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Air Changes
Air Flow
Cold Air Intake
Condensing Heaters
Fans, Blowers, and Compressors

The purpose of this page is to discuss some of the needs and principles of ventilating a glass blowing studio, including comments based on visits to many studios and online comments in various discussion boards.  Several references are made to my hot walls page, so you may wish to start a new browser window (Ctrl-N in Explorer) and load it in that window.

There are three reasons for ventilation in a glassblowing shop -
1. To make the work space comfortable enough to work both for temperature and for removal of the ordinary products of combustion.
2. To assure that any noxious gases from combustion do not build up
3. To keep the temperature of the space, especially near the ceiling, low enough that damage to wood or roofing materials does not occur.
Most of the rest of this page is concerned with point number one.
 2. Noxious gases. It is actually highly unlikely that noxious gases should build up if there is any ventilation at all.  The prime candidates are fuel gas leakage, carbon monoxide, and carbon dioxide.
  The first, Leakage, is taken care of by the odor added to the gas and a willing use of a spray bottle with soapy water in it to check for bubbles on connections. Propane is heavier than air and natural gas lighter and relatively low cost detectors are available for each. The most risky time would be entering a shop that had been unused for a while when gas might collect from a slow leak.
  The second, Carbon Monoxide, is most likely to result from incomplete combustion - a yellow flame - and since CO is flammable, it will be removed by other active flames.  However, it is essentially the same density as air (molecular weight 28, same as nitrogen, N2, which is 80% of air) it will normally mix with air rather than collecting high or low and a carbon monoxide detector plugged into a wall outlet will relieve anxiety.  I prefer the ones with a digital numeric display that shows lower levels of CO before the alarm goes off. 
  The third, Carbon dioxide, is one of the common byproducts of burning (water being the other) and is not poisonous, but enough of it messes up the breathing senses and normally is accompanied by reduction the oxygen in the air, so that first headaches and funny breathing occur, then unconsciousness can occur.  Ordinary air movement will mix CO2 which is heavier than air.  I don't believe a cheap CO2 detector exists, but they are available for under $100.  2006-05-22, 2009-06-14


Ventilation pipes at VetroMany studios are overly hot, making it uncomfortable to work if not actually risking heat exhaustion. The best design seems to be to have separate exhaust fans with separate air supply for the furnaces and the blowing floor and to insulate the metal panels between the two so the heat does not radiate from them.

 However, one particularly nice design that took a lot of planning, involves bringing the makeup air out to the floor and blowing it down on the work floor.  This is shown at the right in the setup from Vetro Glass Art in Grapevine Texas.  This studio also has air conditioning for the next door gallery which blows chilled air down on the bleacher seating at the camera position. 2005-03-06


Bowling Green State UniversityNote that in the discussion that follows, it is assumed that there is NO air conditioning.  If there is then it should be completely obvious that everything is done to keep the AC air where the people are and the hot air where the furnaces are and to avoid releasing the furnace air into the cool stuff or sucking the cool stuff into the furnace space. The setup at Bowling Green State U (right) is inside the arts building and is heated/air conditioned with the rest of the building. As you can see, the entire hot wall is an encased box with large vent stack going off the top (the smaller tubes in the back are filtered exhausts for fuming with noxious chemicals.) If this place has a flaw, it is the limited intake air - when the shipping bay doors are opened a crack, really cold air comes past the camera position in winter.

VENTILATION - A studio's ventilation system should, if possible, not include free standing fans to shift glass dust around the shop. The exhaust point of the system should be near or above the hottest part of the shop to move both heat and furnace fumes out of the shop. A planned entry point for air should be used and when possible two or more should be provided: one which provide for the best air flow across the work area and another which supplies air during winter to ventilate without chilling the workers. Only one input should normally be used at a time so that air flow velocity is as high as possible for best cooling and removal of dust and fumes. The input should be chosen so that it does not normally pull in dust or exhaust from parking cars. If several openings are provided, normally air will flow over the shortest possible route from input to exhaust, including along walls, bypassing the people in the center. A fan blowing into a building normally provides poor ventilation except for the direct air flow within a few feet of the fan; there are many dead pockets where fumes and dust may collect. Heat shields in front of furnaces may also help control the air flow - with enough surface before the furnace area, it may be possible to have two exhaust fans, one primarily for heat and fume control and one for work area air flow. When possible, work with the prevailing wind, setting exhaust fans into the downwind side of the building and intakes on the upwind side. Drawing air from the shadier side or from under a building may result in cooler air for much of the day. 5/30/95 from Hot Glass Bits #25


Sketch of simple plan of glass blowing studioAir can be moved by sucking it or blowing it. While standing in a fan feels good, blowing air into a space is not the best way to ventilate it - sucking air out is much better. Controlling where the air comes in with vent panels (windows) is much easier than other ways. In the sketch at the right I have placed three "windows" A, B, & C, a door, a work bench and a wall between the work space and the hot wall space. Think of the "windows" as places that can either hold a fan/blower or allow the intake or exhaust of air. The door can obviously also do that also, depending on where it leads.  Be aware that adding screening will cut down on the free air flow, by as much as 50% depending on the screening. 2005-03-06

To state it up front, my recommendation is that exhaust fans should be placed at B and C, taking air from A and the Door. (Better would be an intake separate from the door, but I didn't offer that.) Let's look at other choices and see what is usually wrong.

A very common choice is to put an exhaust fan at A and take the air in at C (with B not even being provided.) This works under ideal conditions, because air is crossing the room diagonally and air is being taken out from the hottest area. By placing panels or baffles in the hot wall, it can be kept from being too hot in any one place. It becomes a problem when the air being drawn in C is either already hot (late spring in Texas, not to speak of summer) or so cold it freezes the worker (winter up north.) Shutting down C or if C is too small, results in lack of ventilation, hot spots, etc.

Blowing air in through C produces a good air flow just in front of C, which could be aimed at the bench, but the moving air can be felt for only 5-10 feet in front of the fan and there will be dead spots, especially in the area from the word Door to just below the word Bench in the drawing.

Blowing air in through B produces even less flow because the flow is broken up by the hardware behind the wall. Even if A is open as exit, air flow will be diffuse from the center on unless the wall is almost sealed, which is very awkward in use. The higher pressure behind the wall will force hot air out into the work space.

Exhausting air at C produces an air flow across the whole room, strongest across the center and stronger near C but most of the air will be moving. It is important that C (and any exhaust fan) be shrouded* so that power going into the fan will not be wasted with air simply moving in a short circuit around the fan. I have been in a couple of studios where a fan on brackets has been stuck in a window opening and without a piece of plywood or metal cut to surround the blades, almost all the electricity going into the fan is merely pushing air in a donut shape, about 1 foot outside the window, turn outward radially a foot, head back into the building to be sucked out again, pulling almost nothing from the inside of the building. 
 [* Shrouded means adding material so there is little air space at the tips of the blades - this can be a flat panel with a hole just the size of the rotating blade path, centered on the blade, but better is a tube supported on such a panel, so the fan turns in a short tunnel. The most commonly seen small shrouded fan is the "muffin" fan used for cooling computer equipment. 3005-03-06]

If an exhaust fan is used at A, then air sucked in at B and through any gaps in the hot wall, will flow to the left, taking hot air with it. Ideally A should be higher up, since hot air rises and in the ultimate can be overhead exhaust as long as there is air supplied behind the wall from an intake like B. Many studios use a hood and take the hot air up and out and this is an excellent choice if available.


The air safety and ventilation industry people discuss air changes per hour [AC/hr].  The box below has a suggestion of up to 30 AC/hr which would only be necessary with rather toxic materials.  Another source, soon to be published, suggests 1 to 1.25 AC/hr.  I think I would be happier with 2-4 AC/hr.  To get from AC/hr to air flow per minute, you have to know the cubic feet of the room (height x length x width).  Divide that by 60 minutes per hour to get cubic feet per minute (cfm) for 1 air change, then multiply by the number of air changes.  If you want to see what the make up air flow will be, you divide the cubic feet per minute by the square feet of the intake opening(s). The cubic feet per minute can used to select one or more air movers (fans, blowers).  2005-03-06

Item description Workspace Units Example
Size of room: Height   feet 10
Length   feet 15
Width   feet 10
Volume (multiply 3 above)   cubic
Changes per hour
(Small number 1-30)
  Changes 3
Cubic feet per hour
(Changes times cu.ft)
  cfh 4500
Cubic feet per minute
(divide cfh by 60)
  cfm 75
Air flow through 2x2 foot
opening (divide by 4)
  ft per min 18.75
Miles per hour
 (times 0.01136)
  mph 0.2131
COLD AIR INTAKE - One problem of studios in northern climes is that the winter air being drawn in can be very cold.  The drafts of this cold air can be very uncomfortable and cold reduces the efficiency of the furnace that has to heat air an additional 60F degrees.  The solution goes by the name of heat-recovery ventilator (good article) which have become a lot more available with the construction of sealed homes.  [There is also recuperation which is done with much higher heated air and special furnace construction.]
The solution is to draw cold air in over an air-to-air heat exchanger where thin metal plates divide the outgoing warm air from the incoming cold air, heat moving through the plates as it will.  Such a unit could be home built in several ways including metal tubes inside a box or sheet metal assembled like a commercial box (image from article) If built as part of a ventilation system using blowers that would be used anyway, the added cost would be much lower.  6009-06-14
Reply to e-mail about ventilation. Unless you can live with the sound of a high speed blower, you are going to have to get at least two additional openings, probably at least 2x2 feet each, one for intake air and one for the exhaust, depending on the blower you get.  I would take out a window or take off a door and replace it with another with a hole cut in it to get the changes of air you need.  Pete on the group mentions a change of air in the room every two minutes - a room 20x20 with 10 foot ceilings would be 4000 cubic feet so ventilation would have to be 2000 cfm.  In a room with this change and air moving through 2x2 foot openings, the air would be moving at 500 feet per minute (2000 cubic feet divided by 4 sq.ft.) or 30,000 ft/hr or 5.7 miles per hour.  A 6 inch round opening has only 0.196 sq.ft of opening so air velocity would have to be 10,185 feet per minute, or 611154 feet per hour or 115.7 miles per hour!!!!!!!!
Room size
(L x W x H)
Cubic Volume Vol. per minute
change every 2 minutes
2x2 opening-velocity
change every 2 minutes
20 x 20 x 10 4000 cu.ft 2000 cfm 5.7 mph
10 x 15 x 10 1500 cu.ft. 750 cfm 2.13 mph
12 x 12 x 12 1728 cu.ft. 864 cfm 2.45 mph

Reply to e-mail question below 2002-08
  The "standard" for small studios to reduce and control heat is sheet metal walls between the hot equipment and the worker with separate ventilation on either side.  A number of good and not so good installations can be found on my hot walls page. hotwalls.htm  I feel that a metal wall without insulation behind it is just a huge radiator.
    Many studios are set up with exhaust fans above the hot wall, with or without hoods, but many simply draw air across the working space, which leads to very cold drafts in the winter up north and unusable studios down here in the South in the summer.  I strongly feel that a properly designed studio should make a good attempt at sucking all the heated air out from behind the wall and have full replacement intake behind the wall also.  Then cooling/moving of the air in the work space becomes separate task handled on a much smaller scale.
   Most studios, with or without walls, have some kind of vertical metal panels, either hung or on wheeled stands that can both be positioned to block the openings provided for the furnaces/gloryholes when they are not in use and to block direct radiation on to the worker when in use.  The best picture is in the otherwise un-paneled setup at Hands On Glass on the hot walls page.  Or the roll around at Tacoma.
   The item I have the least experience with is use of a flue, which seems to be working on my new furnace even though undersized.  Properly adjusted, a flue should provide exact combustion control and opening the door should release little or no hot air.  A furnace or gloryhole "without" a flue uses the door as a flue.
    The three sources of heat are convection, radiation and conduction.  Most studios have little problem with conduction as they know the direct flow of heat to the surface of equipment wastes energy.  Convection, the movement of hot air, is more of a problem, the flue and exhaust being considerable solutions as well as radiation as discussed below.  Radiation is strongest from the opening to the hottest glass surfaces.  It is "traditional" in too many studios that the gloryhole be partially open all the time because there is no flue - this moderate opening is a major source of radiant heat and hot air.  A fully closing door can be a major improvement, but even an automatically closing door can help.  Because furnaces are often left on for months at a time, they are more normally flued, if not very well, and the door well closed.  
    Many glory holes are built with access doors that work well only with an assistant and thus get opened early and closed late.  None of my photos show one of these in clear detail, but the left hand glory hole in the lower Bowling Green picture is one and the huge one at Tacoma is another.  Commonly these are a pair of big doors, with smaller doors within and a permanent small hole.  The various doors are commonly opened with a rod with a loop on the end.   I build my doors with the handle in the heat shadow of the door so they can be moved with my bare hand after several hours in use.   [for the record, the really true "traditional" method for controlling the gloryhole radiation is not have a door at all, but to lift U-shaped cast refractory plates into place with arched openings for access.  I saw these in use at Steuben in 1999 during class.  An apprentice gets to rearrange them. <Doors since made pneumatic.>  Of course, in a true production environment, they get changed only when the product changes.]     I have used a number of mechanically enhanced doors, pneumatic and motorized.  I have found them universally too slow to open and awkward to work, including finding the control on the floor.  But I do think it is a good idea to keep the doors closed as much as possible.  My solution would be manually open the door quickly and have a slow moving mechanism to start shutting the door immediately, where I could lift the door off the mechanism to shut it quicker.  I envision a continuous drive threaded rod with a cutoff switch when the door is shut and a "half nut" to engage the screw to carry the door shut.  Until I build this thing, I shall have to remember to "automatically" close the door  when I walk away.   Hope this helps, further questions welcome.
Mike Firth

  Furnace Glass Web Site/Hot Glass Bits
----- Original Message ----- From: "Krake, Ann M." <> To: Mike Firth Sent: Monday, August 05, 2002 3:39 PM Subject: Radiant heat shielding for glass-blowing ovens
> Hello Mr. Firth,
> I am an industrial hygienist with the National Institute for Occupational
> Safety and Health (NIOSH).  We're part of the Centers for Disease Control
> and Prevention, or CDC -  sorry for all the government acronyms!  Anyway, I
> have been doing some research for an OSHA consultant in Oregon who is trying
> to help a local glass blower reduce his exposure to heat stress.  I have
> consulted with our engineering control folks here and have also searched the
> Internet but have not had any luck in finding a specific fix for glass
> blowing operations.  In reading some of your newsletters it seems you are
> well-traveled and researched and I was hoping you might be able to lead me
> in the right direction.  I would really like to help these folks reduce
> their exposures, so any ideas would be appreciated.
> Please feel free to contact me by any of the methods listed below.
> Sincerely,
> Ann Krake
> LCDR Ann M. Krake, MS, REHS
> Public Health Service
> National Institute for Occupational Safety and Health
> 4676 Columbia Parkway, R-11
> Cincinnati, OH  45226
> 513.841.4206
> 513.458.7147 (fax)


Condensing Heat Output
A condensing furnace raises the efficiency of a household heating furnace from about 85% to about 95% by cooling the exhaust gases to the point that the H2O generated in burning is condensed.  When this is done, the gas is so cool that it will not rise in a flue/chimney as normal hot exhaust does.  As a result, a blower must be provided for forced exhaust (and a safety switch to shut down if it fails) but gained is the ability to vent horizontally to a side wall and to use PVC pipe for the exhaust.  In fact, the exhaust is a coaxial pair of pipes with intake air being pulled in on the outside and exhaust pushed out the center, which further cools the exhaust.  The furnace must have a good drain. Ours puts out about five gallons a day in really cold weather.  Further, the liquid is corrosive since the CO2 from burning makes carbonic acid, any tiny amount of sulfur in the gas may make sulfuric acid and although temps in a furnace are not high enough to make a lot, nitrogen can oxidize and that can result in nitric acid in the water.  It is definitely not distilled water.
  The method below is to assume an output, 100,000 Btu and work from that through the chemistry to discover how much water results and then how many Btu's result from condensing that weight of water.

 1 ft3 natural gas + 10 ft3 air + flame = 8 ft3 nitrogen + 1 ft3 carbon dioxide + 2 ft3 water vapor 
 [ 1 nat.gas           + 2 O2 from air      =                            1 CO2                          2 H2O
-95 thermochemical calories (1=4.18 international joules) gas
H2O  -57.8 heat of formation -54.6 free energy of formation
CH4 Methane heat of combustion 210.8 kg. calories per gram molecular wt
 CH4 + 2O2 =  CO2 + 2H2O
heat of formation of CH4 = 2(94.38)+4(34.19) -210.8 = +20.34 kg-cal per gram molecular weight   [doesn't match]

CH4 + 2O2 =  CO2 + 2H2O Primary chemical reaction of burning.
100,000 Btu = 100 cubic feet approx. Natural gas 1000-1030 Btu/cu.ft.
which is 3 m3 approx.  1 m3=35.316 cubic feet
or 2.04 kg. = 2040 gm Methane density (1.013 bar and 15 C (59 F)) : 0.68 kg/m3)
with this is 127.5 mol. Methane 16 gm/mol
yielding  255 mol water and 127.5 mol CO2 Formula above
4,590 gm water and 5610 gm CO2 Water 18 gm/mol; CO2 44 gm/mol
4.59 liters of water (4.33 quarts) per hour. Water 1000 gm/liter
this much water releases 10,378 kJ on condensing
( 9,836 Btu about 10% bonus.)
40.7 kJ/mol condensing
The units are usually kilojoules per mole (kJ / mol)
Sometimes the unit J/g is used. The first unit is technically the most correct unit to use.
The molar heat of vaporization for water is 40.7 kJ/mol. Remember the value!!!

Composition of METHANE:
Density (g/cm3) = 6.67151E-04 = 0.00066715
Gas density (1.013 bar and 15 C (59 F)) : 0.68 kg/m3
The primary component of natural gas is methane (CH4), the shortest and lightest
hydrocarbon molecule. It also contains heavier gaseous hydrocarbons such as
ethane (C2H6), propane (C3H8) and butane (C4H10), as well as other sulphur containing
gases, in varying amounts, see also natural gas condensate. Natural gas
can be identified through the presence of ethane, as all natural gas contains ethane.

A mole is defined in S.I. as Avogadro's number of particles of any kind of substance (atoms, ions, molecules, or formula units). In S.I., this unit is abbreviated mol. The mole is the basic unit of amount of substance. Wikipedia

q = DHvap (mass / molar mass)

The meanings are as follows:

1) q is the total amount of heat involved
2) DHvap is the symbol for the molar heat of vaporization. This value is a constant for a given substance.
3) (mass / molar mass) is the calculation to get the number of moles of substance
1 kg of LPG = 47.0 cubic feet of natural gas
100,000 Btu = 1 therm
1 calorie (dieticians' Large Calorie) = 4.1855 kilojoules
1 Therm = 100,000 Btu (British Thermal Units) = 25,200 kilocalories = 105,474.6 kJ = 105.48 MJ
 1000 kilocalories = 3,968 Btu = 1.163 kilowatt hours
(Large Calories)    3412 Btu = 1 kW

1 kilojoules = 0.94781712 Btu
100 000 Btu = 105 505.585 kilojoules

Energy Source Energy Content Local Price
Natural Gas 37.5 MJ/m3 $0. _________/m3
Propane 25.3 MJ/L $0. _________/L
Oil 38.2 MJ/L $0. _________/L
Electricity 3.6 MJ/kWh $0. _________/kWh
Hardwood* 30 600 MJ/cord $_________/cord
Softwood* 18 700 MJ/cord $_________/cord
Wood Pellets 19 800 MJ/tonne $_________/tonne

Conversion 1000 MJ = 1 gigajoule (GJ)
* The figures provided for wood are for a "full "cord, measuring
1.2 m x 1.2 m x 2.4 m (4 ft. x 4 ft. x 8 ft. ).

Fans, Blowers, and Compressors

There are three groups of devices for dealing with air as stated in the title of this section.  They differ by the path of the air and the pressure they will work against.

Fans bring to mind a desktop pivoting fan which is a very good image, but fans can include anything that looks even slightly similar and has the same air flow.  In this case the air must blow in line with the axis of rotation and there must be blades with space between them.  With few exceptions, the axial blower or fan must not have a lot of back pressure and will work a lot better if the blade is shrouded.  One obvious exception is the turbine compressor used in jet engines, but this requires very tight fittings and very high speed so the air is being rammed in against the outgoing air much faster than it can move - it uses a lot of power and makes a lot of noise.  Shrouding works better because the greatest loss in an unshrouded fan is the air coming off near the tips and immediately turning out and back circulating only through the fan and not through the room. The most common shrouded fan people see is muffin style fan in computers.  An axial blower can have multiple sets of blades and the blades can even over lap so light is not visible through the fan.  Fans are used most often for moving large quantities of air between spaces at atmospheric pressure. 
Centrifugal blowers come in a number of forms of which the most common is the squirrel cage blower.  The common factor is the air is moving perpendicular to the axis of the part that is moving the air.  Typically, the air is drawn in through the side of the unit, turns at low speed to encounter the inside of a spinning cylinder with ribs or vanes around the wall of the cylinder which fling the air out and away where it is captured and aimed through an outlet much smaller than the inlet.  Think of taking an open tin can, drilling a hole in the center of the bottom to take a bolt as an axle and cutting slots up the sides, bending the material of the slots in, then spinning it with a drill.  A variation looks like a plate with ribs running from the center and increasing in height toward the rim, the air being thrown off the rim.   The name squirrel cage comes from the appearance like the tread wheel small rodents use for exercise - I suspect it is not called a hamster tread wheel because that is too precise - or childish. Blowers can have higher output pressures, but not a lot higher and they lose capacity rapidly with increasing pressure.
Aside from the small squirrel cage blowers used on some electrical equipment, the most common blowers of this type are the units used for drying carpet after cleaning or a fire, the blowers used for leaf sweeping and the unit hidden inside a vacuum cleaner.
Compressors have the purpose of increasing the pressure of the air considerably.  Therefore they must have close tolerances and normally produce a relatively low output volume.  This is somewhat unfair as there are blowers intended to ram air into enclosed spaces which are designed to work against pressure and still deliver considerable volume.  The two most common compressor forms are a piston in a cylinder - found in almost every workshop for delivering air up to about 150 psi to work tools - and a diaphragm in a space with valves so that when the diaphragm is moved back and forth in the space air is pulled in through one set of valves and pushed out through others - most often found in moderate volume, moderate pressure paint sprayer compressors - up to 50 psi.  If a compressor is made with lobes that fit closely together in any of several fashions, it can rotate without the back and forth slamming of the piston or diaphragm and force air against pressure. Roots blowers (shown) and Wankel compressors fall in this category.  The very precision required makes these higher cost.

axial fans, centrifugal blowers, inline fans, ventilation equipment, air flow, iris dampers, impellers, fan blades from Continental Fan Manufacturing CFM


Wall mount shrouded fan
Floor mount ducted fan
Squirrel cage blower, drawing
Centrifugal blower commonly used for carpet drying
Centrifugal blower in leaf blower
Lobed compressor

Contact Mike Firth