By Chad Edmondson (JMP) and Norman Hall (RLD)
Today’s HVAC and process engineers have lots of choices when it comes to pipe sizing programs for steam and condensate applications. Put in a load and a pressure and out pops the answer. But ever wonder about how that answer is generated? Or if there are times when it might be suitable to choose another pipe size?
When selecting a pipe size in hydronic HVAC systems, we are concerned about GPM or gallons per minute flow rate, velocity, and pressure drop. The same can be said for sizing condensate piping, so many of you may have wondered if you can use the Bell & Gossett System Syzer to size condensate pipe just like you use it to size heating and cooling pipe. To illustrate why that is not a good idea, consider the following example:
Let's assume we have 1750 pounds per how (PPH) of condensate and we want to select a pipe size.
1750 PPH/ (60 Minutes per Hour) = 29.17 PPM
29.17 PPM/ (8.33 lbs. per gallon) = 3.5 GPM
If we use the System Syzer, we see that ¾” iron pipe is a fine choice for a hydronic system. Steam condensate systems, however, come with some extra challenges. First, the motive force is often gravity, which is simply the pitch of the pipe or low pressure. Second, there’s the presence of air in the pipe. Third, steam condensate is often water at, or very near, the saturation temperature.
When sizing condensate pipe it is important to identify whether the pipe is a wet return or a dry return. A wet return means that the pipe is completely filled with condensate and thus the pressure drop calculations would be similar to hydronic systems. In this case, the only thing we need to watch out for is the pressure drop.
If my example of 1750 PPH or 3.5 GPM is in a wet return and I had 2 PSIG at the inlet to a steam trap and I take a 1 PSIG drop through the trap, there is only 1 PSIG plus any elevation difference as the motive force. If I use ¾” pipe, as shown above, the friction loss is about 3.3 feet or 1.4 PSIG of pressure drop per 100 feet of pipe. If the motive force is 1 PSIG and there is no backpressure, I should not have more than 70 feet of TEL (total equivalent length) or I will back up condensate into the coil or heat exchanger return pipe. Piping returns that are located below the water line of the boiler or the discharge from the pump are common examples of wet returns.
Most condensate return pipes are dry returns and are not completely filled with water. These returns are above the boiler water line and have a combination of water, air, and steam at various times. The pipe sizing has to accommodate all three without backing condensate into a coil.
Dry returns must be sized for pressure drop at the rated flow and provide sufficient room for water flow (condensate), air, and flash steam.
The condensate leaving the coil is assumed to be at the same temperature as the steam that entered it. Once the hot condensate enters the lower pressure of the gravity return line, a small percentage of the liquid will flash into steam right inside the pipe. We use what is referred to as a "condensate pressure pair." The condensate pair gives the maximum steam pressure in the coil followed by the pressure you want in the return line.
So how much volume do we need in the pipe for flash steam?
Water has a specific volume of about 0.016 cu. Ft. /lb. and 0 PSIG steam is about 25 Cu Ft. /lb. That is a lot of expansion! If we do not have the room to expand, we will back up the condensate flow and can even cause some steam hammer. As the steam pressure entering the coil rises, the temperature of the condensate rises, causing even more condensate to flash into steam.
Below is a series of screen shots from the Xylem, Hoffman pipe sizing program. The capacity or flow rate is the same and the acceptable pressure drop is the same. The only difference is the pressure. Higher pressure requires a lot of room for flash steam.
If we select a pipe size based on a (5,0) pair, the program selects a pipe size large enough for the flash from the 227°F condensate down to 0 PSIG at 212°F. The 227°F is the temperature of saturated 5 PSIG steam. If I select pipe size based on a (15,0) pair, the pipe diameter must be large enough to flash the condensate from 250°F down to 212°F, so the pipe size gets larger. The latent energy required to flash the water into steam comes from the sensible temperature in the liquid.
As the pressure or motive force increases, the pressure drop per 100 feet can increase--but be careful. Coils and heat exchangers with modulating control valves may have significantly reduced pressures in the condensate line.
Stay tuned for more on sizing condensate pipes in our upcoming blog that speaks specifically to healthcare applications.