Pressure increase

The pressure at the pump discharge and in the first part of the hose will be a bit higher than before, but the pressure in the downstream part of the hose and the exchanger should be a bit less. If you know the gpm, this could be pinned down a bit closer.

What is the flow?

The flow <5m/s (ie 15 fps) is for pipes (the surface of which will be much smoother). It is to maintain the Re within controllable limit so that the flow is streamlined without turbulence. Also the assumed fluid is water (1 cst)

However in your case you may have to go a bit lower.

UD15

Go ahead, nothing will happen to exchanger. The presure will be in fact slightly less at the exchanger inlet due to more pressure drop in hose because of higher fluid velocity. You should be more worried about new lower size hose.

So long as the hose is not to long [you have a reduction in CSA of 40%] you will see some pressure increase at the pump if the volume delivered remains the same. The velocity of oil in the entry to the heat exchanger will increase with a result of some inefficiency's of heat exchange in this area, but with allowable tolerances in your system it will probably be OK.

Garth thanks for your reply. I have some queries though. What is CSA? you are right about me, seeing some increase on the pump side. How the pressure at the inlet of the heat exchanger? Will it also be higher or would be the same, irregardless of the hose size i use?

CSA is the cross sectional area of the hoses.

Assuming the outlet of the heat exchanger remains the same the pressure will equalise out pretty much the same as if you were using the 1.75" dia hose, it is that you will have more turbulence at the inlet and heat exchangers like laminar flows as much as possible.

At first, is there a must need to reduce the predesigned size of that hose?

Reducing the hose diameter between the pump and heat exchanger from 1.75" into 1.25" (28.6% reduction) shall increases the friction loss (drop in pressure) which will have a bad effect on both flowrate and thermal efficiency of heat exchanger. In addition, this friction loss is a drop in pressure which may be requires a higher pump head to overcome that reduction.

My recommendation - in case of you haven't a logic reply to my first question - don't do it.

If your Hydraulic pump is a positive displacement type, the flow will tend to be the same round the circuit. The power consumption will increase to overcome the pressure increase due to the hose CSA reduction, and will depend on the length of that hose. Also, the pump will then definitely have a prssure relief device and as long as you did not exceed that pressure, the flow into the Heat Exchange will remain the same. The Turbulance effect mentioned will not be different than before the hose change. Because of the increased friction in the hose, there will be a little more heat to dispose off. Don't forget that the oil is non compressible.

If the pump is gear type and not so positive in displacement , there will be a reduction of the flow in the Heat exchange due to the hose change. Also, the pressure at the pump outlet will be higher and if there is a relief valve, it should be checked for the setting. The power consumption will be same or less a little. The pressure at the heat exchange will be less than before and less flow. The heat exchange unit will not suffer, but the amount of heat removed will be less under the same conditions.

Since you have, it seems , already done the hose changing, check the values and experiment to seem if the heat exchange unit is performing and cooling the engine fine....

Thanks Lucke, so am i right to say that even for a positive displacement type, even though there will be a pressure increase in the pump outlet, the pressure inlet to the heat exchanger will be slightly lower when a smaller hose is used?

Most likely. For positive displacement, the same volume must pass through if using oil which is not compressible, and the same rpm.The flow speed will increase in the reduced hose to keep the same volume flow, therefore, the pump outlet pressure will increase (more power to keep the rpm) and the pressure loss in the hose brings the pressure back down at the radiator inlet. The oil will heat up in the hose...etc.

What is the pump? Your cooler should have a by-pass built into it. Why the reduction in hose diameter? Do all your case drain lines go through the cooler? Temp, viscosity, head, volume of resevoir, size/type of exchanger,component placement, distance of travel, and angles, restrictions of piping... all are relative to any modification of original design. What is the power unit, and the reason for the modification? Industrial or Mobile?

i should have been a bit more clearer...i have attached a document. I would like to find out how P1 compares with PA, P2 with PB, P3 with PC and P4 with PD in relative terms ( higher, lower...)...Thank you.

Well this changes the scenario from the first impression.

In this case, PA, PB and PC will be higher than the P1, P2 and P3. PD will be <= to P4 depending on the pump's displacement being absolutely fixed or suffers some backflow due to the pressure ahead. Don't forget that you have reduced both hoses diametres and we don't know the length which will also affect the pressure increases.

Agreed. Before, we were told only of the hose reduction from the pump to the heat exchanger; now we have a smaller hose after the heat exchanger as well. I still don't think this will overpressurize the exchanger, BUT WHAT IS THE GPM? (Sorry for the brief shouting, but this, as well as any of the initial design pressure drops, etc., would be good to know.

This seems a strange one and there are all sorts of good ideas.

My 2p's worth

Why are you reducing the hose size? Pressure drop down a line is a 5th power of diameter and so the change to 1.25" means 5 x the pressure drop through the hoses. (CSA = cross sectional area)

Your flow velocity seems a bit high. In this service I would go for 3 - 5m/s ie 10 - 15 fps, but this is a red herring in terms of allowable pressures.

You have a gear pump which is pretty much a positive displacement pump. In which case it MUST have a relief valve on the discharge. I would sincerely hope that the setting of the relief valve on the pump discharge was the same as or less than the design pressure of the heat exchanger.

If the relief valve pressure is below or equal to the exchanger design pressure then you have no worries regarding over pressure of the exchanger.

However you will cause a whole slew of other problems in changing the hose size. You say its a closed loop but your drawings don't close the loop so its bit hard to say. I assume that exiting the motor the oil goes to a sump tank which is atmospheric pressure.

How is the oil flow into the motor controlled?

I am guessing that the gear pump runs at a fixed speed delivering a fixed flow of oil. In this case PD will be much the same as P4 because you have the same flow through the motor to the same sump pressure and so from the motor inlet the conditions are unchanged. However the pressure drops in the hoses are increased so all the other pressures are increased and you will have a higher pressure at PA

There is an issue with slippage back through the gear pump with higher discharge pressures so you may not deliver exactly the same flow to the motor. (At higher discharge pressures some flow slips back through the gears of the gear pump so you don't deliver 100% of the capacity). Provided this pressure is below the relief valve setting on the pump discharge and the motor is big enough for the delivery (I would expect both these to be true) you should get the same oil delivery

Its not clear what sort of exchanger you have. The oil will be arriving at a higher velocity down the smaller pipe so the entrance to the exchanger will be less effective but provided there is sufficient volume to slowdown the flow I don't expect the exchanger to suffer too much.

Overall I think you should be ok so long as there is the meat in the pump motor. But you need to look at the gap between the operating pressure and the design pressure.

The discharge RV on the pump will protect the equipment in the circuit. The RV must be the same as or lower than the equipment pressure. Also you need to check the design pressure for these hoses.

Overall though I would agree with the earlier poster. Why are you changing the hoses?