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Re: [EnergyPlus_Support] Questions about the Coil Model
Just to clarify regarding part load. Latent loads are not neglected, but the
PLR is calculated based on the sensible portion of the load delivered divided
by full-load sensible capacity.
Mike
On 24 Mar 2005, at 13:15, Richard Raustad wrote:
>
> See comments below:
>
> deserteagle_zhou wrote:
>
> >
> > Hi,all,
> > I have two questions about the DX Coil Model .
> > 1.For the DX Coil Model -- DX Cooling Coil Model, there is a
> > sentence in the EngineeringDoc.Doc ,¡°The total cooling capacity
> > modifier curve (function of flow fraction) is a quadratic curve with
> > the independent variable being the ratio of the actual air flow rate
> > across the cooling coil to the rated air flow rate (i.e., fraction
> > of full load flow).¡± The ¡° the actual air flow rate across the
> > cooling coil¡± seems to be a mean value in a timestep.What does that
> > mean exactly?
>
> You are correct, the air flow rate reported by EnergyPlus is an
> "average" value for the
> simulation time step when the DX system's fan cycles on and off to meet
> the sensible cooling load.
> This average value is determined by the air flow rate across the cooling
> coil when the fan is on
> multiplied by the fraction of the time step the fan operated. So the
> "actual air flow rate across the
> cooling coil" referred to above means the air flow rate when the fan was
> on, NOT the average
> reported each time step.
>
> > How can I define the "the actual air flow rate
> > " from the manufacture's catalogue data in order to get the
> > modifier coeficient?
>
> Manufacturers provide a rating for air conditioning systems at some
> nominal air
> flow rate and some "rated" inlet air condition. At this nominal air flow
> rate and "rated" inlet air
> condition, the manufacturer states a gross and/or net cooling capacity.
> Be careful to use the
> gross cooling capacity as stated by the manufacturer. If the
> manufacturer only provides a net
> cooling capacity, the fan heat must be subtracted to yield a gross
> cooling capacity
> (gross cooling capacity is higher than net cooling capacity).
>
> For this example, lets say its 1000 W gross cooling capacity at 2 m3/s
> nominal air flow rate and
> at a "rated" inlet air condition of 26.7 C dry-bulb and 19.4 C wet-bulb
> when the outdoor temperature
> is 35 C.
>
> The manufacturer also provides performance data tables at air flow rates
> different from the nominal
> air flow rate. Find the "rated" cooling capacity in the chart and note
> the "rated" inlet air conditions.
>
> So at indoor conditions of 26.7 C dry-bulb / 19.4 C wet-bulb and 35 C
> outdoor temperature we have
> from the performance data table:
>
> Air flow (m3/s) Capacity (W)
> 2.0 1000 *** rating point ***
> 1.5 850
> 1.0 657
>
> Next this data has to be normalized to 1 at the rating point as follows:
>
> Divide the air flow column by the rated air flow (2.0 in this example) and
> the capacity column by the rated capacity (1000 W in this example). Since
> capacity is a quadratic function of air flow I will also show the air
> flow rate squared in the example below. You get:
>
> Normalized Capacity Normalized Air Flow Normalized Air Flow squared
> 1.0 1.0 1.0
> 0.85 0.75 0.5625
> 0.675 0.5 0.25
>
> This normalized data set must then be curve fit according to the equation:
>
> Normalized Capacity = A1 + B1*Normalized Air Flow + C1 * Normalized Air
> Flow * Normalized Air Flow
> --- OR ---
> Noralized Capacity = A1 + B1*Normalized Air Flow + C1 * Normalized Air
> Flow squared
>
> When this is completed you will have calculated the regression
> correlation (I used a familiar spreadsheet) coefficients
> for Normalized Cooling Capacity as a function of air flow ratio (ratio
> of actual air flow to rated air flow) as:
>
> A1 = 0.25
> B1 = 0.95
> C1 = -0.20
>
> The EnergyPlus curve object would then look like this:
>
> CURVE:QUADRATIC,
> CapacityAsAFuncitonOfFlow, !- Name
> 0.25, !- Coeff1 Constant
> 0.95, !- Coeff2 x
> -0.20, !- Coeff3 x**2
> 0.0, !- minimum value of x
> 1.0; !- maximum value of x
>
> >
> > 2. From the formula of PLR, what do "sensible cooling load and
> > steady state sensible cooling capacity" mean respectively? How does
> > the program calculate them(Is latent cooling load neglected)?
> > Any suggestion and guide will be appreciated.Thank you in advance.
>
> The sensible cooling load is the load to be delivered by the AC system
> to meet the thermostat
> set point temperature. The sensible cooling capacity is is the capacity
> of the AC unit when it
> runs continuously. So if the cooling load is 100 W and the sensible
> cooling capacity is 200 W,
> then the PLR is 0.5 and the AC unit only runs 50% of the time.
>
> >
> > Eric
> >
> >
> >
> >
> >
> >
> >
> > The primary EnergyPlus web site is found at:
> > http://www.energyplus.gov
> >
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> > http://groups.yahoo.com/group/EnergyPlus_Support/
> >
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> >
> >
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>
> --
> Richard A. Raustad
> Senior Research Engineer
> Florida Solar Energy Center
> University of Central Florida
> 1679 Clearlake Road
> Cocoa, FL 32922-5703
> Phone: (321) 638-1454
> Fax: (321) 638-1439 or 1010
> Visit our web site at: http://www.fsec.ucf.edu
>
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>
>
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========================================================
Michael J. Witte, GARD Analytics, Inc.
EnergyPlus Testing and Support
EnergyPlus-Support@xxxxxxxx
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