How Pressure Drop Affects Tower Packing Selection
Pressure drop is one of the most important hydraulic factors in tower packing selection.
A packing may provide good mass-transfer surface area, but if it creates excessive resistance to gas flow, the tower can lose capacity, consume more energy or operate too close to loading and flooding.
This is why tower packing should not be selected only by:
surface area
nominal size
price
material
packing type
Pressure drop must also be considered together with:
gas velocity
liquid load
tower diameter
packing size
bed height
fouling tendency
operating pressure
If you are still collecting basic process data, start with:
[Tower Packing Selection Parameters → https://www.pxdaier.com/tower-packing-solutions/tower-packing-selection-parameters]
What Is Pressure Drop in a Packed Tower?
As gas flows upward through a packed bed, it must pass around the individual packing elements and through the available void spaces.
This creates flow resistance.
The resulting pressure difference between the bottom and top of the packed bed is the pressure drop.
Pressure drop is commonly expressed relative to packing height, for example:
Pa/m
mmH₂O/m
mbar/m
The actual value depends strongly on the operating conditions and packing characteristics.
Why Does Pressure Drop Matter?
Excessive pressure drop can create several problems.
These may include:
higher blower or fan power
reduced gas throughput
lower process capacity
increased operating cost
greater risk of loading
reduced flooding margin
poor vacuum performance
In some applications, pressure drop is only one of several design considerations.
In others, it can be a critical limitation.
Examples include:
vacuum distillation
large-volume gas scrubbers
low-pressure gas systems
energy-sensitive processes
tower revamps with limited fan capacity
1. Gas Velocity Has a Major Effect on Pressure Drop
Gas velocity is one of the strongest drivers of packing pressure drop.
At low gas velocity, resistance through the packed bed may be relatively small.
As gas velocity increases, pressure drop also increases.
At still higher velocity, interaction between the upward gas and downward liquid becomes stronger.
Eventually, the tower may approach:
loading
entrainment
flooding
For the same gas flow:
Smaller tower diameter → higher gas velocity
Larger tower diameter → lower gas velocity
This is why tower diameter and pressure drop cannot be evaluated separately.
See:
[How Tower Diameter Affects Tower Packing Selection → https://www.pxdaier.com/tower-packing-solutions/how-tower-diameter-affects-packing-selection]
2. Packing Size Affects Hydraulic Resistance
Packing size also influences pressure drop.
In general, smaller random packing provides:
higher specific surface area
more gas-liquid contact
smaller flow passages
But smaller passages can also create greater hydraulic resistance.
Larger packing generally provides:
larger open channels
lower resistance to gas flow
better solids tolerance
lower pressure drop in many applications
However, larger packing is not automatically the best choice.
It may provide lower surface area per unit volume and may be unsuitable if the tower diameter is too small.
For this reason, packing size should always be evaluated as a trade-off between:
mass-transfer efficiency and hydraulic capacity.
Read more:
[How to Choose Tower Packing Size → https://www.pxdaier.com/tower-packing-solutions/how-to-choose-tower-packing-size]
3. Packing Geometry Matters
Two packing products with the same nominal size can produce different pressure-drop behavior.
For example:
Pall Rings
Raschig Rings
Intalox Saddles
Cascade Mini Rings
IMTP-type packing
have different internal structures.
Their differences may include:
void fraction
open area
surface shape
flow channels
number of contact points
liquid spreading behavior
Therefore, a 50 mm Pall Ring should not automatically be treated as hydraulically identical to every other 50 mm random packing.
When comparing packing options, evaluate:
type + size + material + operating conditions
rather than nominal size alone.
4. Liquid Load Also Increases Pressure Drop
Packed towers normally operate with both gas and liquid flowing through the bed.
As liquid flow increases, more of the packing surface and void space becomes occupied by liquid.
This increases interaction between the phases.
At higher liquid load, gas has less effective open space available.
The result can be:
increased pressure drop
greater liquid holdup
reduced gas capacity
earlier loading
lower flooding margin
This is why dry pressure-drop data cannot fully describe actual tower operation.
Real operating conditions must consider both gas and liquid loads.
5. Wet Pressure Drop vs Dry Pressure Drop
Two different pressure-drop conditions are often discussed.
Dry Pressure Drop
This refers to gas flowing through the packing without liquid irrigation.
It can help indicate the basic hydraulic resistance of the packing.
Wet Pressure Drop
This refers to pressure drop while both gas and liquid are flowing.
Wet pressure drop is usually more relevant to actual tower operation.
Because liquid occupies part of the open space and interacts with the gas, wet pressure drop can be significantly different from dry pressure drop.
Therefore, dry pressure-drop data should not be used alone to predict real packed-tower performance.
6. Bed Height Directly Affects Total Pressure Drop
Pressure drop is often described per unit packing height.
This means a deeper packed bed normally creates a larger total pressure loss.
For example, if two towers use the same packing under similar hydraulic conditions:
Tower A has a 2 m packing bed
Tower B has a 4 m packing bed
Tower B will generally have a higher total pressure drop because the gas travels through a deeper bed.
This becomes especially important in:
vacuum service
tall absorbers
multi-bed towers
retrofit projects
When evaluating pressure drop, always provide the actual packing bed height.
7. Fouling Can Increase Pressure Drop Over Time
A tower may operate with acceptable pressure drop when new but gradually develop problems.
Possible causes include accumulation of:
solids
scale
dust
polymers
crystals
biological growth
corrosion products
Deposits reduce the open passages available for gas and liquid flow.
As blockage develops, pressure drop may increase.
This can eventually result in:
reduced capacity
unstable tower operation
flooding
increased fan power
more frequent shutdowns
Therefore, a packing with slightly lower initial efficiency but larger open passages may provide better long-term performance in dirty service.
8. Material Alone Does Not Determine Pressure Drop
Metal, plastic and ceramic packing can all be used in packed towers.
However, pressure drop depends more strongly on the actual geometry than on the material name itself.
Material still matters because it can affect:
wall thickness
structural design
achievable packing geometry
weight
chemical compatibility
operating temperature
For example, a thin-wall metal packing design may provide a high void fraction.
A plastic packing may use a different wall thickness or structure.
A ceramic packing may have a completely different surface and flow path.
Therefore, hydraulic comparison should be based on the actual product design rather than material category alone.
9. Pressure Drop Is Especially Important in Vacuum Towers
Vacuum systems are particularly sensitive to pressure loss.
In atmospheric scrubbers, a moderate additional pressure drop may be manageable if the fan has sufficient capacity.
In vacuum distillation, however, pressure loss through the packing can affect:
column pressure profile
boiling temperature
separation conditions
energy requirement
process capacity
For vacuum service, low-pressure-drop packing is often a high priority.
Structured packing may also be considered where very low pressure drop and high efficiency are required.
But the final choice depends on the actual process duty.
10. Low Pressure Drop Is Not Always the Only Goal
It may be tempting to choose the packing with the lowest possible pressure drop.
But this can also be misleading.
A packing with very large open passages may provide excellent hydraulic capacity but insufficient mass-transfer performance for the required bed height.
The correct selection should balance:
pressure drop
capacity
surface area
wetting
efficiency
fouling resistance
available tower height
Therefore:
Lowest pressure drop does not automatically mean best packing.
The objective is to achieve the required process duty within a safe hydraulic operating range.
11. Pressure Drop in Existing Tower Revamps
Pressure-drop problems are common reasons for tower revamps.
Before changing packing, identify the actual problem.
Useful questions include:
Has pressure drop increased gradually?
Was pressure drop always high?
Did gas throughput increase?
Did liquid flow increase?
Is fouling visible?
Has the distributor been inspected?
Is the packing damaged or collapsed?
Is the support plate blocked?
Has the process composition changed?
If pressure drop increased gradually, fouling may be the real cause.
If pressure drop was high from initial operation, the original packing or tower sizing may be unsuitable.
Replacing packing without understanding the cause can simply reproduce the same problem.
12. Packing Support and Internals Can Also Add Pressure Drop
Packing itself is not the only source of hydraulic resistance.
Tower internals can also contribute.
These may include:
packing support plates
hold-down grids
liquid distributors
redistributors
collectors
demisters
trays or transition devices
If an internal has insufficient open area, it can become a hydraulic restriction.
This is particularly important when increasing tower capacity during a revamp.
The packing and internals should therefore be reviewed as one system.
Quick Pressure-Drop Screening Logic
If gas velocity is high:
Check:
tower diameter
packing open area
packing size
flooding margin
If pressure drop must be very low:
Consider:
larger or more open packing
low-pressure-drop geometry
bed height
internal open area
If fouling is present:
Check:
blockage
solids
packing passage size
distributor condition
support grid
If pressure drop increased over time:
Do not assume the packing size is wrong.
Investigate fouling and internal blockage first.
What Information Should You Provide?
For preliminary pressure-drop screening, provide:
tower internal diameter
packing type
packing size
packing bed height
gas flow
liquid flow
operating temperature
operating pressure
gas composition
liquid composition
fouling tendency
current pressure drop, if available
For the complete data checklist, see:
[Tower Packing Selection Parameters → https://www.pxdaier.com/tower-packing-solutions/tower-packing-selection-parameters]
Use the DAIER Tower Packing Engineering Assistant
The [DAIER Tower Packing Engineering Assistant → LINK TO TOOL PAGE] can help organize key operating parameters before preliminary tower packing screening.
It is useful when reviewing:
tower diameter
gas velocity
packing size
liquid load
packing volume
replacement projects
If you are new to the tool, see:
[How to Use the DAIER Tower Packing Engineering Assistant → https://www.pxdaier.com/tower-packing-solutions/how-to-use-the-daier-tower-packing-engineering-assistant]
Final pressure drop, flooding capacity and separation performance should still be verified using the actual packing data and project operating conditions.
[Use the DAIER Tower Packing Engineering Assistant → LINK TO TOOL PAGE]
Specs and test data available upon request.