Chilled Water Cooling Coil Behavior
Chilled Water Cooling Coil Behavior
(OP)
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.
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.





RE: Chilled Water Cooling Coil Behavior
RE: Chilled Water Cooling Coil Behavior
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.
RE: Chilled Water Cooling Coil Behavior
RE: Chilled Water Cooling Coil Behavior
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.
RE: Chilled Water Cooling Coil Behavior
(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.
RE: Chilled Water Cooling Coil Behavior
RE: Chilled Water Cooling Coil Behavior
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.
RE: Chilled Water Cooling Coil Behavior
Your room conditions need you to cool the supply air to 55°F, not lower.
RE: Chilled Water Cooling Coil Behavior