Temperature rise of a coolant - Generators / Motors 3

[NickParker]

I understand the concept of temperature rise in windings, but I’m unsure about the temperature rise in a generator’s cooling system. There are two key temperatures involved: the cooling fluid’s inlet and outlet temperatures. The difference between them is the temperature rise of the coolant.
My understanding is that if this temperature difference is large, it means the cooling effect is higher, and if the temperature difference is small, the cooling is less effective.
The specification states that a minimum temperature rise of 10K is required for the cooling fluid, but the vendor's datasheet shows a coolant temperature rise of 15K. Based on this, the 15K provided by the vendor should be better than the 10K specified, Is my understanding correct?
Additionally, I’ve noticed some motor specifications that limit the cooling water outlet temperature to a value like 37°C, and also restrict the coolant temperature rise to a certain level, like 7K.

Why do the specs limit the cooling water outlet temperature, and why do they restrict the temperature rise to specific values like 7K?

 

[waross]

Higher numbers may indicate that the winding is overloaded.
By higher numbers I mean the actual temperatures, not the specified limits.

--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[zeusfaber]

My understanding is that if this temperature difference is large, it means the cooling effect is higher, and if the temperature difference is small, the cooling is less effective.

That is sometimes true. It depends what happened to cause the change.

If something happens to reduce the coolant flow (doesn't matter whether the blades fell off the circ pump impeller, thousands of baby mussels grew in the coolant pipework or one of the lads throttled down on a flow valve to "improve" the delta T), you will see an increase in the temperature difference between inlet and outlet, accompanied by a reduction in cooling effectiveness.

On the other hand, if something happens to insulate the coolant from the heat source, then your understanding holds true.

 

[Gr8blu]

Insufficient flow = higher temperature rise in coolant (more time exposed to heat)
Insufficient pressure = higher temperature rise in coolant (more time exposed to heat)

External factors which may limit allowable coolant differential between "in" and "out":
1) whether the system recirculates, or is once-through
2) whether the "in" temperature is ALREADY "high" relative to the material it's trying to cool
3) whether the coolant is a PRIMARY or SECONDARY coolant in the system

For most rotating machines, a liquid (like water) is a SECONDARY coolant because it does not directly contact the parts to be cooled. That is done by the PRIMARY coolant (usually air). Temperature rise in a machine is calculated relative to the PRIMARY coolant. If the "hot" secondary coolant is 37 C, the minimum PRIMARY coolant temperature will be roughly 8 C higher - which means the PRIMARY coolant minimum temp (also called "ambient") is 45 C. Note that this is higher than the industrial "standard" maximum ambient condition of 40 C.

If the secondary coolant is not a liquid but is instead air, the differential between hot secondary and cold primary coolant may well be 15-20 C, with correspondingly higher "ambient" conditions for the machine's internal components. This is becuase the ehat exchange mechanism for air-air is not as effective/efficient as liquid-air.

Converting energy to motion for more than half a century

 

[che12345]

'...The specification states that a minimum temperature rise of 10K is required for the cooling fluid, but the vendor's datasheet shows a coolant temperature rise of 15K. Based on this, the 15K provided by the vendor should be better than the 10K specified, Is my understanding correct?
I have the following opinion for your consideration.
1. Machine A. The "heat loss" is say X kW . Cooling fluid dt (T[sub]out[/sub]-T[sub]in[/sub])= 10 K; at certain volume and flow rate.
2. Machine B. With the same volume and flow rate, the cooling fluid dt is 15 K. That is, machine A is of lower heat loss (kW). Or machine A is of higher efficiency.
Che Kuan Yau (Singapore)

 

[NickParker]

If the "hot" secondary coolant is 37 C, the minimum PRIMARY coolant temperature will be roughly 8 C higher - which means the PRIMARY coolant minimum temp (also called "ambient") is 45 C. Note that this is higher than the industrial "standard" maximum ambient condition of 40 C.

.
I came across an old discussion thread here,

where it mentions that the maximum allowable outlet temperature (API 541) is 49°C. Does this imply that the cold primary coolant (air) temperature is approximately 56°C, which seems significantly higher than the typical ambient temperature of 40°C?

Does a high temperature rise in the water cooling circuit indicate high efficiency of the heat exchanger, or does it suggest high heat loss from the motor?
I'm confused now.

 

[FacEngrPE]

To state the responses above a bit differently.

From Pumps and Systems website

TEWAC (Totally Enclosed Water to Air Cooled, IC81W, IP54/56)
The motor internals are cooled by air circulating between the heat exchanger and the rotor / stator. The heat exchanger is cooled with water or some other heat transfer fluid.

On a large motor purchase I would specify the motor manufacturer to select a heat exchanger that respects the thermal limits of their motor's insulation class, and other design constraints, when selecting a heat exchanger per API 541. (Disclaimer I have not actually purchased one of these, I have purchased the ODP (Open Drip Proof, IC01, IP12) version).

TEAAC (Totally Enclosed Air to Air Cooled, IC611, IP54/56)
Has a much larger heat exchanger, Air is not a good a heat transfer fluid as water.

In all cases the energy rejected from the motor is unrelated to the temperature of the circulating cooling fluid. Energy rejected, and carried away by the cooling system is related only to the motor's electrical (and possibly mechanical) losses.
The purpose of the cooling system is to keep the insulation temperature below the point where it's service life would become unacceptable. Per NEMA MG-1

 

[edison123]

Design of electrical machines cooling system is more of an art than science. The only goal of any air/hydrogen (primary coolant) to air/water (secondary coolant) cooling system is to have the machine inlet air/hydrogen temperature under 40 deg C, the limit defined by standards.

In air/hydrogen to water coolers, the lower the inlet water to outlet water temperature difference, the better is that cooling system. In general, outlet to inlet temperature difference doesn't exceed 5 deg C in a well-designed heat exchanger.

Cooling of hot water from the heat exchanger to cold water is an additional integral part of cooling system design.

When the inlet temperature of primary coolant (air/hydrogen) does not exceed 40 deg C, then any temperature rise of the winding to their rated class falls under machine designer's scope.

Muthu

http://www.edison.co.in

 

[Gr8blu]

OP: The liquid coolant differential is dependent on a couple of things.
First - how much differential can the overall system take? What I mean is - is the system recirculating, such that the "hot" liquid from this part of the process becomes the "cold" liquid for the next part? If so, chances are that the allowable differential is small (on the order of 3-5 C). Likewise, for certain environmental considerations (such as expelling "hot" coolant into a sensitive liquid environment such as a small stream or bay), the limit may be artificially lowered.
Second - how much flow and pressure are available for the liquid side of the circuit? A reasonable rule of thumb is that operating at 100 cfm per kW of loss on the primary "air" side will result in an air temperature rise of 18.3 C between in and out. Similarly, a flow of 1 USGPM per kW of loss will result in roughly 3.2 C rise in the liquid. These are close approximations based on the thermal properties of the coolant materials.

If I saw a specification that listed 49 C as the max allowable output temperature on the liquid side, I'd be designing for an inlet air temperature in the 55-60 C range. And I'd be asking the spec writer if they made a typographical error. Ending up with a 17 K differential means the coolant circuit is only using about 20 percent of the "normal" flow - which is a very inefficient process.

Converting energy to motion for more than half a century

 

[NickParker]

a) If the system operates as a single-pass (non-recirculating) setup, the temperature rise can be "unrestricted", constrained only by the flow rate, pressure, and the heat exchanger's material limits. Is my understanding correct?
b) From API 541 - How does specifying the minimum temperature rise as 11k minimizes the use of cooling water?

 

[Gr8blu]

@Nick:
If the system operates in "single pass" mode, exit temperature may still be constrained. For example, putting too much heat into a local environmental area such as a stream or lake (and thereby killing all the fish).

Minimum allowed temperature rise (of the liquid coolant) corresponds to MAXIMUM flow (volume and/or pressure).
Maximum allowed temperature rise (of the liquid coolant) corresponds to MINIMUM flow (volume and/or pressure).

Converting energy to motion for more than half a century

 

[FacEngrPE]

In this case the maximum coolant temperature rise is also constrained by the thermal limits of the motor windings. Coolant flow rate is much less constrained.

 

[edison123]

API 541 table

those pressures should be ≤ and not ≥.

Muthu

http://www.edison.co.in

 

[NickParker]

Maximum allowed temperature rise (of the liquid coolant) corresponds to MINIMUM flow (volume and/or pressure).

Is the high temperature rise in the water cooling circuit specified to lower pressure in the tubes to prevent leakage or to conserve water usage?