# Re: [EnergyPlus_Support] Questions about the Coil Model

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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
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:

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
>
>
>
>
>
>
>
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--
Senior Research Engineer
Florida Solar Energy Center
University of Central Florida
Cocoa, FL  32922-5703
Phone:   (321) 638-1454
Fax:     (321) 638-1439 or 1010

UCF - From Promise to Prominence: Celebrating 40 Years

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