June 19th, 2023 | Posted in Air Cleaners
When working to very close tolerances, variations of 10 or 20 in shop temperature can make a difference in critical part dimensions. For example, the thermal expansion coefficient of stainless steel (which varies somewhat depending on the alloy) is approximately 5.2 to 9.6 millionths of an inch/inch diameter per degree Fahrenheit[i]. Therefore a 20 degrees F difference can make a dimensional difference of 118 to 192 millionths, or about 1 or 2 tenths of a thousandth per inch of part diameter. For rough work, that may not make much difference. For high precision work, however, a few tenths makes a significant difference.
In order to offset the potential for undesired part-size variation due to ambient temperature fluctuations, many shops have installed air conditioning. This stabilizes shop temperatures and, as a corollary, stabilizes part dimensions.
While working in an air conditioned shop is a real plus in the summer, keeping the doors closed to maintain stable part dimensions means that the haze of coolant mist in the shop tends to accumulate to the point that the mist may represent a health hazard. Health hazards such as this were recently addressed – and they are not trivial. In order to reduce the concentration of coolant mist in the shop air, it is a common practice to open the shop doors to ventilate the shop. If you have installed air conditioning in the shop to maintain part dimensional stability, what does it cost you in terms of wasted electricity to open the shop doors for ventilation? Using a 100 ft. by 100 ft. space as an example, the costs are calculated below:
1,000 ft.2 requires 21,000 BTUs per hour for air conditioning[ii]
A 100 ft. by 100 ft. shop space is 10,000 ft.2.
Therefore a 10,000 ft2 space requires 21,000 BTUs X 10, or 210,000 BTUs per hour
Figuring an ordinary machine shop shift of 10 hours, that comes to 210,000 BTUs per hour times 10 hours, or 2,100,000 BTUs per day
1 BTU = 0.000293071 kilowatt hours[iii]
Therefore 2,100,000 BTUs requires 615.5 kilowatt hours per day
Average cost of electricity is $0.14 per kilowatt hour[iv]
612.5 kilowatt hours per day X $0.14 per kilowatt hour = $86.16 cost per day to air condition a 100 ft. x 100 ft. shop space.
The number of days in an “air conditioning season” depends on the local climate, but 84 days (4 months at 21 weekdays days per month) is used here for convenience.
84 days X $86.14 per day = $7,237.70 of electricity costs per air conditioning season.
Note that not only are we losing $7,200.00 per year out the door, we are also losing the part dimensional stability that we sought in the first place by installing air conditioning!
Another point worth considering is that accumulations of coolant mist on the HVAC coils reduces their efficiency with resultant needless increases in air conditioning costs.
If you are exhausting unfiltered air to control coolant mist rather than opening the shop doors you can go to the “Cost of Air Conditioning Loss due to Exhaust” calculator.
Clearly, the better solution to coolant mist in the shop air is installation of effective mist collection devices. Air Quality Engineering offers a wide range of mist collection equipment. Contact them for guidance on keeping the shop air clean while keeping the doors shut.