It has been said that air compressors make better furnaces than they do pneumatic power systems, and there’s a good reason for that.
What is an air compressor? In a sense, it is a “converter”. Specifically, it is a machine that converts a form of INPUT energy into POTENTIAL energy, which can easily be stored and used elsewhere via a compressed air network (piping system). The energy that you input might come from an electric motor, or it might come from a gas or diesel engine, or perhaps it’s supplied only by a battery, which in turn is charged by a third power source…but whatever the case, the fact is that no conversion process is 100% efficient.
Every time you convert power from one “type” to another, you LOSE power through the conversion process. How efficient is the process? Well, if you have an electric motor, you can read the rated efficiency on the motor nameplate itself. The motor nameplate power is the motor’s OUTPUT power, NOT it’s electric input power. The efficiency rating tells you how much power you will need to put INTO the motor to turn it at load vs. what % of the power you will get back OUT at the motor shaft. Now, we all know the First Law of Thermodynamics which states “Energy can be neither created nor destroyed.” If that’s true, then where does the lost energy go? Well, in the case of the motor, it is expressed from the motor itself as heat. Put your hands on the powered motor, and you can feel it radiating from the housing. This is the 2nd Law of Thermodynamics in action: “Energy always flows from areas of high concentration to areas of low concentration”. There is, at the moment the motor is working, a high concentration of energy at the motor shaft. There is a low concentration of energy around the motor itself. That heat then is dissipated into the environment around the motor, warming it up as the motor works. Modern motor efficiencies vary with horsepower (kW) rating, and range from a standard efficiency of (approximately) 85.5% to efficiencies in the mid 90% range with a premium efficient model…. But how efficient are air compressors?
Did you know: On average, it takes 4x the compressor input power to pneumatically power a tool that would use ¼ of that compressor’s energy. In other words, a 4 hp air compressor can power a single 1 hp pneumatic tool. Why? Because as much as 94% of the power that goes into an air compressor is expressed NOT as pneumatic (potential) energy, but as WASTE HEAT. Compression is an exothermic (heat-generating) process.
How much heat does an air compressor give off, when in operation? While compressor types and individual model efficiencies allow for some variation, the rule of thumb is that PER horse power, a compressor will reject 3,600 Btu/hr (1.055 kW) of heat.
It is critical that engineers and plant managers in charge of compressor room design understand this. In the Industrial world, where compressors are often put out on the plant floor rather than in dedicated rooms, there is an opportunity to dissipate that heat in a large space, or easily repurpose it within the plant in colder months. Regardless, it is often more a commodity or at worst at nuisance that can be mitigated by the opening of a door or a window. In an enclosed space, however, that “waste heat” can pose a real problem.
“Enclosed spaces” are the norm in the oilfield, where compressors are installed in dedicated “instrument air buildings” at remote, usually unmanned, sites. What determines the instrument air building size is what you might call the “Law of Minimal Clearances”. Space is money, square footage has a cost, and therefore the smaller the building, the lower the cost, and the easier the sell. The client (or the project engineers) will seek to find the solution that costs their client the least amount of money, and that’s often the smallest possible package….. So if a compressor needs 1 meter (39”) of clearance on a minimum of (3) sides for proper operation and serviceability, then that is exactly what it will get. If the compressor itself has a 36” x 60” footprint, then you can imagine that the building in which it is housed will be only a little larger than that…but now think of the heat load that compressor represents on that building.
A Case in Point:
Our 36” x 60” 40 hp compressor is installed in a building that is only 88 square feet, and it will be throwing (40hp x 3600 Btu/Hr = 144,000 Btu/hr or 42.2 kW of heat continuously into the structure while it’s running. The building is insulated per code (R12 walls, R20 roof, R30 floor) and the compressor is ducted out of the building, to mitigate the heat load. At a -25oC ambient (winter) outdoor temperature, the required heat to maintain a +20oC INSIDE the building for the compressor and operator comfort (should one stop by to say hello), is only (approx) 9,000 Btu/Hr, which means our compressor is generating 16x more heat than you require to keep the building comfortably warm. If, during the day, the temperature outside rises to -10oC, then you’ll need only 5,500 Btu/Hr to maintain that same 20oC inside the compressor room – less than 1/26th of the heat load the compressor itself represents. How can you possibly balance that rejected heat with the compressors required inlet air, and not have massive temperature swings in the compressor room that compromise the compressor’s operation?
There is really only ONE solution. In a large space, as we said, the “waste heat” from the compressor can be easily dissipated, but that’s not the case in an Instrument Air Package. Here, we need to monitor and control the heat recovery process inside the building CONTINUOUSLY. Ducting out all of the waste heat isn’t possible – the cooling fan on a compressor is sized to move all that heat from the compressors coolers to the ambient environment. In the case of our 40hp machine, that cooling fan will be moving approximately 100 – 200 cfm per HP across the coolers. Even on the low end of that scale, if you are evacuating 4000 cubic feet of air per minute out of a compressor building that is only 800 cubic feet in size, then you’re doing one complete air change every 12 seconds. No heater can possibly keep up with that kind of wind tunnel.
How do we then monitor and control heat recovery? With a heating and ventilation PLC and thermostatically controlled exhaust and heat recovery dampers. By modulating the inlet and exhaust louvres, we can constantly adjust the inlet and outlet air flows from the building and maintain a temperature inside to within a programmed tolerance; typically +/- 2 degrees C. This not only protects the equipment from freezing up or overheating, it also protects your investment.
When it comes to heating and ventilation, don’t go “cheap”. An emergency call out in the middle of the night in the hinterland of oil and gas country sure isn’t. But also don’t just let that ‘free’ heat escape to the elements all winter long – your utility provider may not like the decrease in billings, but your CFO certainly will!
Reimund (Ray) Krohn is Central Air Equipment’s Product Specialist for Engineered Projects. He has been working in the compressed air industry for 22 years and is a US Department of Energy certified AirMaster+.
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