Heat Exchanger Sizing Question 1

[Sam M]

Im looking to purchase a heat exchanger to cool oil in circulating lube oil system.
This will be cooled by the shop water line.

I had initially suggested these types.

https://www.mcmaster.com/heat-exchangers/heat-exchangers-5/heating-cooling-capacity~560000-btu-hr-/

https://www.mcmaster.com/heat-exchangers/heat-exchangers-5/heating-cooling-capacity~280000-btu-hr-/

Heat Capacity = 70HP/170,000 BTU
Oil Flow Rate = 21 GPM
Shop Water Flow Rate = 15 GPM
Water In = (60F - 90F)
Water Out = N/A (Water Will be Dumped out)
Oil In = 150F
Oil Out = 100F (Going Back into the system)

What is a good-sized heat exchanger for this application?

 

[MintJulep]

Once through water to drain for cooling is irresponsible.

 

[Compositepro]

Once through cooling with city water is illegal in many US jurisdictions and can get very expensive.

 

[LittleInch]

Why water and not air cooled?

Heat capacity of your oil?

Look closely on how they've calculated the heating and look at your flows and temperatures which seem to be higher or lower than mentioned in the website listing.

For off the shelf HXs, you need to be within their stated limits or it won't work.



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[EdStainless]

Do the math, for a simple one shift operation this is about 70,000gal/yr
If you work 6 days a week x 3 shifts it is more like 200,000gal/yr
Even when I worked places with private wells we used cooling towers and recirculated the water.
There are plenty of helps out there to size the HX.
The biggest issue is the variation in cooling water temperature.
Whether this is morning to afternoon or season to season it is a big factor.
There are different ways to deal with this, from multi-speed or variable speed pumps to multiple HX in parallel.
You need to hire someone that know how to do this.
There are skid mounted pre-engineered packages available with cooling tower and HX bundled.

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[Sam M]

Thank you all for the feedback.
Ideally my boss would like to use water for oil cooling.
What additional information is needed to help narrow down some options?

 

[Gr8blu]

Sam M Be aware - even if your local jurisdiction allows once-through flow on your secondary coolant (i.e., water side), there will be limits on the maximum temperature of that coolant as it exits the heat exchanger back into the environment. It is not uncommon for this restriction to be in the range of 3 C (5 F) above the inlet temperature. Note that if you opt for a recirculating secondary system, this severe constraint goes away (somewhat) as the cooling tower will generally handle a wider hot-cold variation.

Converting energy to motion for more than half a century

 

[EdStainless]

You would still use water, just not dump it.
You need a small cooling tower.
These are usually fiberglass (or some other non-metalic) and they have a fan and a circulation pump.
Some of them have a two speed pump motor so that you have high and low cooling options.
This circ pump would run water through the HX to cool the oil.
You need to calculate the heat load to size the cooling tower and then you can size the HX.
Yes, it is more complex. And the cooling tower will require some ongoing makeup water and chemical treatment.



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P.E. Metallurgy, consulting work welcomed

 

[georgeverghese]

If your cooling shop water supply can be at 90degF, then getting oil cooled down to 100degF wont be possible unless you choose a full countercurrent plate heat exchanger (and even then it may not be easy). Besides, shop water will have to be clean and free of solids if you choose a compact plate HX. This may perhaps be the most you can expect - temps in degF :
Option A - 2(or more) tube side pass TEMA U HX
Water in / out : 90 / 100
Oil in / out: 150 / 110

Option B: - Full countercurrent shell and tube HX
Water in / out : 90 /100
Oil in / out : 150 / 100

Option C : Full countercurrent compact wide plate HX - clean non fouling service only on both sides.
Water in / out : 90 / 100
Oil in / out : 150 / 95 to 100

For option C, a compact spiral HX could handle a more fouling service, and can be cleaned also. Allow some 5psi dp minimum for water side and same for oil side.

 

[Sam M]

I appreciate the feedback - i tried using the basic heat exchanger equation but the unknown was obviously the exit water temperature. The shop water does not quite get to 90 F - That was more of a guess of ambient temperature. Ill play with the numbers but i think the missing input to the heat exchanger equation would be the limitations of how hot exit temperature can be on the water per city specs.

 

[LittleInch]

Still going for once through complete waste of treated water?

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[Sam M]

Thats right complete waste - my city regulations are 95F max for waste water.

 

[Christine74]

The first thing you need to do is an energy balance to determine your water outlet temperature. This is useful because it tells you whether you need countercurrent flow or if mixed co-current/countercurrent flow is possible as you would have with a multi-pass shell and tube heat exchanger. It also tells you the required temperature approach.

Just eyeballing this it looks like you need pure countercurrent flow so a small fixed-tubesheet shell and tube heat exchanger might work but you'd probably need somewhere around 200-300 sq. ft. of surface area.


-Christine

 

[jari001]

To address the calculation method part of the question, using the NTU method is my advice when an outlet stream temperature is not known.

Having the water max temperature be 95°F and a situation where the cooling water is anywhere near 90°F is going to be challenging, as others have mentioned. I'm surprised your boss doesn't want to investigate air cooling and I think it may be necessary to show why it should be considered given that it may be a more cost effective solution to your problem. Did your boss give a reason for discounting air cooling?

I'm assuming you're confident that your drain pipe can't handle 95°F with no problems in their current state because someone did a routine borescope inspection within the last 5-ish years (forgive me, it's a recent sore subject).

 

[MJCronin]

Consider the purchase of a small used STHX. Ensure that it is cleaned, painted and HYDROTESTED, as part of the sales agreement

Your temperature demands are modest ....

It sounds like the junkyards would be full of HXs in the size and materials you desire

MJCronin
Sr. Process Engineer

 

[Sam M]

Hello all,

Thank you for the feedback.
In doing the basic heat exchanger equation, i came up with the following.
Using my original temperature values and having a max runoff water temp of 95F (Also replacing inlet water temp with 85 F max)

Q (Heat Transfer Rate) = 175,000 BTU/Hr
U (Heat Transfer Coeff) = (25 - 250) BTU/Hr*F*Ft^2 (Common measurements I found online for my situation)
A (Heat Transfer Area) = Unknown
dTm (Log Mean Temp Diff) = 30.79 F

∴ A = Q/U*dTm

A(Umax) = 28.41 ft^2
A(Umin) = 227.34 ft^2

Looks like Christine74 was fairly spot on.
Am i on the right track?

Best Regards,
Sam M

 

[georgeverghese]

Oil mass flow = 4050 kg/hr, assuming rho = 850kg/m3
Oil Cp = 2.5kJ/kg/degC, assumed
Oil dt = 27.8degC
Duty = 4050 x 2.5 x 27.8 = 281 600 kJ/hr = 267 000 Btu/hr

 

[LittleInch]

80kW of heat rejection.

What happens now or is this a new installation?

What is cooling the rest of the machine?

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[davefitz]

air cooled would be more environmentally sound, but provisions should be made for fouling due to accumulated dirt , and tube spacing to allow external cleaning with high pressure water jets . At most power plants the air cooled is initially provided but often replaced with water cooling to improve reliability but at an environmental cost, this lesson is learned after siezing and destroying bearings that overheated due to overheated oil. The cooled oil stream should have thermocouples wired to an alarm to alert operators that it is time to clean the HX.


"...when logic, and proportion, have fallen, sloppy dead..." Grace Slick

 

[EdStainless]

Dave, we had equipment that ran an air cooler (fin-fan) followed by a water cooled HX.
During low load or cool weather the F-F was plenty of cooling (in cool weather even with the fan off).
When the outlet temp was too high the water flow valve opened to get us additional cooling.
This kept both units small and it minimized water use.

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, consulting work welcomed