Temperature Affects on PD Blowers

The most important thing to remember about maintenance on any PD blower is that the temperatures must be controlled. The minimum inlet temperature for any Gardner Denver blower is minus 20 degrees F because below that temperature the cast and ductile iron parts of the blower become very brittle. The other consideration is high discharge temperature. When the discharge temperature is excessive, the parts expand to the point that contact occurs. The contact itself causes frictional heat and causes further growth of the parts leading to greater contact and eventual failure.
 
A common misconception is that a relief valve will always protect a blower from failure. The relief valve located in the discharge line is only detecting the pressure at that point and not the differential pressure across the blower or the actual compression ratio across the blower. The discharge temperature will follow the compression ratio so as the compression ratio goes up; the discharge temperature goes up accordingly. 
 
Consider a blower at an altitude of 1,000 feet above sea level operating at 15 PSIG on the discharge with a small inlet pressure loss of 5.5 inches of water due to the inlet filter and inlet piping. There are 27.71 inches of water in 1 PSI so the 5.5 inches of water pressure drop is equal to 5.5 divided by 27.71 or a loss of 0.2 PSI. The atmospheric pressure at 1,000 feet above sea level is 14.16 PSIA so the inlet pressure to the blower is 14.16 PSIA minus the loss of 0.2 PSI or 13.96 PSIA. The compression ratio across the blower is the absolute discharge pressure divided by the absolute inlet pressure or in this case (14.16 plus 15 PSI = 29.16 PSIA) on the discharge divided by (14.16 minus 0.2 = 13.96 PSIA) on the inlet for a compression ratio of 2.088. 
 
The term Equivalent Sea Level Pressure (ESLP) is used to determine what discharge temperature will occur as a result of operating at the same compression ratio but operating at sea level instead. By subtracting 1.0 from the compression ratio and then multiplying that answer by 14.7, we find the ESLP. In this case (2.088 –1.0) times 14.7 equals an ESLP of 16.0 PSI. So the discharge temperature of this blower is equivalent to a blower operating at 16 PSIG at sea level (14.7 plus 16 PSIG = 30.7 PSIA) divided by 14.7 PSIA equals a compression ratio of 2.088.
 
Now suppose that this blower inlet pressure drops as the inlet filter gets very plugged with dirt and the pressure drop across that blower goes to 54.4 inches of water or 2 PSI. Then the inlet pressure to the blower becomes (14.16 minus 2 = 12.16 PSIA) and the compression ratio is now (14.16 plus 15 PSIG = 29.16 PSIA) divided by 12.16 PSIA for a compression ratio of 2.398. The ESLP equals (2.398 minus 1) times 14.7 or 20.6 PSI. So now the discharge temperature of the blower will be the same as if that blower was operating at sea level at 20.6 PSIG since (14.7 plus 20.6) divided by 14.7 is 2.398.
 
So, the relief valve is still sensing only 15 PSIG but the discharge temperature is considerably higher because the compression ratio is equivalent to running the blower at 20.6 PSIG. Filter pressure drop / inlet pressure drop should be measured so that elements are cleaned or replaced when the pressure drop reaches a point that will cause excessive compression ratio and excessive discharge temperature.  

• Temperature Affects on PD Blowers