HOWO 20 ton water and foam fire fighting truck

Structure of Foam Fire Trucks

I. Extinguishing Mechanism of Foam

Foam has a low relative density, good fluidity, strong durability and fire resistance, low thermal conductivity, and high adhesion. These physical properties allow it to quickly cover the surface of burning liquid, isolating flammable vapors, air, and heat transfer. Combined with its cooling effect, it extinguishes fires.

II. Structure and Working Principle of Foam Fire Trucks

Foam fire trucks consist of a truck chassis and specialized equipment on its superstructure. Most foam fire trucks produced domestically and internationally use Class II general-purpose truck chassis provided by vehicle manufacturers. The specialized equipment mainly includes a power take-off (PTO), water-foam liquid tank, equipment box, pump room, fire pump, vacuum pump, foam proportioning device, and fire cannon.

Its working principle is as follows: The PTO outputs power from the truck chassis engine, which drives the fire pump through a transmission system. The fire pump and foam proportioning device mix water and foam in a specific ratio, which is then sprayed out by the fire cannon and foam extinguishing gun to extinguish the fire. Therefore, the truck chassis must meet certain load-bearing capacity and engine power requirements.

2.1 Power Output Device

Most domestically produced foam fire trucks use a main engine power take-off (PTO) system. The PTO can be arranged in various ways, but currently, medium and heavy-duty foam fire trucks mostly use two types: a sandwich-type PTO (front-mounted transmission) where the PTO is placed between the main clutch and the transmission, and a driveshaft-type PTO (rear-mounted transmission). The sandwich-type PTO extracts power from the main engine and transmits it to the water pump via the transmission system, driving the pump and achieving dual-action operation. This type is currently the most common.

2.2 Water-Foam Tank

The water-foam tank is the main container on the foam fire truck for carrying the extinguishing agent. Depending on the development of the fire protection industry, various materials can be used for its construction. In the 1980s and 90s, polyester fiberglass was commonly used; now, high-quality carbon steel and stainless steel are increasingly being used.

2.3 Equipment Box
The equipment box mostly uses a welded steel frame structure, lined with all-aluminum alloy plates or steel plates. In recent years, the internal layout of equipment compartments has been categorized into four types: fixed divider type, where each divider frame is fixed and non-adjustable; movable divider type, where the divider frame is made of aluminum alloy profiles with internal patterned aluminum plates, allowing for adjustable spacing; push-pull drawer type, where the dividers are designed as pull-out drawers for easy access to equipment, but are more complex to manufacture; and rotating rack type, where each divider is made into a small, rotatable equipment compartment, a type commonly used in imported fire trucks. 

2.4 Fire Pumps

Currently, the fire pumps configured in domestic foam fire trucks can be broadly classified into three categories: atmospheric pressure pumps (low-pressure pumps), i.e., single-stage centrifugal pumps, such as BS30, BS60, BS40, R100 (imported), etc.; medium- and low-pressure combined fire pumps, multi-stage centrifugal pumps. Examples of fire pumps include 20110/20140, 20110/30160, 20110/35170, and KSP1000 (imported); high and low pressure pumps include NH20, NH30 (imported), and 40110/6130. Fire pumps can be configured as either centrally located or rear-mounted.

2.5 Pump Room

Like equipment boxes, pump rooms are typically all-rigid frame welded structures. Besides housing the fire pumps, they usually also have space for storing pump-related equipment for firefighters to operate.

2.6 Foam Proportioning Mixer

The foam proportioning mixer is the main equipment in an air foam extinguishing system that draws in and delivers foam concentrate. It mixes water and foam in a specific ratio, typically 3%, 6%, or 9%. Currently, most foam proportioners produced in my country use a 6% foam concentrate ratio. Mixers are generally available in three specifications: PH32, PH48, and PH64. In recent years, many high- and low-pressure pumps and medium- and low-pressure pumps introduced and manufactured in China have adopted ring pump-type air foam proportioning devices, which are integrated with the pump design.