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Great air-conditioning on board: Buschjost valves are the backbone of interior ventilation

Air-conditioning systems are now considered as indispensable standard features of modern buses because passengers react very sensitively to uncomfortable temperatures, draughts or stale air. As a result the demands upon air-conditioning systems are correspondingly high. Buschjost has developed a drive and motor control valves specially for controlling ventilation flaps and temperatures with the goal of achieving higher levels of travel comfort.

Passengers expect pleasant temperatures and an appropriate climate in a bus. In order to create this optimum climate, the German association of transport operators, the Verband Deutscher Verkehrsunternehmen (VDV), recommends that cooling systems should be capable of achieving a temperature for the passenger compartment of a public transport vehicle of 3 Kelvin below the outdoor temperature. In addition there should be a separately controllable area for the driver. In the view of the VDV, the temperatures to be achieved during the cooler seasons in the passenger compartment and the driver’s working area should be 18 °C and 18 to 25 °C respectively. Similar recommendations apply to comfort in coaches. Furthermore the whole of the interior of the bus should have as homogenous a temperature field as possible with a tolerance of up to ±2 Kelvin.
It is particularly important that the heat and cold energy generated by the air-conditioning system must be evenly distributed in order to achieve a homogeneous temperature distribution in the bus interior. Buschjost motor control valves and servomotors for ventilation flaps have been specially designed for commercial vehicle air-conditioning systems.

Buschjost has developed a new servomotor (9663.02400) which allows ventilation and fresh air flaps to be positioned with high precision and kept in that position. On this DC gear servomotor the feedback potentiometer for precisely determining the position is placed directly on the drive axis. The high torque drives are fitted with a high-precision gear and designed for rotary movements of up to 180 °. Because of their high torque they are also optimally suitable for the precise control of large ventilation flaps. They operate very quietly with a starting torque of up to 350 Ncm and a holding torque of up to 15 Nm in all installed positions.

In recent years the demand for comfort energy in buses has grown tremendously. The combination of low gear play and a highly precise feedback potentiometer leads to a smaller need for readjustment and therefore to a reduced energy demand. Only the infinite adjustment movement requires energy. As soon as the ventilation flaps have reached the desired position they are held exactly in this position without further power consuming.

The ventilation flap drives operate using the same control electronics as the motor control valves used in the water and cooling circuits in order to make the design of the ventilation control in bus air-conditioning systems simpler and more cost-effective. In practical terms this means that only one control is necessary for the ventilation flaps and the motor control valves. There is no permanent demand for electrical energy such as with pulse valves nor is an additional, expensive pneumatic form of energy for actuation required as with pneumatic valves.

The long-life drives are manufactured to protection class to IP 69. Gold contacts, which provide a high level of contact reliability, are used to ensure trouble-free connections to the power supply and the control system, even in climatically difficult environments.

Motor control valves are the backbone of every heating and air-conditioning system in commercial vehicles. They form the interface between the cooling water circuit and the interior compartment heating system or air-conditioning system. The demand profile requires a light but very robust and durable construction that will function under the severe environmental conditions inside the engine compartment and cope with temperatures up to 110 °C. In addition the design must be very compact yet easy to service as the space for installation in vehicles is very restricted.

In the development of the valve housing, Buschjost opted for a resistant plastic that could be processed in large numbers using modern injection moulding techniques. Compared with units made from non-ferrous metals, this technology is clearly superior and more energy-neutral.

There are two different types of motor control valves available. One is rotary valves and the other ball valves. The latter are manufactured as 2- or 3-way valves. With rotary valves, one control disc continuously opens two flow passages. The matched geometry of the disc pairs creates a quasi-linear flow rate characteristic. Driven by the above described powerful drives and benefiting from their contamination-tolerant ceramic discs, the valves close absolutely tightly. The infinitely positionable drive holds the throttle cross-section constant with no power consumption.

In the 3-way motor ball valves, the special passage geometry of the ball results in a flow rate characteristic that ensures maximum flow, even in the mixing mode. The valves are suitable for use with many kinds of fluids such as water or other coolants and for a temperature range of -30 °C to 110 °C within a pressure range of -0.5 to 2.5 bar. They have been designed strictly in accordance with the relevant standards and comply with the new regulations governing EMC immunity contained in EN 61 00-6-2.

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