[Equest-users] CHW coil model documentation
Erik Kolderup
erik at kolderupconsulting.com
Wed Jan 29 10:17:33 PST 2014
Aaron,
Thanks very much for your detailed response. Looks like my case is one
where the default equest CHW coil curves don't work. I do have a fairly
high CHW dT of about 20F. So the actual conditions are 40F CHWS and about
60F CHWR at design load. And as noted below I have constant airflow and a
48F SAT setpoint. I suppose I would need to come up with a new CAP-FFLOW
curve to give realistic flow at low load.
In this case I did my own "post-processing" flow calculation based on some
points from the manufacturer's coil selection software. The chart below
shows how those results compared to the eQUEST results with default coil
curves in this case.
[image: Inline image 1]
So just a heads-up to folks to take a look at hourly CHW flow and CHWR
temperature values to make sure they are reasonable in your model.
Otherwise pump energy may be significantly underestimated.
-Erik
*Erik Kolderup, PE, LEED AP*
erik at kolderupconsulting.com | 415.531.5198 | www.kolderupconsulting.com
On Mon, Jan 27, 2014 at 5:33 PM, Aaron Powers <caaronpowers at gmail.com>wrote:
> Erik,
>
> I think you'll find the DOE2 documentation a little lacking as model
> descriptions are concerned. Relevant keywords can be found in the Volume
> 2: Dictionary reference roughly from pages 394-398 (March 2009 version).
> The documentation are good at describing how to use the program and how
> keywords relate to the program, but there really hasn't been an in depth
> description of the models or underlying algorithms since the DOE2 Engineers
> manual, which was released in 1982 for version 2.1a. Consequently, a lot
> of this document is out of date, especially with respect to the water side
> equipment.
>
> For almost all its HVAC heat transfer components, DOE2 uses empirical
> curves rather than models based on the NTU method, for example. This
> empirical method has been shown to be superior to other alternatives in
> many cases.
>
> In particular, there are three main empirical curves which are used to
> control cooling coil valves. These three expressions relate the coil
> capacity as a function of entering air wetbulb, entering CHW temperature,
> airflow, and CHW flow. The product of these three curves (for a given WB,
> CHWST, airflow, and waterflow) multiplied by the design cooling capacity
> gives each hourly operating capacity. Mathematically, it looks like:
>
> [image: Inline image 3]
>
>
> The * in the above equations denote a normalized value with respect to
> design conditions. These together with a few extra air-side calculations
> and two energy balances complete the coil model. The entering wetbulb is
> essentially imposed on the system, so, on an hourly basis, the CHWST, cfm*,
> and gpm* are selected to match the hourly capacity with the hourly load.
> The control scheme determines precisely how this is accomplished. It's
> important to note that these curves are normalized to compute to 1.0 at the
> following values:
>
> WBa,in = 65 deg-F (not the ARI condition)
> CHWST = 44 deg-F
> cfm* = 1.0 (i.e. 100% design flow)
> gpm* = 1.0 (i.e. 100% design flow)
>
> There are some details I've left out, but this is how it works from a high
> level.
>
> Now more specifically about your situation. As you've stated, it's a CAV
> system with a specified CHWST, so DOE2 has only the ability to modulate
> water flow through the coil to match the load. Another tricky part of
> these curves is that they are not accurate over all inputs. For instance,
> if you were to plot the default curve for cap3* above, you'd see that the
> coil still has about 25% of its capacity even with zero flow. This is
> something that would not happen if you used the NTU-effectiveness method.
> Thankfully, DOE2 keeps the inputs in range, but it doesn't warn you, and
> I'm not sure it's ever mentioned in the documentation. Of importance here
> is that DOE2 linearly interpolates between 30% and 0% flow for the cap3*
> equation above, and it only allows wetbulbs in the range of 59 to 73 for
> the cap1* equation above.
>
> Just to have some numbers to work with, let's say your coil has a design
> capacity of 100,000 Btu/hr and a design water dT of 10 deg-F, which gives a
> design CHW flow of 20 gpm. As you stated, your conditions require 60 deg-F
> entering air temp (as controlled by t-stat), constant airflow, and 40 deg-F
> entering water temperature. Now let's say that you're at a very low load
> like 10% of design, or 10,000 Btu/hr. Letting the entering WB = 59 deg-F,
> the first two capacity curves immediately evaluate to:
>
> cap1* = 0.94541
> cap2* = 1.0 (it's a CAV system)
>
> So to meet the load, we need the value of cap3* to satisfy:
> 0.94541*cap3* = 0.1
> In other words, we need to select gpm* such that cap3* = 0.10577. This
> corresponds to a gpm* of about 5.3% or a gpm of 1.06. At 10,000 Btu/hr, a
> gpm of 1.06 will produce a dT of around 18.9 deg-F across the coil. So you
> can see how large or even physically impossible dT's can happen.
>
> I'm baffled though as to how you're able to get 80 deg-F coming off the
> coil. The only way I can create this condition is by setting the design dT
> to something like 18-20 deg-F. Maybe if you post your inp someone can find
> where this is coming from.
>
> Good luck,
>
> Aaron
>
> ---------- Forwarded message ----------
> From: Erik Kolderup <erik at kolderupconsulting.com>
> To: equest-users at lists.onebuilding.org
> Cc:
> Date: Fri, 24 Jan 2014 12:23:53 -0800
> Subject: [Equest-users] CHW coil model documentation
> Anyone know where to find documentation of DOE2.2's chilled water cooling
> coil model? I see keywords listed in the help system for the CHW model,
> such as the CHW-CAP-FEWBEWT curve, but I haven't found any documentation
> about how all the curves are used together to calculate coil performance.
>
> I ask because my hourly outputs show physically impossible results. I have
> a single CHW coil with 2-way valves. At low load the CHW flow drops, which
> is expected. But it drops so much that the reported leaving water
> temperature is far higher than the entering air temperature. For example, I
> get 80F water leaving the coil when the entering air temperature is only
> 60F. The heat balance works out when comparing the air-side and water-side
> loads based on the hourly outputs. But the leaving water temperature is
> much too high, i.e. flow too low.
>
> For reference, the system has 40F CHW entering the coil, 48F air leaving
> the coil (hospital operating room), and constant airflow.
>
> Thanks for any insights.
>
> *Erik Kolderup, PE, LEED AP*
> erik at kolderupconsulting.com | 415.531.5198 | www.kolderupconsulting.com
>
>
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