Designing aerospace and defense air conditioning systems for extreme environments

Aerospace and defense air conditioning systems and environmental control units (ECUs) are deployed in some of the most extreme climates:

  • Blazing deserts, where sand and record-setting temperatures threaten the integrity of the system.
  • Arctic tundras, where temperatures drop well below 0°F for long stretches of time.
  • Coastal bases, where corrosive seawater can wear down vital equipment.

But regardless of the climate, the mission of the ECU remains the same: Cool the sensitive equipment so defense systems, UAVs and spacecrafts remain operational and can perform critical tasks such as space travel, surveillance and threat detection. If not properly designed and integrated with the overall system, inadequate thermal management can risk the entire mission or operation.

And with launch dates, intelligence gathering and millions of dollars on the line, ECU failure isn’t an option.

In this article, we’ll discuss the importance of MIL-STD certifications and ruggedization for these challenging locations. By following these engineering recommendations, you’ll ensure consistent temperature and air pressure while protecting vital equipment and ensuring the safety and comfort of aircraft personnel.

The difference a MIL-STD certified provider can make

Before getting into engineering specifics for particular environments, it’s important to discuss Department of Defense military standards (MIL-STD), or informally known as “MilSpec”. In addition to internal quality control measures, ECU providers can choose to adhere to MIL-STD when engineering their system.

MIL-STD-810, which addresses environmental stressors on the systems’ materials, can generate confidence in the overall durability of the material system design. But not all MilSpec equipment is created equal. An important distinction is noting whether an ECU provider simply “designs to meet” the standard, or is actually certified with MIL-STD-810.

For example, a provider may design and build an ECU for a defense application to be used in a hot desert location. They will follow the guidelines for that particular standard (Method 510.4 for Sand and Dust, Method 501.4 for High Temperature). But a provider that is MIL-STD-810 certified will build following those guidelines and conduct testing (either in-house or with a third party) to ensure the product will actually stand up to the high heat and abrasive sand.

For the most reliable product in an extreme environment, seek out a provider that has gone through the (very strict) certification process. The result is a more reliable ECU, proven to withstand the demands of your environment.

Relevant MilSpec standards for extreme aerospace environments include:

  • MIL-STD-810F, Method 514.5, Figure 514.5C-3, Table 514.5C-V11 (Vibration)
  • MIL-STD-810F, Method 510.4, Procedure 1 and 2 (Sand and Dust)
  • MIL-STD-810F, Method 506.4, Procedure 3 (Dripping Rain)
  • MIL-STD-810F, Method 509.4 (Salt Fog)
  • MIL-STD-810F, Method 501.4 (High Temperature)
  • MIL-STD-810F, Method 502.4 (Low Temperature)

A certified manufacturer will help you to determine which MilSpec standards are relevant for the application, and also whether to ruggedize your unit.

When building a ruggedized unit, your ECU provider will reinforce piping, mounting, fans, compressors and other systems and utilize material such as flexible rubber hoses for vibration absorption. This will ensure the system can sustain prolonged exposure to heavy vibration during transportation and withstand your particular harsh environment.

A note on IP-rated components

A qualified ECU provider will guarantee that all components within the AC system can handle the environment. Major components and subassemblies (such as compressors, power and cable assemblies, fans and enclosures for electrical control components) should be rated at a minimum of IP54 to withstand continuous exposure to the elements without any harmful effects.

How to cool in extreme hot environments

extreme hot environments

Air conditioning systems that must cool ambient air ranging from 120°F to 150°F are considered extreme hot environments. In addition to meeting the high temperature MilSpec (Method 501.4), your ECU provider should follow these engineering and design measures.

Refrigerant for high heat

In non-extreme conditions, ECUs and air conditioning systems can operate with R407C refrigerant. But once the ambient temperature passes 120°F, the refrigerant should be switched to R410A, as it can exhibit greater refrigeration capacity. For applications reaching up to 150°F, R134A is best because it’s designed to remain stable even under extreme temperatures.

As with any change in refrigerant, the system will need to be engineered to handle the different refrigerant (i.e. high-pressure compressors and other components capable of withstanding the higher pressures).

Lead-lag system for additional cooling capacity

A lead-lad system (multiple ECUs working in tandem) is traditionally designed to provide cooling redundancy and increase the lifespan between service cycles. But it can also function as a source for extra capacity in very hot settings. There, it can come into play by giving an extra burst of cooling in extreme situations.

The system can turn on both ECUs and provide an extra burst of cooling in extreme situations, such as a particular blistering stretch of days or an impending heat wave in an already hot locale, and make sure the sensitive equipment doesn’t overheat. This capability will require enough power to support it, but this is something your ECU provider should be able to address in the design phase.

Necessary over-sizing of units

In extreme hot temperatures, the performance of any AC system will fall off gradually; above 100°F the system will lose about 10% efficiency approximately every 10°F, with the performance degradation starting to become significant above 110°F. This must be taken into account early enough in order to properly size the unit.

For example, a five-ton cooling unit will no longer be able to provide five tons of cooling in a 130°F setting. The ECU provider will need to design at least a six-ton system to offset a 20% loss in cooling capacity and meet those cooling demands.

Airflow capability

Project engineers are often able to predict the tons of cooling capacity needed to meet the heat load as they draft specifications for their systems. But an equally important — but sometimes overlooked — component is the airflow capability.

Inaccurately scoped airflow prevents successful cooling. Not enough airflow (cubic feet per minute, or CFM) or too much external static pressure can lead to condensed water freezing, which then blocks off the airflow coils. Too much airflow will blow condensation off the evaporator and down the ductwork, where it can cause rusting, mold and other mechanical problems down the line.

The rule of thumb is 300 to 500 CFM per ton of cooling. For example, if you were to operate a 3-ton ECU for a UAV cooling system, then approximately 900 to 1500 CFM would be appropriate.

A qualified ECU provider will bring their experience to the table and make more precise recommendations on an appropriate airflow — and point out if you have started specifying a system without the sufficient airflow to actually cool your heat load.

How to cool in extreme cold environments

extreme cold enviroments

Maintaining precise set temperatures and humidity parameters in low ambient environments — those under 50°F and going as low as -30°F — poses as much of a challenge as extremely hot environments. Aerospace equipment still generates heat and must be cooled down, even when it’s well below freezing outside.

In addition to meeting low temperature MilSpec (Method 502.4), your ECU provider must provide a solution to maintain pressure in the system. Air conditioning systems have a high pressure and a low pressure side; as outside temperatures drop, the system loses its ability to maintain pressure.

Engineers can use the receiver method or the fan speed method to keep consistent pressure in these sub-zero conditions.

Receiver method to maintain pressure

The receiver method involves adding additional refrigeration components. Generally speaking, the outside coil of an AC system is used to remove heat from hot high-pressure refrigerant gas. As it cools, the high-pressure refrigerant gas condenses down to a liquid state. But in extreme cold environments, temperatures and pressures are so low that the outside coil can freeze and lose its capacity to reject heat from the refrigerant.

To address this, your ECU provider will have the “receiver,” which works with a three-way low ambient control valve, store extra refrigerant to cover up sections of the coil. This causes the usable area of the coil to get smaller and smaller — so only a fraction of the coil is exposed to the outside temperature. This prevents coils from freezing and pressure is still maintained.

Fan speed method to maintain pressure

A second method is controlling the airflow through fan speed. Temperature and pressure sensors will automatically turn the fans on and off until the pressure has reached an acceptable level. This allows the system to maintain enough pressure to allow the unit to cool properly.

Both methods are effective at controlling pressure and mitigating the concern of low pressure in extreme cold environments. The receiver method is more complex and costly because it requires more copper, refrigeration piping and overall design work, but allows for more precise control of the system’s pressure. The fan speed method only requires the addition of simple electronics. But it won’t be as efficient as the receiver method because it is not as precise; the system simply turns the fan on and off as the pressure rises and falls and is continually in the state of adjusting the pressure levels.

How to cool in corrosive environments

Corrosion poses a risk for AC equipment in humid, coastal or ocean environments. In addition to the Salt Fog MilSpec (Method 509.4) (Salt Fog), e-coating the equipment is a viable solution.

E-coating, or powder coating, involves covering any parts of the system that can come into contact with the corrosive environment. This “environmental control package” includes coating the evaporator, condenser coils and stainless steel hardware. This coating will help the unit stand up to harsh environments and contaminants and offer anti-corrosion protection at the highest level.

Achieve set points with precision-engineered ECUs

Aerospace and defense cooling equipment faces challenging environments — but not impossible. By working with a certified and experienced ECU provider, you’ll have a partner with the expertise to implement precise and customized designs for any situation. Start a conversation about engineering a cooling system to precisely fit your aerospace or defense application.