Crew Cooling in Armoured Vehicles

Introduction

The current generation of armoured vehicles has been derived along a line of development that has constantly sought greater mobility, improved protection and more sophisticated weapons and detection systems. The resulting vehicle has a very powerful engine enclosed in many tons of metal, with weapon and detection systems consuming most of the current that the powerful on-board generating system can provide. After being in the theatre of operation for more than a few days, the large mass of metal acquires a bulk temperature, which at the climatic extremes of most design specifications will result in a crew-space environment of at least 65°C. This extreme is many hundreds of kilowatt-hours of heat movement outside of the relatively small environmental envelope in which the crew can survive or operate effectively. Added to this problem are several other thermal inputs, including waste heat from the engine compartment, a constant supply of fresh air (possibly through an NBC filtration pack), and a significant amount of waste heat from electrical or hydraulic systems.

The necessary crew environment

Within the potential temperature extremes of the crew-space environment, the crewman must be able to operate the vehicle and its systems effectively, or at the very least, if he is simply a passenger in an APC, he must be capable of fighting at the end of the journey. Regardless of race or colour, the acclimatized crewman's normal blood (or deep-body) temperature is 37°C. Within the space of one degree above or below that norm, the crewman will begin to lose concentration and accuracy in the task at hand, as his mind begins to become concerned with his discomfort. From one to two degrees away from the ideal blood temperature, operational effectiveness will tend towards zero, while three degrees of divergence will leave the crewman approaching total incapacity and probable death. The ideal environmental condition to maintain the crewman at both peaks of comfort and performance is thus one which will maintain his blood temperature at 37°C.

Body cooling mechanisms

The in-built human mechanisms to achieve control of deep-body temperature, arise from the conversion of food into energy, which results in a steady heat gain within the body. This heat is initially dissipated through conduction from the skin into the surrounding ambient. At rest, the crewman needs to dissipate around 150 W, for which the skin-to-air heat exchange function operates with a heat gradient of around 4°C - meaning that the target 37°C deep-body temperature, can be maintained with the skin at about 33°C. When exerting effort, more cooling is required, possibly up to several times the 150 W needed at rest; this has the effect of increasing the necessary skin-to-air temperature difference similarly, which quickly becomes impractical. Under such stress, a second phase of blood cooling commences, where the skin exudes perspiration, and evaporation of the water content provides a much more powerful heat removal action. This effect however, is limited by the local humidity, resulting in a maximum acceptable Wet-Bulb Temperature for the human animal of little more than 35°C. Fortunately, this maximum is rarely exceeded as a natural ambient, but very positive steps are necessary to limit the crew ambient in armoured vehicles well below such extremes.

Ideal crew environment.

Figure 1 illustrates ideal Crew Enclosure Conditions. This chart shows a required Operator Enclosure Environment for agricultural and forestry vehicle cabs [1], which are also ideal for the case under review. The relatively small “desired zone” is defined between 25°C dry-bulb, with a wet-bulb temperature between 17 and 22°C (indicating a Relative Humidity (RH) of 44% to 77%), and 27°C dry-bulb, with a wet-bulb temperature between 13 and 19°C (15% to 46% RH). These conditions are chosen as ideal, to enable the operator’s body to comfortably achieve temperature regulation without need of perspiration, while operating the vehicle.

Environment heat inputs

Within two or three days of arrival into any climate, the mass of the vehicle will normalize to its surroundings, and take up the average temperature. As evening approaches, the closed down vehicle interior will begin to cool in the falling ambient, lagging by an hour or two. At the coldest hour of the night, the interior will still be several degrees warmer than outside due to the time lag. It will begin to warm as morning arrives. As soon as the sun rises, the outer surface of the vehicle will begin to absorb the infrared radiation and gain temperature more quickly than the local ambient, passing the heat to the interior, until the midday peak is reached. The time lag will again occur before the interior temperature starts to fall during the afternoon, and if still closed down, the cycle will repeat. As a result of external ambient temperature changes, the vehicle interior will cycle from some degrees above the local maximum, to a few degrees above the minimum.

Solar radiation effects

In warm and hot climates, solar radiation will provide from 500 to 1100 Wm-2 to the upper surfaces of the vehicle [2] (for most vehicles, from 5 to 8 m2 of plan area above the crew space). At a noonday peak, the crew compartment roof can have a thermal input of from 2.5 to 8.8 kW, resulting in temperature gain of the roof structure from 10 to 25°C over ambient, depending on paint finish and colour, which of military necessity will exclude beneficial highly reflective or white paint. In spite of the insulating effects of anti-spall linings or crash padding, this input will raise the temperature of the closed vehicle interior by 5 degrees as a minimum, and by up to 15 or 20 degrees at worst. In a sunny ambient of 50 to 55°C, the closed down crew space can be expected to reach 65 or 70°C.

Engine bay effects

In order to maximize the performance of the engine cooling system within a minimum space envelope at high ambient temperatures, it is very usual for the cooling elements of the radiator, intercooler and oil coolers to be placed in an incoming ambient airflow, which then flow across the engine and transmission bays before being ejected into the environment at around 90 to 100°C. After operating for an hour or two, the vehicle engine can be expected to heat the crew-to-powerplant bulkhead to at least 80°C, demanding effective insulation if several more kilowatts of power are to be excluded from the crew space.

Fighting compartment auxiliaries effects

The crew space is surrounded with auxiliary systems for fire control, communications and the operation of the turret itself including gun elevation and stabilization, rotation and possibly ammunition handling, all of which consume power at varying efficiencies, with the inevitable result that several more kilowatts are released into the crew environment. (Note that this heat gain is virtually independent of local ambient temperature.)

NBC collective protection effects

NBC collective protection equipment, being safety critical, is usually located in a dedicated compartment. It is common for the equipment to be mounted to a hinged, armoured cover so that the complete system can be swung clear of the vehicle for maintenance access. This has the effect of turning the NBC structure into an extended secondary surface heat exchanger into which heat is conducted from the vehicle skin, with the result that the system mass tends to attain the external skin temperature. When ambient air is drawn through the system in climates with high solar gain, that air becomes heated by at least 10°C. Further, the fan necessary to move the air through the filters and distribution system, heats the air flow by adding it’s power into the equation, with the result that the system adds to the vehicle heat input by up to 2 kW.

Ambient heat effects

A final source of heat input can arise from the external ambient, around areas of the vehicle hull not exposed to solar heating, such as the floor of the hull, and the hull walls under the tracks and behind the wheels. If the hull interior temperature is below the external ambient, heat will flow into the vehicle at a rate dependent on the temperature difference. For simplicity in this exercise, the effects are ignored.

Summary of heat effects

Below are summarized typical major continuous heat inputs into the crew-space interior of an armoured vehicle, in ambient temperatures of 20 to 25°C, and 50 to 55°C:

Ambient: 20/25°C 50/55°C

Heat input from:

Solar radiation 2 kW 8 kW

Powerplant 1 kW 2 kW

Auxiliaries 4 kW 4 kW

NBC system 1 kW 2 kW

Crew heat output 1 kW 1 kW

Ambient effects (ignored) - -

Total input approximation: 9 kW 17 kW

Crew space cooling by ventilation

During training and in routine vehicle movement under non-combat conditions, it is normal for armoured vehicles to be used with all hatches open to provide both unrestricted vision and a plentiful supply of fresh air ventilation. In a climate of 20/25, the crew space can be maintained within the target environment zone with a ventilation flow of 100 to 150 ls-1; the heat input of 8 kW is removed by the ventilation air being heated by around 5°C while flowing through the vehicle. It can be seen however, that the ideal environmental envelope is only just achieved, and an increase in ambient of only 5°C will result in that ideal zone being exceeded; the beginnings of discomfort and the associated reductions in human performance will become measurable. An increase in ventilation flow could be achieved by use of a powered distribution system, but even this would limit use to relatively low ambient temperatures with low humidity. Simple ventilation cannot achieve the target environment below 28°C, with humidity above 40% RH.

However, the need to close down the vehicle for NBC operation with the minimal airflow necessary to achieve acceptable system filter life, eliminates any possibility of ventilation methodsto control the crew environment in any warm climate.

Powered crew cooling

There are three essential elements in a Powered Crew Cooling System:

• A cooling module, capable of adequately cooling in the maximum specified ambient temperatures, a suitable volume of re-circulated air into which can be mixed a supply of fresh air from an inlet filter or NBC pack
• An air distribution system, able to carry the cooled air to each crew station without an excessive amount of heat gain, and to deliver that air at the crewman so that a mini-climate is formed around him, able to control his deep-body temperature.
• A control system, able to regulate the air temperature around the ideal envelope, and prevent excessively cold air from being generated in climates lower than maximum, and a means of controlling airflow volume to a comfortable level.

The cooling module

The cooling module will contain essentially a means of circulating the cooled air, with enough force to overcome the resistance of the cooling and distribution systems, and a means of extracting heat from the processed air. There are two main methods for achieving this:

• The Vapour Cycle, in which a refrigerant gas is compressed into a hot, high pressure form, which is then cooled into a high pressure liquid by a condenser heat exchanger, losing heat into the ambient air being passed over it. The liquid gas is then piped to the evaporator, where it is suddenly dropped in pressure through a control valve, to become a very cold vapour that can be passed through the cooling coil in the re-circulating airflow. Heat is extracted from the airflow into the gas, which is finally returned to the compressor to restart the cycle.
• The Air Cycle, in which the re-circulating air itself is turbine-compressed into a hot, high pressure form which is cooled through a radiator to become warm high pressure air. This air is expanded by making it drive a turbine wheel, and is then exhausted as the cold, cooling airflow to be distributed to the crew stations.

Of the two systems, the Vapour Cycle is based on simple, low technology automotive industry components, manufactured in bulk and hence relatively low in cost. The system is also low in energy cost, providing about 2 kW of cooling for each 1 kW of input power. The Air Cycle system on the other hand, needs use of relatively high technology turbine machinery to be effective, which implies high cost. The essential elements of the system may already exist on the vehicle if either of the main or auxiliary engines are gas-turbines, in which case the Air Cycle becomes the favoured candidate for crew cooling. The energy cost of the Air Cycle system is nearer to 1 kW of energy for each 1 kW of cooling.

Whichever system is used, it will be shown that from 6 kW to 10 kW of cooling will usually be required, to deal with an air circulation rate of up to 150 or 200 ls-1, including an input of fresh air from the open hatches, air filter or NBC pack of 50 to 100 ls-1. The cooled air should be delivered into the distribution system at around 16°C, allowing for a few degrees of heat gain in passing round the hot vehicle, before it is discharged at the crew not too much below a comfortable 20°C. Note here, that if the air is cooled to, say, 16°C, the wet bulb temperature cannot exceed that figure; in humid conditions, the air will be produced at 16°C dry and wet bulb, indicating 100% RH. All surplus moisture received into the air processor will be drained away for collection or loss as distilled water condensate.

The air distribution system

The air distribution system, though simple in concept, presents several challenges to the designer.

• First, it must carry the cooled air at full volume with minimal airflow resistance around the vehicle, to arrive at a location close enough to the crew station to discharge air at the crewman so that the main flow is directed at his lower face and upper torso. The air is best issued out at between 18 and 20C, drawing in only a little local air to arrive at the target area at 20 to 22C. Spill over around the crewman should then provide the major part of his body with an envelope of protection below 28C, whatever conditions in the hull may be. Outlets should be adjustable for flow direction.
• Second, the distribution ducts must limit the amount of wild heat pick up from the hull and the local environment, to avoid dissipating the cooling effect.
• Third and with more difficulty (the more so in retrofit applications), the ducting must avoid all the other systems installed around the vehicle, and not reduce vital headroom in access areas, while achieving all other objectives.
• Finally, for turreted vehicles, a means must be found to supply the two or three men in the turret with cooled air from the hull while meeting the above requirements, or alternatively, to supply the driver and any other hull-based crewman from a turret mounted cooling system.

Each crewman should ideally receive 15 to 20 ls-1 of cooled air, while the driver (and also the loader in an MBT) should receive up to double that amount to combat the amount of physical exertion inherent in his duty. Passengers in an APC configuration can tolerate a reduced flow of 10 to 15 ls-1, since they are close together and can share common envelopes.

Experience has shown that the re-circulating air can be expected to return to the cooler some 5°C lower than the outside ambient temperature, given a reasonable level of hull insulation, crash padding or anti-spall lining. As a result, a wide range of air temperatures will exist in the hull, with a very complex pattern of isotherms. For example, in an MBT such as Challenger, in an midday ambient of 41°C, 63% RH, the following typical temperatures would be found:

• 20C at the cool air discharge;
• 28C around the crewman;
• 36C at the re-circulation inlet;
• 50C out of the NBC pack;
• 60C in stagnant corners around the roof; and
• 70C adjacent to the engine bulkhead.

For this example, assuming an NBC flow of 75 ls-1 into a total cooling flow of 150 ls-1, the heat extraction from the NBC flow would be just over 6 kW, and from the rest of the air re-circulated, just under 2 kW, to give a total of 8 kW. A mean temperature inside the vehicle would be very difficult to calculate, and virtually impossible to measure. Note that no effort has been made to cool the whole of the vehicle interior, since such an action would immediately start to draw unlimited heat loads out of the hull walls, which in turn would attract further inputs from the ambient.

The control system

The control system need not be complicated, and needs little more than an on/off switch, basic control of the circulation fan speed, and an air thermostat either in the outlet or return air stream. Design of the cooling heat exchangers and the coolant flow rates should achieve the required air temperatures at maximum duty, and the air thermostat can then cut the system on or off to prevent over cooling in lesser climates. Complex electronic controls are best avoided for reasons of simple maintenance, and elimination of RFI or EMP problems.

Significant engineering problem areas

Most of the problems associated with engineering a Crew Cooling System into an armoured fighting vehicle, are perfectly normal to the design engineers in the relevant disciplines, and acceptable solutions to the requirements of space envelope, weight, reliability, durability and maintainability can be achieved. Two major problems exist though, to tease the system conceptual engineer: System Power Supply, and Turret to Hull Rotary Duct Joint

System power supply

Ignoring the relatively minor power requirements of the system auxiliaries such as fans and control circuits, a significant amount of power is needed to drive the compressor in the vapour cycle system with up to 5 or 6 kW. This level of power cannot be taken from the batteries (as also with the NBC system), and is beyond current limits for electric motors using the vehicle 28 V DC electrical system, even if an engine (main or APU) is running. Driving the compressor mechanically from the main or auxiliary engine is not difficult, but which engine should be used? A solution that fixes the gas system in the hull is ideal for an APC or IFV vehicle. In an MBT, most of the crew stations revolve with the turret, and rotary joints for the gas pipes, or for the bulk of the cooled air duct flow, are very difficult to achieve. The location of the NBC pack for fresh air supply into the air handler is also very relevant, and if this happens to be turret mounted, it becomes essential to mount the whole cooling pack on the turret, in which case the power supply presents a major difficulty. Solutions so far used have included a dedicated turret mounted diesel engine, or an hydraulic motor taking power out of the turret rotation and gun elevation circuit only when spare capacity is available. For the Air Cycle system, the air handler must stay of necessity with the parent gas turbine, this further influencing the location of the NBC pack.

Turret to hull rotary duct joint

The design of a rotary duct joint in the air distribution system of an MBT, while not in itself a problem, is made difficult by the need for a very small space envelope in an already crowded site, since it is usual that the joint has to be around the electrical slip-ring connection into the turret. This is sometimes even further complicated by rotary hydraulic connections. The site, on the floor at the turret centre of rotation, will usually obstruct the gun breech at maximum elevation, and available space will be further restricted; the introduction of such a feature in retrofit installations may well be impossible. This difficulty becomes of key importance at the concept stage of design, since it may preclude a hull installation for the sole reason that it is impossible to arrange an effective air delivery system to most of the crew in the turret. For a turret mounted cooling system, only the drivers air supply has to pass through the joint, which can thus be smaller in dimensions. In the final event, it may become necessary to eliminate the rotary joint, and supply the drivers bay from turret basket mounted ducts blowing from behind the driver at intervals of 90 or 180 degrees of rotation.

For smaller turrets, such as those of an IFV vehicle, the problem is much reduced, since air can be blown into the turret from several outlets positioned below the turret ring.

Conclusion

The inside of an armoured vehicle is a most inhospitable environment for the human body, so much so that it is surprising that relatively few modern AFVs have been developed to include an effective Crew Cooling System. It is not possible for a crew to operate an AFV in even a warm climate, closed down under NBC conditions, and survive for more than an hour. Even without NBC restrictions, it is difficult to imagine survival of the crew in a midday desert battle, without heat stress becoming a deciding factor in the battle outcome.

References

Author

John Bridger has recently retired from Gallay Limited, of Wellingborough, where he was Technical Director from 1982 to 1997. He now acts as a Consultant in Heat Management.