Chilled Water Coil – Confusion About Delta T and Heat Transfer

[Josef M.]

Hi everyone,

I'm a mechanical engineer currently working as a site manager. From time to time, I need to do some HVAC-related calculations, and this particular one is driving me a bit crazy.
I'm hoping someone can help me understand what I'm missing here.

I have a chilled water coil system with inlet and outlet water temperatures, as well as air-side inlet and outlet temperatures across the evaporator.
Here's what's confusing me:
If the chiller lowers the absolute temperature of the water but keeps the ΔT (temperature difference) the same, the amount of heat removed (Q) stays the same—assuming a constant water flow rate.

For example, with water going from 10°C in to 12°C out or from 20°C in to 22ºC out, the heat transfer comes out the same. That doesn’t seem intuitive to me, especially if the air temperature around the coil is constant.
So my questions are:

• How is it possible for the same amount of heat to be removed in both cases, assuming the environment and airflow are the same?
• Why don’t we consider the air temperature around the coil in these calculations?
• If the surrounding air is, say, 25°C, wouldn’t colder water remove more heat due to a larger temperature difference, even with the same ΔT? Shouldn’t that mean you’d need less water flow to achieve the same Q? How is the same flow, with lower temperatures removing the same heat? if the temperature difference is bigger to outside, and the time that water goes through is the same, how can it be that the Q removed is the same?

Sorry if this is a silly question—it’s more about curiosity than an urgent issue. I'd really appreciate any insights.

Thanks!

 

[MintJulep]

I think theres a fourth - Heat removed from the water by the chiller package.

The thread discussion is about an air-to-water coil, so that's where I drew the imaginary dashed system boundary line in my head. But yes, if you want to understand the entire system then you need to include the chiller and condenser, etc.

Item 3 to me is simply the bit between items 1 and 2.

Yes, but it's a passive bit, so its performance is a function of the inlet and outlet temperatures of both streams (and other things).

The amount of heat that it can exchange between the two streams is proportional to the LMTD of the streams.

Consider the (hopefully obvious) limiting case of air inlet temperature = water inlet temperature. Heat transfer = 0.