Self Cleaning Inlet Air Filters Puffing Air Temperature


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In frame 6B machine we are using self cleaning inlet air filters. Air for cleaning of these filters is taken from compressor 17th stage (CPD), and then it is cool down(with the help of air to air cooler) to 30 to 35degC. Then it is passed through alumina filters which absorbs moisture content in the air.

Thus we are puffing the filters with an air of 30-35degC temperature and6-7kg/cm2 pressure. As we are using paper type filters, these absorb moisture during rain and filters DP goes high.

So can we directly give the air from CPD (temp at air processing unit is 80-90 degC) to filters for puffing (bypassing air to air cooler and moisture separators) whenever filters absorb moisture?

I believe the problem with your suggestion is two-fold: First, you need to remove the moisture from the air, and cooling it before running it through the "desiccant" is the best way to do so.

Second, I don't know if you could just bypass the cooler and run the hot air through the dryer/desiccant. You would have to know what the maximum air temperature the assembly could withstand.

Finally, I don't know if the solenoids on the discharge nozzles can withstand the higher air temperature.

And, I don't believe it's going to be as effective as you think it will be.

If you don't have them installed on the inlet filter house, you should look into what are called "rain hoods" which are pretty effective (if the inlet filter house isn't oriented such that it faces into blowing wind and rain--something which should have been considered during the initial layout and planning of the plant) at keeping rain from impinging directly on the inlet filters. Rain hoods were a purchased option, something the purchasers of the turbine and auxiliaries would have had to pay additional monies to purchase and have them installed.

Rain hoods can be modified to make them more effective (if already installed), and they can be added to most inlet filter structures. And there are different (though usually more expensive) filter materials which aren't as subject to swelling and reducing air flow when wet. You should contact the manufacturer of the inlet filter structure supplied with your turbine to see what they can recommend or offer, and speak with other inlet filter structure manufacturers as well; you may find that a new inlet filter structure with different filters/filter arrangement may save money in the long term--especially if the current arrangement/filters is causing a drop in output during annual rains.

Hope this helps!

> Will it not increase CTIM also?

CTIM is measured at, or just outside of (next to), the axial compressor bellmouth. Some newer units measure ambient temperature with sensors on the outside of the inlet filter structure, but that's a newer instrumentation addition for a lot of machines, which I've not seen on any GE-design Frame 6B heavy duty gas turbines.

Increasing CTIM will not help dry the inlet air filters. Increasing CTIM will decrease the efficiency of the unit--warmer air is less dense air and if the air density is lower the mass flow-rate through the axial compressor and turbine will be lower. And if the mass flow-rate is reduced the power output of the unit will be reduced.

> And will increase heat rate?

To increase efficiency of the unit, one actually wants a lower heat rate. Increasing the heat rate will reduce the efficiency. (Heat rate measures the amount of fuel burned per unit output, so to increase efficiency one wants to reduce the amount of fuel burned per unit output.)

The self-cleaning filter system operates by setting off a small "puff" of air in the center of the filter airflow. This, in turn, induces a momentary reverse flow of air--a rather large reverse flow of shor duration--through the filter, which serves to knock off the larger particles of dirt from the outside of the filter. Some self-cleaning systems can have the pulse duration varied, but, making it longer will reduce the efficiency of the reverse flow, which, again, is only intended to knock the larger particles of dirt off the outside if the filter. And, some self-cleaning systems can vary the time between pulses, but there is a limit to how fast the air piping can "re-charge" between pulses to provide a sufficient pulse to induce the reverse flow.

The "puffs" are so small that they would not increase the compressor inlet air temperature by any measurable amount.

Finally, by "re-circulating" air from the axial compressor discharge one is further reducing the air flow through the unit (the turbine in this scenario) and that reduces the mass-flow through the unit, which reduces the power output of the unit (for the same fuel flow-rate). Taking a small amount of air and using it in the self-cleaning inlet air filter system, doesn't reduce the mass-flow significantly, but if one were to run the self-cleaning system continuously and with long duration pulses and short durations between pulses the overall efficiency is going to be reduced, even if only by a little bit. The self-cleaning inlet filter systems are only intended to <i>extend the period between filter changes</i>--and that depends on the type of dirt and contaminants in the air, as well as the amount of humidity in the air. Excessive humidity will eventually make the smaller contaminant particles adhere to the filter surface, and they will not be removed by pulsing. And, in reality, only the large particles will be removed by pulsing--not the smaller particles, which will be trapped in the filter material. "Self-cleaning" is kind of a misnomer; the filters aren't "cleaned" (as in near new) when pulsed. Only the larger particles/contaminants are removed by the pulsing.

Only by having clean inlet filters, a clean bellmouth and IGVs, a clean axial compressor and by ensuring ALL air flow through the axial compressor also goes through the turbine will efficiency by maximized.

Lower compressor inlet air temperatures increase efficiency (at Base Load); that's why various inlet air cooling methods are used on many units (evaporative coolers; chillers; foggers). Inlet air heating, including Inlet Bleed Heating, increase CTIM by recirculating a portion of axial compressor discharge air <i>downstream of the inlet air filters</i> into the axial compressor inlet. This increases axial compressor inlet air temperature, AND it also reduces the efficiency of the unit by reducing the mass-flow rate through the turbine section. (And, the energy used to compress--and heat--the air extracted from the axial compressor discharge is lost when that air is recirculated, at a lower pressure, to the inlet air stream. So, there's a kind of double-whammy loss when axial compressor discharge air is recirculated to the axial compressor inlet.)

Hope this helps!