Air-conditioned cabs: warm feet, cool head
As far as air conditioning is concerned, the most advanced tractor cabs are on par with the most modern cars. And you can stay warm (or cool) even with the engine off, thanks to the installation of additional passenger compartment autonomous heating/cooling units, with considerable economic and energy advantages. New devices provide operator protection at all stages of work
At first they were simple shelters against the weather. Then came closed cabs. Nowadays, maintaining the highest level of operator comfort in the driving seat of the self-propelled agricultural machines is a must (primarily tractors), in practice a good selling point for models that are not quite the market leaders.
The ergonomic optimization of the passenger cabs of tractors (as well as combine harvesters, vertical mixers, sprayers and self-propelled harvesters, etc.). brings together many factors: ease and safety of access to and exit from the cab; interior space; visibility towards the front and of the equipment connected to the 3-point rear attachment; arrangement, the identification and force to operate the controls; vibrational and noise levels; range of accessories ... There is no doubt, however, that the microclimate has historically been the first object of attention by designers, taking into account that the related stress can heavily influence the performance of the driver, as well as threaten his health, in particularly serious cases.
Ensuring the best microclimate in the tractor is not exactly simple, considering that the machine can work in extreme temperatures from approximately -20 to +40 °C. Due to the very large glazed surface area and the relatively small volume the cab is subject to an intense greenhouse effect (with hot weather) and an equally fast loss of thermal energy (in case of low temperatures).
Air or water
From the point of view of heat transfer efficiency, it is well known that air and water do not offer the same performance. In general, both fluids are used for heating, one in the gaseous state and the other as a liquid. To counter the problems caused by a harsh climate, it involves both advantages and disadvantages. On vehicles, the air heaters provide a comfortable temperature in the driver cab and an effective defrosting of the glazing (especially the windshield) before starting the vehicle engine. The air is channeled directly into the cab, with a thermal power generally between 1 and 2 kW.
On the other hand, in addition to bringing the cab to an ideal temperature, water heaters can improve the performance of the internal combustion engine in the early stages of operation, avoiding risky cold starts and reducing the thermal stress on the mechanical parts. These circuits are integrated into the main liquid cooling system of the vehicle. Among other things, with typical thermal powers of 2-4 kW, they not only heat the engine structure, but also the fuel, the service oils and the lubricants.
The evaporative air conditioner
The most common physical principle adopted for conditioning the climate of a closed environment is that of the transition of a gas from the liquid state to a gas and vice versa. An alternative to this solution, both economical and eco-friendly, is to cool the air using the principle of water evaporation, i.e. the natural mechanism that regulates the temperature of the human body, creating a condition of well-being directly on the person, allowing the proper evaporation of moisture present on the skin, and therefore removing heat from the body. Evaporative air conditioners operate on this principle, using the evaporation of water to cool the air. The technology on which evaporative air conditioners are based is cheaper (using up to 80% less than the traditional solution) and also eco-friendly because it does not use gases, oils, or other chemical agents.
The feeling of climatic well-being perceived by the human being is influenced by four factors: the ambient air temperature, the relative humidity (i.e. the ratio between the water vapour contained in an air mass and the maximum that it could contain in saturation conditions), the ventilation, and the moisture density. The evaporative air conditioner acts on all of these factors: on the surface of the filter, the air is deprived of part of the water vapour (RH 40-60%) and consequently it cools. It is then fed into the cab with a very low density (with tiny nebulized droplets of Æ 0.002 µM), which allows the distribution of water over a larger area, promoting a more rapid evaporation, further facilitated by forced ventilation.
The air circulation
The climate in the passenger cab of the tractor is controlled by a forced flow of air, which must be carefully directed and distributed by a series of vents, located in different areas of the cab. In addition, in the case of a cold and/or wet climate, the same air must prevent the formation of condensation on the glazing and provide defrosting, in case any ice is deposited on the outer surface of the doors and windows.
It is therefore necessary that the vents are carefully positioned, promoting a surface channeling on the windscreen, which is the most important glazed surface in terms of visibility. In addition, for the greatest operator comfort, it would be necessary to differentiate the temperature of the air channeled into the high and low areas of the cab, pursuing the old, but still valid saying "cool head, warm feet".
Nowadays, even on agricultural tractors this is a reality, since the more advanced models allow the adjustment, in some cases even with digital controls, of the temperature, the magnitude of the flow and the orientation of the air based on the needs.
The pressurization
The physical principle is well known in meteorology: moist air masses move from areas of high pressure to those of low pressure. Therefore, if you want to avoid unwanted pollution of the air quality in a given environment, it is necessary to confine it and keep it at a pressure higher than the outside environment. This condition is called pressurization. It is the same trick (albeit adopted for slightly different purposes) that is activated in aircraft cabs, to allow us to breathe normally during a flight at more than 10,000 m of altitude. In tractor cabs, pressurization is necessary, or at least highly recommended, when making pesticide treatments, because the edges of the doors and windows cannot be airtight. This is the only reasonably practicable solution to ensure that the air entering the passenger compartment, which is the one creating the pressure, is not polluted, because it passes through activated carbon filters. Fortunately, the desired effect is achieved with minimal overpressure, in the order of a few millibars. The relevant regulations specify that the ventilation/air conditioning system should be able to provide at least 0.5 mbar (50 Pa) of positive differential between the inside and the outside of the cab. If, however, there is a pressurization control circuit, which can report any anomalies, the minimum limit is reduced in derogation to 0.2 mbar (20 Pascal), thus avoiding the oversizing of the fan that creates the air flow and stressing the efficiency of the activated carbon filters. For this purpose, Masi Conditioners Molinella (BO) has devised Polaris, a closed cab pressurization control unit, characterized by a high sensitivity of 0.002 mbar (0.2 Pa). With a LED traffic light visualizer it is able to monitor the pressure differential, indicating acoustically and visually any malfunctions.
Engine off, heating (or cooling) on
In cold climates, the running engine usually ensures a viable source of heat to maintain a comfortable temperature inside the cab. Problems arise in the early stages of use of the machine, when the internal combustion engine has not yet reached its operating temperature, and in case of long work interruptions (that is, if the engine is turned off) such as to cool down the cabin quickly and significantly. In this case, an additional heat source must be provided, with sufficient potential to quickly restore an acceptable microclimate.
Webasto-Diavia has developed an integrated solution for heating/cooling of work vehicles (agricultural tractors included) with the engine off, capable of maintaining a constant cab temperature of about 21 °C, regardless of the weather outside. The application is also able to keep the engine warm when it is turned off (or to warm it up if it is cold), so as to reduce the thermal stress that it would undergo in case of very low start-up temperatures.
The advantage in economic terms is much more tangible if the typical use of the machine provides for frequent and/or prolonged breaks. Webasto-Diavia has calculated that the working time during which the engine of different vehicles is kept idling, maybe just to keep the cab warm (or cool), ranges between 3 and 60% of the total. It concluded that the savings achieved by switching off the combustion engine and activating the system described could reach 7,000 euro/year per vehicle.
