Knorr-Bremse ZB44 Series, Service Manual

6

Introduction

1

1 2 

Basic principles

The ZB44.. range of air processing units combines 
the functions of a conventional air dryer with those of a 
pressure regulator and multi-circuit protection valve in one  

monoblock

 housing.

1 2 1 Features

• Classic air drying functionality
• Multi-circuit protection valve functionality
• Integrated unloader function
• Integral safety function (air dryer)
• Exhaust safety function (Multi-circuit protection valve, 

EC

)

• Two integrated pressure sensors (Optional)
• ESS Compressor control (Optional but recommended)
• Regeneration either via reservoir or by time switch
• High pressure tyre inflation with safety valve (Optional)
• System pressure tyre inflation (Optional)
• Air suspension supply (Optional)
• Heater (Optional)
• Oil separator (Optional)
• Silencer type (Optional)

1 2 2 Functionality

The air processing unit is fitted in the compressor discharge 
line of an air braking system to remove water, dirt, carbon 
and oil from the compressed air and therefore prevent it 
entering the vehicle air reservoirs during the 

Charge

 phase. 

It controls the system pressure (unloader function). It also 
maintains a pre-set pressure in the unaffected circuits of a 
multi circuit braking system in the event of a failure in one 
circuit (multi-circuit protection valve function).

When the increasing air pressure in the braking system 
reaches a preset value (cut-out pressure), the unloader 
valve signals the purge valve to open and the air processing 
unit enters the 

Purge

 phase. This allows the trapped dirt 

and condensate to be expelled and the desiccant to be 
regenerated with clean, dry air from a separate purge 
reservoir or with air from the brake system using an integral 
timer. During this phase, air delivered by the compressor is 
discharged to atmosphere and does not enter the braking 
system. When, due to normal usage, the air pressure in 
the braking system drops, to the closing pressure of the 
unloader valve, it signals the purge valve to close and the 

Charge

 phase begins again.

1 2 3 

Air dryer element

Drying function

During the 

Charge

 phase, compressed air enters the air 

processing unit through port 

1

 and passes through the 

oil separator element to remove particles and oil from the 
air using the cyclotron principle. This helps to maintain 
the quality of the desiccant bed, prolonging its life and 
effectiveness. It continues up the outside of the inner 
desiccant cartridge and then passes down through the 
desiccant bed to remove the water vapour. The now clean, 
dry air opens the delivery check valve, flows through the 
multi-circuit protection valve, and is delivered from ports 

21

, 

22

, 

23

 and 

24

 to charge the vehicle braking system.