Written on: May 8, 2013 by George Carey

than low pressure steam.

To calculate the velocity (how fast the steam is moving), there is a simple formula used in the industry. It is based upon a couple of factors—the BTU/H expressed as lbs. per hr. of condensate, the pipe size and the cubic volume of the steam. If I had a 100,000 BTU/H boiler and it was making steam at 0 psig and the steam main was 2″ its velocity would be 33 feet per second, or approximately 22 mph. If I cranked up the pressuretrol to 10 psig, the steam’s new velocity would only be 20 feet per second or approximately 13 mph! Don’t get confused into thinking higher pressure steam is going to speed up the delivery of heat to the building!

of radiation.

through baseboard.

For this convection to occur, there are three factors to consider: 1) surface contact area; 2) temperature difference between the water and the inside wall of the tubing; 3) the convection coefficient which is calculated based upon the properties of the fluid, the surface area’s shape and the velocity of the fluid. I think if you use your mind’s eye, you can visualize the following: as the stream of water flows through the baseboard; the outer edge of this stream is in direct contact with the tube’s inner wall. This “rubbing” against the wall creates drag, which means the water that is touching the inner wall of the tubing is moving slower than the “core” or inner stream. Because of this, the temperature of this outer layer of water becomes cooler than the inner stream. This drop in temperature impacts the rate of heat transfer—it slows it down. Remember, one of the factors that affect convection is temperature difference! In fact, a good visual is to think of this outer layer as an insulator that impacts the rate of heat transfer from the hotter inner stream of water to the tubing’s inner wall. This is especially true when the speed of the water approaches laminar flow instead of turbulent flow. So in effect, the faster the water flows through the tubing, the thinner the outer boundary layer or insulator becomes, thus increasing the rate of heat transfer from the hotter inner “core” water to the tubing’s inner wall. You can confirm this by taking a look at any baseboard manufacturer’s literature and check out their capacity charts. They typically will publish their BTU output per linear foot based upon two flow rates: 1 gpm and 4 gpm. The BTU output is ALWAYS higher in the 4 gpm column.