Chilled Water Cooling Coil Behavior

[FL Engineer]

I've been wondering how a specific coil behaves under conditions other than its design conditions, and particularly relating to part-load conditions and the change in the sensible heat ratio (SHR) of the coil. If anyone has any good reading materials on this, I'd be happy to do my own research. I could also understand this better if anyone has any generic coil performance tables that cover a wide variety of conditions that I could analyze.

So let's take a look at a scenario for example. We have a single-zone variable-air-volume system with a minimum outdoor air requirement for ventilation. A separate fan for the OA intake is ensuring that we always have our 200 cfm for ventilation. Let's say at design conditions the total (mixed) supply air is 1000 cfm with an OA component of 200 cfm (20% OA). Let's also assume that our OA is 93 DB/79 WB and our return air is 75 DB/63 WB. This gives us a design supply air of 1000 cfm at ~78.5 DB/66.5 WB (0.0112 humidity ratio) for the coil, which will cool the air to about 54 DB/54 WB.

Now at part-load conditions our supply air fan slows down. Let's say our supply air is now 800 cfm. With our OA remaining at 200 cfm, we now have 25% OA and our supply air is 800 cfm at around 79.5 DB/67.5 WB (0.0117 HR). If our supply air drops to 600 cfm, we have 33% OA for 81 DB / 69 WB (0.0124 HR).

Now, I realize that at all of these conditions, the total enthalpy of the airstream decreases along with the total supply air, so our total capacity is lower, but the SHR also decreases as our relative latent load increases. I also believe that the SHR of the coil's performance decreases since the relative surface area of the coil increases with a lower supply airflow (I don't have a great reference for this, so please correct me if I'm mistaken here). So will the coil be able to maintain it's 54 DB / 54 WB setpoint for all conditions as we decrease airflow from 1000 cfm to 200 cfm, while maintaining a constant 200 cfm OA? If not, at what point will the coil fail to maintain it's setpoint, and why? Can a different coil be selected knowing that the coil will have to experience a wide range of conditions? I'm just unclear on the interaction between coil geometry and its SHR at various conditions in general, and haven't been able to find any good materials to read up on the subject, so I've come to you guys for clarity.

This post has gotten long enough so I'll leave it at this, but I'm also curious about things like how the coil's performance changes in response to reduced chilled water flow from a modulating valve (again particularly interested in SHR).

Thanks.

 

[DrRTU]

You are generally correct on coil performance based upon air flow but may be extrapolating the data too far off the performance curve. Your worst case makes the coil 100% OA at low flow. I doubt you will be able to achieve your desired wb over the complete range w/o reheat or face/bypass. I suggest you run the coil selection on all conditions. I guess you will not get a valid selection due to low air or water flow.

https://www.trane.com/content/dam/T...ptimization/APP-CMC054-EN_course material.pdf

 

[FL Engineer]

Thanks DrRTU, I'm going to review that link. I actually think I found the accompanied lecture on Trane's youtube channel.

Can you elaborate on what you meant by "I doubt you will be able to achieve your desired wb over the complete range w/o reheat or face/bypass."? Thanks.

 

[quark]

In a mixed condition, the enthalpy difference between coil inlet and outlet conditions is approximately 9.3 BTU/lb and with 100% OA, it is 20 BTU/lb. So, the coil has 2.5 times more capacity when working with OA (9.3*5/20). This is not your problem. Looking at your coil exit and return conditions, you have a moisture load of 0.0007 gr/lb of air. if you want to remove this much moisture with 20% of flowrate, you need to cool down OA to 40F, then reheating is needed. If RH is not a requirement, your scheme is ok.

 

[ChasBean1]

FL Enginner, nicely articulated post!

You noted that the enthalpy decreases as your total airflow decreases, and this is correct. It drops from 32.3 Btu/lb at the 1000 cfm condition, down to 31.7 Btu/lb at the 800 cfm condition, etc.

With the decreasing supply airflow and the constant outdoor air component (200 cfm), I would anticipate that your cooling coil will be capable of maintaining the effluent dry bulb temperature that you mention, 54°F, which means your dew point will be below 54°F, which means you’ll be able to dehumidify as you need under all conditions.

 

[FL Engineer]

I'm still not clear on why some posts here say that the air would have to be cooled to below 54°F and then reheated. I apologize if I'm missing something obvious here. Would anyone mind drawing that process on a psych chart to illustrate it? Cooling to 40° then reheating to 54° would result in approximately 54/46 DB/WB. I do understand that reheat is necessary to dehumidify the ventilation air without over-cooling the space.
(Edit: I guess what I don't understand about the reheat thing is that you have to pass the 54/54 condition to get to the 40°F condition, right?)

Just from looking at a psych chart, I'd came to the same conclusion that ChasBean1 stated, I was just wondering if there are any barriers due to the coil geometry (or other factors) that could prevent cooling air with a high humidity ratio (or grains/lb) to the ~54/54 condition even if the total enthalpy of the air-stream is lower.

 

[ChasBean1]

Time out: "Cooling to 40° then reheating to 54° would result in approximately 54/46 DB/WB." No. Cooling to 40 then reheating to 54 would result in a 40°F dry bulb. And a max dew point of 40°F. What's missing is what your space needs..

 

[FL Engineer]

@ChasBean1 Well I'm assuming that by cooling the air to 40° that it will be essentially saturated so basically 40/40 DB/WB. If you reheat to 54° from that point then you are around 54/46 -- and yes, as you stated your dew point would be 40°F at this condition.

In most of my typical applications a 54/54 supply air condition is sufficient, and there is no need to further reduce the moisture content of the air. The percent of outdoor air (or the coil entering air temperature) shouldn't affect that. Or am I still missing something here? I left out space needs since this is more of a general question and that seemed like extraneous information, but in most applications I'm aiming for a 75/63 room air condition with approximately 50% relative humidity and a dew point which would prevent sweating diffusers.

 

[ChasBean1]

You’re wasting a hideous amount of energy.

Your room conditions need you to cool the supply air to 55°F, not lower.

 

[FL Engineer]

Yes, that's my point. I was wondering why other posters brought it up.