Double suction risers

[Peter_1]

In case of a double suction riser with the possibility of a reduced capacity, many literature describes that the smallest tube is calculated for the smallest capacity and the biggest one for full capacity.
What's wrong with and is it even not better to calculate the smallest one for the smallest capacity but for full capacity taking both lines for the return of the gass?
So dividing the full flow over both lines so that both have optimal speed.

The more I go back to the basic, the more I see things which I doubt more and more.

[taz24]

In case of a double sutcion riser with reduced capacity, many literature describes that the smallest tube is calculated for the smallest capacity and the buggest one for full capcity.
What's wrong with and is it even not better to calculate the smallest one for the smallest capacity but for full capacity taking both lines for the return of the gass? So dividing the full flow over both lines so that both have optimal speed.

The more I go back to the basic, the more I see things which I doubt more and more.

I thought that the purpose of the double riser was to get oil back. If that is the case then increaseing the size of the risers will effect the oil flow?

Cheers taz.

[Peter_1]

It's indeed done this way for proper oil return.

See page 11, typical arrangements under 2. http://www.heatcraftrpd.com/resource...s/H-IM-64L.pdf

[US Iceman]

...better to calculate the smallest one for the smallest capacity but for full capacity taking both lines for the return of the gass?

So dividing the full flow over both lines so that both have optimal speed.

This is the same way I see the problem Peter. At part load the larger line (with the P trap) is sealed off with oil. Therefore, the gas and oil have to rise up the smaller riser.

When the load/capacity increases, the higher gas velocity pushes the oil out of the P trap and now you have gas and oil returning through both risers.

Since these are risers and primarily used for oil return, the total cross sectional area of both risers may be smaller than one line sized for low pressure loss.

You have to pay a penalty for using risers (single or dual), so it may be better to spend more time on where to place the equipment. But, I think you already know this.

The more I go back to the basic, the more I see things which I doubt more and more.

You cannot believe every thing written in books, that's why it is important to understand the basics. Sometimes, the people who write the books don't fully understand what is being done.

[Peter_1]

Sometimes, the people who write the books don't fully understand what is being done.

Ahhh, are you refering to someone

[US Iceman]

Ahhh, are you refering to someone

Not directly....

[taz24]

It's indeed done this way for proper oil return.

See page 11, typical arrangements under 2. http://www.heatcraftrpd.com/resource...s/H-IM-64L.pdf

Sorry Peter you have lost me!!
I do not know if you asked a question or made a statement.
From your conversation with Iceman it seems you were making a statement?

taz.

[Peter_1]

Well Tazzy, it was in fact a question because I read it different in several publications( also in my old school books) from what I expected it should be.

It seems for me right sizing the smallest one for the smallest capacity but then at full load choosen in such a way that both lines have optimal speed for the proper oil return.

It's because I have to teach these in class next Tuesday and also in the course the pupils (is this the correct word) have, it's explained line A for minimum load and line B for max load.
So I want to tell it the right way.

[US Iceman]

It seems for me right sizing the smallest one for the smallest capacity but then at full load choosen in such a way that both lines have optimal speed for the proper oil return.

This is the the way I would do it. The actual size of the two risers is determined by the capacity reduction of the compressor. The smallest riser is for minimum load, while the larger riser is sized for full load minus the small riser capacity.

Also, it's important to note the volume of the trap in the large riser. The oil charge of the compressor has to be increased by a similar volume. On large traps, this may be an issue you have to deal with.

Using VFD's and risers would be an interesting experiment though.

...also in the course the pupils (is this the correct word)

That's right Peter. Students would be another word to use.

[monkey spanners]

Just checked in Dossat book pg449 and states that "The small diameter riser is sized for the minimum load condition, whereas the combined capacity of the two pipes is designed for the maximum load condition." which agrees with what you think is right.

Cheers Jon

[taz24]

Well Tazzy, it was in fact a question because I read it different in several publications( also in my old school books) from what I expected it should be.

.

I agree with you Peter.
Sorry to throw confusion at you
Cheers taz.