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Solair
12-09-2007, 10:07 PM
Hello.
Anyone familiar with "Priniciples of refrigeration" by Dossat and Horan ?.
On page 373 it gives frictional losses in equivalent feet of pipe for "sudden enlargement " on a reducer.

Why does the tables show a longer length in feet for an enlargement then for a contraction ?.
Surely the pressure drop is lower here and the equivalent length in pipe should be shorter.
Can anyone explain ?.

Lowrider
13-09-2007, 09:05 PM
Your way of thinking is wrong.

In an expansion the pressure drop is larger since it expands because of the larger diameter (and thus area) so the total drop is larger.

In a contraction the pressure there is some pressure needed to contract, but unless speed is really high this is a small amount!

Solair
13-09-2007, 09:17 PM
Thanks for the reply.
Why then does it go on to explain on the same page and I quote " Consequently, as the pressure drop across the discharge line increases because the pipe diameter is to small, the discharge pressure also increases".

It also states " The pressure drop generated across the discharge line is reduced by selecting a larger pipe diameter for the application ".


It has always been my thinking that larger bores reduce pressure drop and smaller ones increase pressure drop.
Are the tables wrong in this case ?

Lowrider
13-09-2007, 09:24 PM
well it must be the fact I don't speak English by nature. But any way, since the line is too small the gas get's to much drag (resistance) from the walls and thus the pressure will rise as will the pressure difference. I think pressuredrop is a wrongly used term in this, but again it could be in the language!

And indeed a smaller pipe will have a smaller pressure difference then a larger one if the same amount is going trough it.

The Viking
13-09-2007, 09:46 PM
" Consequently, as the pressure drop across the discharge line increases because the pipe diameter is to small, the discharge pressure also increases".


Aha!
A nice discussion about minute shades of the English language.
Good, as a foreigner, I will enlighten you.:D

Look at the above sentence again but this time visualise a compressor with a length of pipe attached to the discharge port.
Where do you measure discharge pressure?

Now you see why a small bore pipe will increase both resistance and pressure.

(If you didn't get it, the discharge pressure is measured by the compressor, the increased resistance in the pipe will reduce the pressure downstream but increase the pressure up stream of it....)

Lowrider
14-09-2007, 07:30 PM
No need to explain about the pipes, I understand it, the language is the problem!

Am I wrong in thinking pressure drop is not a suitable name for it?

Solair
16-09-2007, 03:32 PM
Can anyone shed any more light on this ?.

To give one example in the tables for nominal 2" pipe a sudden enlargement of 1/4" gives rise to 8" of equivalent length of pipe where as a sudden 1/4" contraction gives rise to only 4" feet of equivalent feet of piping.

Why does increasing the pipe diameter result in a greater pressure loss according to the tables ?.
Many thanks.

paul_h
16-09-2007, 04:45 PM
Can anyone shed any more light on this ?.

To give one example in the tables for nominal 2" pipe a sudden enlargement of 1/4" gives rise to 8" of equivalent length of pipe where as a sudden 1/4" contraction gives rise to only 4" feet of equivalent feet of piping.

Why does increasing the pipe diameter result in a greater pressure loss according to the tables ?.
Many thanks.
In the first example (2" into 2-1/4") gives a pressure drop equivalent of an extra 8" length. eg doing that expansion means the refrigerant has more pipe to expand in, so loses pressure, the same as extending the pipe another 8"
The second example shows that making the pipe smaller would affect the piping less, as it would be a restriction, (equivalent of a 4" extension) but wouldn't upset the balance so much as there's no expansion of refrigerant.
edit: how much pressure is out of your garden hose? (bare hose, not with a nozzle).How much presure is out of your garden hose 2 metres away after the water has expanded from the outlet? How much more is there if you restrict the outlet a bit?

Solair
16-09-2007, 05:07 PM
Thanks for the reply.
I think you have misunderstood my question.
The tables in "priniciples of refrigeration" (are you familiar with this book ?) relate to pressure drop created by friction.

These tables ( ashre) indicate that a particular fitting such as a reducer ( sudden enlargement or sudden contraction), connection or valve has as much friction as a piece of straight pipe of the same diameter, having a length equal to the number listed in the tables.
This means that when a table gives say an equivalent length for piping of 8 feet then the pressure drop has increased and increases the load on the compressor accordingly.

Why do the tables indicate a larger pressure drop (ie more friction ) for a sudden enlargement ?.
My thinking has always been when you increase the pipe size then you reduce friction hence reducing the load on the pumping mechanism.

Solair
16-09-2007, 05:47 PM
Here are the said tables -.

http://i127.photobucket.com/albums/p140/tricksybucket/Pressuredrop2.gif

Solair
19-09-2007, 12:07 AM
Please can someone help.

http://www.refrigeration-engineer.com/forums/showthread.php?t=9485

Are the tables incorrect ?.
Thanks for your time.

Solair
19-09-2007, 01:08 AM
The information on page 50 in this pdf link would appear to conflict with what Dossat and Horan have published. :confused:

http://www.mcquay.com/mcquaybiz/literature/lit_systems/AppGuide/AG_31-011_01082007.pdf

paul_h
19-09-2007, 11:09 AM
Thanks for the reply.
I think you have misunderstood my question.
The tables in "priniciples of refrigeration" (are you familiar with this book ?) relate to pressure drop created by friction.

These tables ( ashre) indicate that a particular fitting such as a reducer ( sudden enlargement or sudden contraction), connection or valve has as much friction as a piece of straight pipe of the same diameter, having a length equal to the number listed in the tables.
This means that when a table gives say an equivalent length for piping of 8 feet then the pressure drop has increased and increases the load on the compressor accordingly.

Why do the tables indicate a larger pressure drop (ie more friction ) for a sudden enlargement ?.
My thinking has always been when you increase the pipe size then you reduce friction hence reducing the load on the pumping mechanism.
There is a pressure drop when you expand a pipe because whatever is in the pipe has more space available to take up. Hence a pressure drop expressed in the extra metres of the smaller pipe it's equivalant too.
The larger pipe doesn't improve flow, because it can only flow as much as the smaller pipe feeding it can flow, hence the result is the same flow, but at lower pressure.
the pdf you linked is talking about tee joins, not reducers.
Dossat talks about friction being one of the causes of pressure drop, but that's not the only source of pressure drop, expansion causes a pressure drop too.

ella
19-09-2007, 04:18 PM
Why not to ask author? You definitely can find his contact in the book. Or look at the internet. This is regular practice to provide readers option to send feedback.

ella
19-09-2007, 04:21 PM
Why not to ask them? You have a link to publisher.

Solair
29-09-2007, 10:57 PM
Just a quick update.
I contacted ASHRAE and a technical services engineer has forwarded a note to the technical committee regarding the special fittings losses tables.
So hope to receive an answer in the next couple of weeks.

Pooh
01-10-2007, 01:11 AM
Solair
I beleive you are getting mixed up between pressure drop due to frictional losses and and pressure drop due to increase or decrease in pipe size.

In a length of pipe the pressure only drops due to frictional losses due to the length of pipe ie. the pressure will be higher at the start of the pipe than at the end however if you reduce the diameter of the pipe the decrease in pressure after the reduction will be reasonably small in comparison to increasing the diameter due to the refrigerant being able to expand to take up the extra space available. Look at your gas laws, can't remember which one but it is p1 over v1 = p2 over v2 or something like that. I will look it up when I am sober and post the full answer.

Ian

Ravi
01-10-2007, 05:15 AM
Paul and Pooh, both answered your question. The pressure drop in a fitting is (conventionally) the sum of frictional losses due to the fitting length and the pressure drop due to directional change (incase of turbulent flows). The directional change may lead to vortex formation and thus causes further pressure drop. The directional changes are more predominant in an expander as the stream lines are pulled apart.

However, great confusion arises when you calculate pressure drop in a piping system as per equivalent length method. As you consider the fitting losses equivalent to some pipe length and then use Darcy's formula, it looks (superficially) like the entire pressure drop is due to friction alone. There is great redundancy in this method and a slightly better and predominantly used method is Crane's k-factor method (Refer to Crane's Technical Paper 410). You treat linear lengths with Darcy's equation and losses in fittings as a fraction of velocity head, in this method.

However, the k values are constant for any given pipesize and Reynold's number and this contributes to significant errors when flowrates are high. Hooper (2-K method) and Darby (3-K method) further modified presure drop formulae and they are the latest methods that can be used to calculate pressure drop emperically (How far they stand against the CFD analysis is a different discussion altogether). Refer to Chemical Engineering Journal for the above said methods, if you are interested.

Solair
03-10-2007, 12:24 AM
Thanks for the replys but with all due respect I shall await the outcome from the ASHRAE technical committe as the Engineer who contacted me has forwarded my query to them.

Pooh
03-10-2007, 12:32 AM
Solair
sorry mate why dont you just say we all dont know what we are talking about, why bother asking a question if you are not going to beleive the answer given especially taking that there are some of the most qualified and experienced refrigeration professionals in the world posting on this site.

Ian

Solair
03-10-2007, 01:13 AM
Sorry if I have caused offence Pooh but the engineer I made contact with at ASHRAE seen fit to forward my query to the ASHRAE technical committee and if they confirm what you and others have stated then I shall apologise.

Solair
09-10-2007, 12:01 AM
I have had some time to study through the postings given here and its starting to make sense now. Many thanks and I apologise if I offended anyone.


Just came across these tables.

http://www.iklimnet.com/expert_hvac/piping_eq_length.html

They show quite a wide variation in the data with regard to sudden contractions and sudden enlargements when compared to the earlier tables by ASHRAE that I posted.
Why are flow velocities not considered in these tables when applying the equivalent length in pipe to these losses ?.

Kind Regards Solair.

Frudge it
04-11-2007, 01:11 PM
Hi Solair ,
Reading your problem , You have my full sympathy.I trained on a Dossat Manual - "Principles of Refrigeration" too.I had the same problem you are having with dossat , and overcame it this way.Pressure is requred to overcome Resistance.Pressure drop is the loss of pressure in any given pipe diameter Or Length caused in overcoming resistance to flow in a pipe. Resistance is Inversely Proportional to Pipe ID Surface Area , at any given Pressure / Gas Velocity , therefore the Greater the Area of the Pipe ID , the lower will be the resistance.Taking any given fitting with a Small ID in one end and a Large ID in the Other end. , our terminology of " enlargement " and "Contraction" merely changes in response to the "Direction of flow" into / out of the fitting.The fittings behaviour & characteristics of Pressure loss / drop function according to the Increase / Decrease of ID surface area & resultant resistance.Perhaps overelaborate BUT if you grasp the basics this way , ALL types of situations ca be unravelled quickly in the fittings area.Hope it helps...