4 replies to this topic

[Cwsrt]

Hi everyone!

 

 

Our new plant has two sets of air driers (100% sparing). We are now studying what operating scheme will be the best for our air driers considering desiccant life, equipment reliability, and start-up duration of the spare unit in case of tripping. Our options are as follows:

 

1. One set on stand-by and switch manually when problem occurs on the set in service.

 

2. Regular and time-based switching between the two sets.

 

3. Run the two sets in parallel (2 x 50% Operation).

 

Personally, I think the third scheme is better since one drier set will be capable of running at 100% in case of tripping or maintenance of the other one. For the desiccant life span, it think it will be the same for both #2 and #3, since the desiccants will be exposed to less moisture at 50%. One disadvantage I can think of is that the driers will be more prone to contamination since both will be operational.

 

I would like to consult you regarding this concern. I'm also not sure if my assumptions are valid.

 

*Note: Switching of driers would require manual opening of the drier's isolating B/V. 

 

Thanks in advance for your replies.

 

Regards,

CWSRT

 

 

 

 

 

 

[Bobby Strain]

Congratulations! You are among the few who can properly spell desiccant. But dryer is the noun for your equipment. Drier is a condition of less moisture.

 

Bobby

[curious_cat]

How long will your desiccant last before it needs regeneration?

 

To me Option #2 sounds the only viable plan. Especially in #3  won't both dryers be spent sort of at once? You might be in a soup then if this is a continuous process. 

 

Unless this is a very long cycle process with very infrequent regeneration. 

[ankur2061]

CWSRT,

 

The scheme #3 has its own disadvantages. I can think of the following:

 

a. Desiccant's have an operating life. That means desiccants need to be replaced after a few thousand hours due to their deterioration. Operating the 2 dryers simultaneously at 50% load gives you less control on monitoring the condition of the desiccant. There is every possibility that the 2 desiccant beds may end up deteriorating simultaneously requiring replacement for both the beds at the same time. This could prove to be an operational problem as well as a logistical problem in maintaining inventories of the desiccant material. 

 

b. Sudden and drastic load changes are also detrimental to the performance and life of the desiccant bed. For example, if for any reason you need to shut down one bed and you quickly ramp up the load on the parallel operating bed it could cause air drying performance problems as well as shorten the life of the bed.

 

c. If the air quality from the drying system is not acceptable, then it is difficult to pinpoint which desiccant bed is not performing since you are operating both beds in parallel at equal loads.

 

IMHO, options #1 or #2 are better suited and are generally the norm for operating air dryers. Automatic switching from one dryer to another has now been commonly adopted when moisture specifications in the air need to be strictly controlled. This could be time based, depending on the expected behavior of the desiccant bed over a period of time or it could take a signal from the moisture analyzer installed at the outlet of the dryer system to switch from one bed to another.

 

Hope this helps.

 

Regards,

Ankur.

[Cwsrt]

Thanks for the insights Ankur and curious cat!

 

I think you have good points in identifying the operational disadvantages of operating the driers in parallel. We'll certainly take that into account when deciding on the air dryer operating scheme.

 

Thanks also for the brief English lesson Bobby. It also took me a while before getting used to the spelling of "desiccant".

 

Best regards,