Heat gain of AC pump motor vs DC 1

[cdxx139]

Hello Electrical Engineers,

We are changing out 3 600 HP DC motor pumps and changing them over to AC. My job is to provide the cooling in the room for this massive heat gain, which comes out to about 500,000 Btu/hr, which may require a 50 ton chiller @ 1.2 kW/ton is 60 kW.

The existing pump house with the DC pumps do not have any cooling system. The DC rectifiers are in another building.

The client is questioning if we dont have a cooling system now with DC motors, then why do we need them when we change them out to AC motors.

My electrical is weak, so if there is an explanation, primer or white paper that I can be directed to, to help, it would be appreciated.

knowledge is power

 

[BrianPetersen]

Are you assuming that the entire power output of the motors is being converted into heat?

If the motors will be doing the same thing in future that the existing motors are doing now, the heat loading situation shouldn't be any different.

The only way there will be an increase in heat loading is if the efficiency is poorer with the new drive system than with the old one, and if that's the case, why are you doing it?

Client's statement is on the right track ...

 

[itsmoked]

The new setup should be considerably more efficient. Often with pumps that large there is so much cold water, well coupled to the room, that when the motors are running the water is moving and it's extracting truckloads of heat so no additional cooling is needed and, in fact, you may need a coat to hang around long.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[Compositepro]

Electric motors are 95%+ efficient in converting electrical energy to mechanical work. In this case the work goes into moving water. Eventually all of the energy becomes heat, but all of the work energy ends up as heat in the water not the room. Unless the water circulates in a loop that never leaves the room, this heat is a non-issue. Only the 5%- losses from the motors heats the room.
Whether the motor are AC or DC has no bearing on the thermodynamics of your system.

 

[jraef]

Are there drives involved? You say the DC "rectifiers"' are in another building, so are these pumps running full speed all of the time then? If so, is the DC speed a lot lower than the AC motor speed so they are adding VFDs in the same room as the pumps? 1800HP worth of VFDs would reject somewhere around 75-100kW of heat but could the drives (if any) be where the rectifiers were?

If no VFDs are involved in this, I'm inclined to agree with Keith that you are leaving out the cooling effects that the fluid through the pumps and piping will provide (assuming you are not pumping hot water).


"You measure the size of the accomplishment by the obstacles you had to overcome to reach your goals" -- Booker T. Washington

 

[iop95]

DC motor use natural cooling and no need a separate cooling system.
You need to put in same room with AC motor an VFD. So, need cooling for it.
At 5% total loss, as you calculate, need about 500.000BTU/h cooling, when working at nominal power. Need to check efective power needed/used by pumps and pump program/duty.

 

[waross]

Assuming that the ambient temperature in your location is not expected to rise above 40 degrees Celsius, (104 degrees Fahrenheit), you may be able to ventilate. Ventilation is a lot cheaper that cooling both for initial cost and for running cost.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[GroovyGuy]

Hi ccdx,
The losses from 3 x 600hp AC motors should be no more than 76,000 BTU/hr each (or 228,000 BTU/hr total).
Where does the 50T of AC come from? I would have thought that (say) three 6T Bard wall-mounts should be more than adequate to cool the motor room.

If the motor/pump room is not equipped with AC now, I see no reason why AC is required because of a motor change out. BTW, there must be some existing means of heat removal currently installed (ie inlet or exhaust fans).

Are all three pumps expected to run at the same time? I would have thought that one would be an installed spare.

Per waross's comment above, unless these pumps are installed in a very hot clime, you can cool the motor room with air-changes. The motor's will be rated for 40C ambient temperature. I have also noticed that in (most) water pump rooms a lot of heat is removed due to the presence of the cool water piping, pumps, etc.

Regards,
GG

 

[cdxx139]

Thank you for the insight, Here is additional background that will answer specific questions

GROOVYGUY & COMPOSITE PRO & BRIAN PETERSON- I used ASHRAE formulas for heat gain of a motor. See attached. Equation (4) is when the motor is in the conditioned space but the driven machine is outside. (A majority of the heat goes into the water being pumped in my example. All 3 pumps run at once. I am using 2000 HP to also cover additional smaller pump loads, electrical equipment etc. Using 1.0 for both motor use factor and motor load factor. I am using 90% for motor efficiency until the final pumps selections are made. This should be closer to 93% for 600 HP motors, but will finalize when actual new pumps are selected. This calculates to over 500k Btu/hr. Using the other 2 equations, the heat gain would have been over 5 million BTU's.

WARROSS - The outdoor air requirement would come to almost 60k CFM of outdoor air, assuming a 9 degree delta (104F - 95F). The underground structure does not allow this amount of air to be brought into and exhausted as a possibility.

IOP95 & JRAEF - VFD's will not be used.

ITSMOKED - I was able to credit about 200k BTU/hr with Fourier's Law on thermal conductance of a hollow cylinder. I am not sure how realistic this credit is, but even with it, there will still be a significant heat gain into the space (over 300k BTU/hr).

What I am being told, is the reason the current space with DC motors is not being cooled, is because the heat into the space happens at the DC motor rectifiers, which are located in another building, and can no longer be there; which is one of the reasons for this project.

I cannot fathom the space not requiring cooling with these size motors.





knowledge is power

 

[GroovyGuy]

Howdy cdx,
90% to 93% efficiency is way too low for a 600hp motor. Depending upon the number of poles it should fall between 95% to 96%. If you do in fact have a 93% motor, don't forget to evaluate the life cycle costs, including the cost of energy to operate the motor and cool the space. BTW, how many poles are these motors?

I fail to understand why, if cooling is currently not required for the DC motors, why it will be required for the AC motors? I would have assumed that the difference between the motor efficiencies would be minimal, and hence the losses should be similar.

What are the min/max outside air temperatures? Don't forget that year round cooling will be required. AC units don't like to work much below 0C, although with hot-gas bypass you may be able to run a little lower.

Regards,
GG

 

[cdxx139]

You are correct Groovy,

I just found a chart that said minimum efficiencies for 500 HP motors should be in the 95% efficiency, so I update my calculation to reflect that and am more line with your estimate.

I also fail to understand why cooling is currently not required, but we both did the calculation and its too significant to ignore.

My max outdoor air temp used was 95F. We are suggesting to tap the process water being pumped as a cooling medium for an AHU to cool the pump house.



knowledge is power

 

[LionelHutz]

The DC motor losses are not limited to the rectifier. The DC motor itself will have internal losses similar to what an AC motor has and these losses are already be contributing to the heat load in the space. So, you should concentrate on trying to figure out if the current DC motors efficiency is higher then the AC motors efficiency which means there could be an issue. I just quickly looked up a Baldor 200hp DC motor and the efficiency peaked at 92% so I'm doubting the 600hp DC motors are more efficient then the replacement 600hp AC motors.

I just don't see why this motor change would require more cooling for the installed space unless the DC motors are not cooled by the air in the space (ducted or liquid cooled for example).

 

[jraef]

I don't see it either. To me you have two paths here:

1) You run your calculations and end up adding cooling, which will not do harm to the equipment but consumes energy and budgets, maybe for no good reason other than assuaging some nagging doubts.

2) You accept the empirical evidence in front of you, that the existing motors do not require added cooling, so as long as the replacements are the same (or better) efficiency, there is no way they will now need cooling.


"You measure the size of the accomplishment by the obstacles you had to overcome to reach your goals" -- Booker T. Washington

 

[itsmoked]

Are the DC motors still running the show? Have you been in the room with them while they're running steady state? This situation cries out for a look-see. There could be something you haven't been told that would make a big difference. Earth cooled walls, some air turnover? It could be very cool air, etc., etc. All the dams I've been in are pretty chill even small rooms with huge transformers with no cooling supplied, were not 'warm'.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[waross]

The efficiency of a DC motor assumes a supply of Direct Current and does not include the rectifier losses.
If you have been given an efficiency figure for the system including DC motor and rectifiers, this percentage will be correspondingly lower.
There is something wrong with your numbers.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[cdxx139]

Thank you for the input

Lionel - we don't have the existing pump efficiencies. But from ASHRAE equation (a governing international engineering body), the existing pump would need to be over 98% to not require space cooling. I don't know pump motor efficiencies, but from the age alone of the pump motors, this seems impossible.

I will ask the Process engineer if perhaps the DC motors were water cooled? I am at a loss.

Jraef - I have to lean towards #1 from an engineering standpoint. To accept the evidence, even when it goes against the physics, just seems irresponsible.

Itsmoked - The DC are still running, but only for a couple of hours, once or twice a year, with no long term schedule of when the next running will be.

Maybe I need to look at the credit I was applying for the large pipes with cool water running through them.

Waross - I have not been given any motor efficiency numbers, maybe due to the age of the pumps? Almost 100 years.


knowledge is power

 

[jraef]

"Maybe I need to look at the credit I was applying for the large pipes with cool water running through them."

One issue you may be missing is that in motors, a significant portion of the heat is in the rotor, which can pass into the shaft. In fact on smaller motors designed as TENV (Totally Enclosed Non Vented), the shaft is the ONLY method of removing heat from the motor. On average it's considered that 25% of a motor's total losses are what are called I[sup]2[/sup]R losses (resistance heating effects) in the rotor alone, and this rotor percentage tends to increase with motor size because the I[sup]2[/sup]R losses decrease in the stator. So on a direct drive / close coupled* pump, the water flow through the pump (volute and impeller) acts as a heat sink for the motor shaft, which is a heat sink for the rotor and if the rotor is cooler than the surrounding air because the water is so cold, it can also remove heat from the stator. In other words the cooling effect is more than just the pipes in the room. From the outset, >25% of your losses are going to be removed by the pump itself, then more by virtue of the pipes in the room.

To illustrate Keith's earlier point, I was once in a pumphouse in Prescott, AZ with a single 800HP well pump running full speed. We were there for commissioning, but a forest fire broke out nearby so they had to run the pump and we just stuck around to watch it all day. By afternoon the outside temperature was over 100F but after running the pump that long in our little cinder block building, the room got cold enough that we all wished we had brought jackets! (Nobody thinks of doing that on a hot day in Arizona).

*If it is belt driven for some reason, this would not apply.


"You measure the size of the accomplishment by the obstacles you had to overcome to reach your goals" -- Booker T. Washington

 

[itsmoked]

Excellent point Jeff!

I recently bought a small completely silent pump for an aquarium application. You literally can not hear a thing while it's pumping 20gal/min. The primary way they managed to do this was thru no fan, no openings, but the shaft and the shaft side of the pump just being metal touching the pumped fluid. Interestingly running that pump raised the 300 gallon tank 2 degrees F.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[jraef]

Yep. My hot tub pump motors use totally metal pumps (as opposed to plastic, which would be more corrosion resistant), because it aids in heating the water by utilizing the pump motor heat. My heating element is dead right now and they are hard to replace, so I disconnected it and heat the tub up by running the motor all day when I know I'm going to use it. In the summer, it's more than enough heat transfer to hold the tub at 102F. Gotta get it fixed before it starts getting cold though...


"You measure the size of the accomplishment by the obstacles you had to overcome to reach your goals" -- Booker T. Washington

 

[GroovyGuy]

Hi cdx,
I think that you just answered the mystery, wrt the cooling issue;

" The DC are still running, but only for a couple of hours, once or twice a year"

Obviously the pumps (motors) do not run long enough, or frequently enough, the heat up the room. If the future duty cycle is similar to the past, perhaps you do not need to worry about it?

Regards,
GG

"Wish I didn't know now what I didn't know then." -- Bob Seger