Parallel flow through a sensor

[MacomMech]

This is my first post, hopefully I have the correct forum.

I am trying to fit a sensor that can take a maximum flow rate of 4 l/min into a lubrication system that has a 60 l/min flow. The easiest way I can think to do this is to design a simple manifold and hook the sensor up parallel to the main flow and put a restriction in the main flow causing back pressure to push oil through the sensor. Where I am struggling is arriving at the dimensions of the restriction, calculating flow through the sensor as temperature varies, and arriving at an overall pressure drop.

Main Flow 60 l/min
Oil Viscosity 320 cSt at 40oC
Sensor pressure drop 1bar at 320cSt oil
Sensor tube ID 4mm
Main Flow Inlet 1.5" 38.1mm


Thank you

 

[BigInch]

Initially size the sensor line with a x-sectional area proportionally smaller to the flow, ie. sensor tube area 4/60 x mainline area. Use a flow equation to check the pressure drop in the tubing, then size the orifice to give the same pressure drop in the parallel segment of mainline.

http://virtualpipeline.spaces.msn.com

"What gets us into trouble is not what we don't know, its what we know for sure" - Mark Twain

 

[jmw]

However, take care to allow for temperature changes with corresponding viscosity changes and consequent changes in pressure drop.
As each path may have different flows the amount of heat loss from each may differ. Thus in the lower flow rate line if it loses heat more readily than the higher flow rate line the viscosity in that line will be higher, the headloss will tend to increase or the flow rate to decrease. A drop in flow rate will allow further depression of the temperature.
This is more often a problem with higher viscosity fluids where even small temperature changes can result in significant viscosity changes.
You may need to look at insulation or at using some kind of flow regulation. What you need to do is target equivalent temperatures in each flow path when the flow rate is optimum.

JMW

http://www.ViscoAnalyser.com

 

[MacomMech]

There was a link in another thread to a bypass loop calculator. Over the past week I have been punching numbers into it and looking at the results.

BigInch the sensor tube diameter is fixed, but from my research over the past week:-

To get the oil to flow through the bypass the pressure drop in the mainline between the bypass loop has to be equal to the pressure drop in the bypass loop including the sensor. This can be done by restricting the flow in the main line.

If I was to use a pump in the bypass, then it has to be sized to overcome the pressure drop of the bypass loop. Pressure drop in the main line will remain relitively unchanged.

I know this is pretty basic stuff, but my fluid mechanics lectures 22 years ago I'm sure didn't cover anything as useful as this. Thanks guys.