidity, specific humidity, mixing ratio, enthalpy) calculations
3. Thermal calculations for bleeding-in ambient air to cool down the process airflow to protect a baghouse
4. Thermal calculation for heated-up bleeding-in air to mix with the process airflow to avoid condensation
5. Insulation and/or heating elements calculation for baghouse, cyclone separator, and ductwork 

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