4 replies to this topic

[PE123]

Hi all,

I am in the process of evaluating the suitability of a twin lobes roots blower for a ultra low temperature application.

Suction Condition
Pressure = 1.13 kg/cm2A
Temperature = -65 degC
Mass flow = 350 kg/hr (Rated)

Discharge Conditions
Pressure = 2.1 kg/cm2A (info by vendor)
Temperature = 14 degC (Info by vendor)

Capacity Control Method
As the mass is varying, there is kick-back valve to maintain a min flow thru the blower
At the min flow, the discharge temperature is 100 degC ( Info by vendor )

Above the background of my project.
However, I have 2 concerns

1) Any possibility of the blower seizing up due to thermal expansions of the lobes ?
2) Any possibility of the lubricating oil freezing up due to the ultra-low temperature ?

What can I check to ensure that the above 2 scenario do not happen ?


Thanks for sharing your experience
fsk
PE

[Dazzler]

Hi fsk
These are my thoughts.
Your main source of advice should be the vendor in this case. Hopefully they have a good reputation.
A. You could ask the vendor for reference to a similar application that they have provided for.
B. The blower may need grease rather than oil. You could ask for the technical datasheet of the recommended lubricant, then you can discuss it together.
C. Some metals may have smaller coefficients of thermal expansion. Ask them about the materials and how they were chosen and judged to be suitable for -65 up to 100dC.
D. I wonder if they are counting on some constant internal or external recirculation flow to provide a little heat to keep the average inlet temp higher than your process supply flow at -65dC, therefore making materials selection easier for them.
E. If your application is extreme, it may be worth asking the vendor also if they anticipate an special or restrictive operating procedures - such as slow ramp up/down to/from full capacity so as to avoid thermal shock.
F. Get a proposal from another vendor of similar equipment as a technical comparision.
Good luck.
Dazzler

[agorag]

Always start with a schematic diagram. Also state the gas compo, polytropic index, compressibility ratio, speed of driver & driven rotors, lobe dia, shaft dia at bearing, type of shaft sealing.

Roots blowers are constant volume (positive displacement) machines, and are comparable to screw compressors. So, refer API619 for better understanding of the specifications.
From your data, pressure ratios appear OK. But for this compression, the temperature rise appears to be quite high.

> You obviously need a material that is strong enough at this temperature. API619 has materials and material test requirements (charpy test) for such low temperature service. The upperside of the temp doesn't seem concerning.
> Since the gas is already at -65C, I dont understand your concern on lubrication. There are hundreds of lubricating oils that wouldn't form wax and are meant for low temp service.
> Since you donot have a cooler in the recycle line, you should be more concerned with the shaft seals at the low flow condition and look for ways to 'quench' the recycled gas with the process gas.

[PE123]

Dear agorag,

Thanks. I have uploaded my schematics.
API 619 is certainly useful for me.

1)Regarding my concern on the lubrication, it is precisely because the temperature is low that I am concerned about the lubrication freezing up. The pour point of some low temperature service lubricant in my company is ~ -41 degC. This brings the possibility of the lubricant freezing up when handling gas of such low temperature (-65 degC).

2) As some schematics show, I do have "quench" cooler. To cap the discharge temperature at 100 degC, a min load of 270 kg/hr has to be maintained by the recycle stream.

I am pushing for a roots blower to be implemented because being positive displacement in nature, risk of cavitation is zero as compared if I uss a centrifugal compressor. Also the pressure ratio (~2) is low, thus a roots blower should suffice.

However, there are some concerns about the reliability of a roots blower for such a low temperature ethylene application, (freezing of lubrication, blower seizing etc )

Any more advice on roots blower is welcomed.
thanks

Best Regards
fsk

Attached Files

•  4875_001.pdf   254.73KB   24 downloads

[agorag]

I dont see a reason for you to be concerned about. Low temperature carbon steel should be specified (I think ASTM A 35x series) for the casing & rotor. As stated, charpy tests should be specified.
There are many grades of dewaxed lubricating oils & greases(Mitsu, Total, Mobil...) that have the capability to work at these subzero temperatures.
Check the blower manufacturer's credentials. Depending on the load at the bearing, they must be able to recommend the right lubricant. Depending on the shaft dia, grease may also be specified by the manufacturer, since the speed is low. Since you've not provided adequate details, a better reply is not possible.
For the sake of discussion, in this case, temperature at the bearing will be much higher than the gas temperature and is hence not a problem.