Convey-air conditions in a pneumatic- dilute pressure convey-system.

The convey-air conditions to be considered are :

- absolute pressure

- temperature

- relative humidity (RH)

There are four locations to consider the conditions of the convey-air.

Location 1)

Intake conditions of the compressor. In the worst case ,the RH should be considered as 100%.

Location 2)

Pressurized air at the outlet of the compressor. Here the pressure and the temperature are increased, whereby the RH decreases or stays 100% with or without condensation.

In case an air cooler is used, this should also be taken into account.

Location 3)

At the mixing location the convey-air and the fly ash are mixed at the compressed-air -pressure.

The heat-content of air and fly ash is equalized over both, resulting in a mixture-temperature.

The resulting mixture temperature is close to the fly ash temperature, due to the higher specific heat content of the fly ash.

Location 4)

In the silo, the air has completely expanded again to atmospheric pressure and the condition can vary between the original conditions and those determined by the circumstances

in the pipeline such as, warmer by the fly ash, cooler by heat-exchange with the surroundings, condensed water reacted with material, vapour-water reacted with the material, reaction speed, residence time in the convey line, etc

It is most likely, that condensation takes place at the mixing point in the installation.

However, the condensed water is evenly spread over the conveyed fly ash and very low in mass compared to the conveyed fly ash therefore it is not noticeable.

This complies with the experience in cement conveying installations, where the cement even bonds with the condensed water.

The above is easy to calculate.

A simple cooler with condensate drain reduces the amount of condensed water, but is not a must.

The allowable air temperature in respect to the butterfly valves depends on the used seat material.

Special material like VITON can handle up to 200 degrC, which is approx. the compressed air temperature at the outlet of the compressor at 2.5 bar.

Other parts must be fit for the temperature, they experience. Consult the suppliers.

Best regards

Teus ■

The relative humidity (RH) of air depends on pressure and temperature.

Assume the starting condition at 1 bar(absolute) and 25 degrC with a RH=100% (atmospheric).

Then this air is compressed to 2.5 bar(o) = 3.5 bar(a) with a temperature of 200 degrC

The higher temperature causes the air to become dryer and the higher pressure causes the air to become wetter.

The combination of these two effects is air of 0% RH. In fact, the water vapour is overheated.

When this air is mixed with fly ash with a temperature of 40 degrC, then the mixture temperature will be approx. 45 degrC, resulting in a RH of 100 % + 0,00279214 kg/kg condensation.

========================================================================

In case the air is cooled in a pre cooler down to 20 degrC, the condensation is 0,0159382 kg/kg (which is drained) with a RH of 100% at 3.5 bar(o)

When this air is mixed with the fly ash of 40 degrC, the mixture temp is approx 35 degrC and the air has a RH of 41% and no condensation takes place.

have a nice day

Teus ■

Hello Calvin,

Glad to see you making good use of the forum.

I hope Teus will give you further comments on your condensation estimate.

Just things for thoughts:

1. Always use worst case scenario for design. Your compressor may provide max. pressure of 3.5 bar, but the line pressure vary with solid loading. Consider the lowest/highest line pressure and use that to look at how significant the condensation can be at the coolest/hottest mixing temperature.

2. What happens when the line is empty (abnormal operation or during startup)? You would have no ash on the pipeline and hot conveying air close to 158oC blowing into your dust collector (if there is one). So when choosing valves and filter bags, this is to be considered.

From an ex colleague of yours. ■

oulet temp (K) = (inlet temp(K) )[(outlet pressure/inlet pressure) ^[(1.4-1)/1.4]

to calculate the compressed air temp.

inlet temp = 28degC

inlet ab. P = 1bar

oulet ab. P = 3.5bar

outlet temp = 158 deg C

Please advise the equation is right or not. (EQUATION IS OK FOR AN ADIABATIC COMPRESSOR)

Fly ash = 80tph

Compressed air = 33.8 Nm3/min (2028 M3/HR # 1690 KG/HR)

If fly ash temp is 28deg C and 0.8J/kg/K (SPECIFIC HEAT)

100%humidity, air temp is 200deg C, the mixing temp is will be near 34 deg C. (33.3 degrC)

(CORRECT)

Intake air moisture content = 0.024 kg/kg, RH 100%

Convey air at 3.5 bar(ab) and 33.3 degrC, moisture content = 0.0091 kg/kg, RH = 100%

Condensation = 0.024 – 0.0091 = 0.0149 kg/kg

Equals 0.0149 * 1690 = 25.1 kg condensed water per hr

25.1 kg water / 80000 kg fly ash * 100 = 0.031 %

This amount of condensed water is spread out over the surface of 80000 kg of flyash and will pose no problem for the functioning of your pneumatic conveying system.

To branco,

I always enjoy it, when someone uses this forum with results.

When the pressure is lower, due to a lower SLR, the compressed air temperature is also lower.

In addition, the convey air at a lower pressure can contain more water, resulting in less condensation at less fly ash.

Success

Teus ■

A screw compressor with internal compression reaches abt. 200 degrC outlet temperature at approx 2.5 bar delivery pressure.

In my first reply, I was unaware of the compressor type therefore I assumed this figure.

Forget about the 200 degrC.

I did not account for condensation heat.

Calculate the pipeline between the compressor and the pneumatic conveying installation as a heat exchanger.

Whether this will cause condensation, you can check that afterwards.

If you are worried about condensation, install a simple water separator with drain just before the conveying system.

The 2 bar pressure drop over an 8” – 120m pipeline seems very high.

Is there f.i a pressure reducer in between?

5.5 bar at a dew point of 62 degrC forms condensation in contact with a surface of 62 degrC, which is the inner wall. The outer wall must be colder to force the heat transport.

The tank is also cooled by the colder fly ash entering the tank.

If the system is running with no solids transfer, then the air pressure will be much less.

The compression temperature is then also very low.

The dust collector at the end of the pipeline has to be designed for the mixture temperature of the air and fly ash at the end of conveying.

Any other occurring temperature is normally lower.

success

Teus ■

Dear Calvin.wck

Is the screw compressor an oil filled compressor or an oil free compressor?

I assume an oil-filled compressor, because of the high pressure of 5.5 bar.

In that case, the compressed air temperature must be lower because of the cooling effect of the oil and at the same time, the condensed water is drained in the compressor unit.

Why not check the compressor data sheet for temperatures?

Calculating the pipeline as a heat exchanger is calculating the heat resistance of the gas/pipe boundary, the pipe wall and the pipe outer wall/surrounding.

The outer pipe wall transfers heat by conduction and radiation.

Once you have set up the heat resistance equation, the heat loss along the pipeline can be calculated using an integral equation.

The result is then the tank temperature, which you could not know.

Practice has learned that there is no condensation problem in pneumatic conveying installations as described here (even when cement is conveyed).

If you are the first one who experiences such a problem, just install your water separator in the pipeline and inform the forum.

BR

Teus ■