Relationship between flow rate and heat transfer

The greater the flow rate of cold water, the more effectively it can carry away the heat transferred from tap water, thus speeding up the cooling of the tap water! If the flow rate is low, heat cannot be removed in time, and thus the cooling effect will naturally decline! But the cost has to be considered! Find the optimal balance between cost and efficiency!

Both cost and efficiency need to be taken into account

The inlet and outlet temperatures of the frozen water pipe are assumed to be constant. Q = C*M*(T1-T2); the greater the flow rate, the larger M becomes, and thus the more heat can be exchanged. Assuming that the heat exchange efficiency remains constant, the better the cooling effect will be.

The flow rate inside the frozen water pipe affects the outlet temperature, and it also has an impact on the heat transfer coefficient, right?

The heat transfer resistance is mainly concentrated outside the tube, and measures should be taken to enhance heat transfer on that side. Increasing the flow velocity inside the pipe will raise the film coefficient there, but it may not be very helpful for this problem.

I really thank everyone for their answers! However, if I consider the inlet temperature and outlet temperature of the chilled water (which are not very different from each other) to be the same, does that mean it has nothing to do with the flow rate? Thank you all again!

On the contrary, Q = M*Cp*ΔT; assuming that the heat input remains constant and the inlet and outlet temperatures are considered the same, so that ΔT = 0, then the flow rate M must tend to infinity. Therefore, if heat transfer is optimized, ΔT≥3~5°C. I believe the reason why the inlet and outlet temperatures are so similar is that the heat transfer amount Q is very small. What Haiyou said on the 6th floor is correct – the heat transfer resistance is concentrated on the outside of the pipes, so measures should be taken to improve the flow in the water tank, such as installing stirring mechanisms. If possible, please provide information on the quality of the water in the tap water tank, its temperature (inlet and outlet temperatures, or the desired cooling temperature), the inlet temperature of the chilled water, and the flow rate of the chilled water. This way, we can work together to find a solution.

Thank you to all the sea friends for their help! I’ve been very busy lately, so I can get online only late. Thank you very much for everyone's attention. Actually, it’s like this: my company needs to establish monitoring standards for the cooling system of a pulse transformer. The existing design involves a row of chilled water coils that extend into the tap water tank in order to cool the water contained within that tank ； At the same time, a pump is located on the water storage tank to draw out the tap water from the tank and use it to circulate and cool the pulse transformer; after cooling, the circulating water returns to the original tank. Now, any known conditions can be assumed, such as the inlet temperature of the frozen water, t1, the outlet temperature, t2, and the flow rate, m1 ； Water quality M and temperature T of the tap water ； Pump water flow rate in m2, circulating return water temperature T', coil length L, with inner and outer diameters of r1 and r2 respectively. If other conditions are needed, you can add them; since I haven’t measured the specific parameters, I had to use letters as substitutes. Thank you to all the sea friends for attending! I’m at a lower level and don’t have enough permissions to send attachments; sorry about that! It would be much better with a diagram!

In this case, increasing the flow rate to enhance heat transfer effects will not have a very significant impact; it is better to try to increase the heat exchange area, such as by lengthening the pipes or enlarging their diameter

If that’s the case, one can try using finned tubes as the heat exchange tubes

When the flow rate is high, the flow velocity inside the pipe is high as well; the thermal resistance of the pipe wall is low, which increases the heat transfer rate and thus the amount of heat transferred

As mentioned above, one additional point: the chilled water should be below zero degrees, right? Heat exchange is carried out directly through the pipes with tap water; if the flow is poor, an ice layer can easily form on the surface of the pipes, reducing the efficiency of heat exchange. Therefore, if the tap water is stagnant, stirring should be increased.

In this heat exchange process, convection occurs inside the tube as forced convection; outside the tube, it is natural convection if there are no measures such as stirring (otherwise it is also forced convection). The overall heat transfer coefficient K = 1/(1/ai + 1/ao + δ/λ + Ri + Ro), where the values within the parentheses represent the heat transfer resistance inside the tube, the heat transfer resistance outside the tube, the heat transfer resistance through the tube wall, and the fouling resistance inside and outside the tube, respectively. In any case, an increase in the flow rate within the pipe leads to a higher velocity, and as a result the convective heat transfer coefficient inside the pipe increases (turbulence is usually easily reached, and this coefficient is roughly proportional to the 0.8th power of the velocity). The overall heat transfer coefficient also increases, but the extent of this increase depends on the relative values of the convective heat transfer coefficients inside and outside the pipe (this relative value essentially determines whether the heat transfer process is governed by the thermal resistance inside or outside the pipe). If there is no mixing outside the pipe, then once the flow rate inside the pipe reaches a certain value, its effect on the overall heat transfer coefficient can be ignored.