# Curve Coefficients

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Hello,

For the curve COOL-EIR-FT the two inputs for a water loop heat pump system are "entering wetbulb temperature" and "entering water temperature". I understand that the water temperature is the condenser water, but is the entering wetbulb temperature for the evaporator side?

John Grando LEED AP

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I'm basically trying to figure out how eQUEST comes up with the total Electric consumption for a DX coil. I thought it was the below

Total Capacity * Capacity Curve [T] * Total EIR * EIR Curve [T] * EIR Curve [PLR] but it's not coming out exactly the same. Does anyone have any input for what I'm missing. The attached .xls is an example for one day. The total capacity is 79.983 kBTU/Hr and the EIR is 0.2722.

John Grando LEED AP

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Hi John,

understanding is that COOL-EIR-FT's temperatures are the outside drybulb
(at the condenser/heatpump) and the entering wetbulb temperature at the
evaporator (mixed air conditions). This mirrors how packaged equipment
efficiencies/capacities are presented in manufacturers' literature. See
attached discussion for an example.

I understand COOL-CAP-FT and COOL-SH-FT curves work from the same
temperatures.

To your second question... I'm with you!

I'd like to see the equation for "hourly electricity consumption" for an
air-cooled DX process written out in its entirety showing all the
affecting - I was searching for something analogous to [Volume 2:
Dictionary > HVAC Components > CHILLER > Chiller Energy Consumption] for
airside DX cooling systems a while ago and also came up empty-handed. I
have a hunch the effects of the 3 bypass curves (which are a mystery to
me) may have something to do with the hourly calculated total capacity
and/or EIR though... perhaps those need to work their way into your
equation?

Regarding the hourly results you're using to check your work... I'm not
sure whether those are the correct figures to work from, but I'd caution
to be mindful of the wonky effects of altitude on calculated
capacities/airflows when working with hourly outputs - recent discussion
on the list brought more to light on this topic - and the effects are
more pervasive than I originally thought...

~Nick

NICK CATON, E.I.T.

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Joined: 2011-09-30
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John

You may have already looked over what the 2.1 manual has to offer as far as
explanation on this.

IV.71

H. Calculate the cooling part load ratio.

Some central systems control the operation of the central cooling
coil based upon temperature signals from the zone thermostats
(COOL-CONTROL = WARMEST). Other central systems control the
operation of the central cool ing coil based upon the coil exit
air temperature (COOL-CONTROL = CONSTANT, RESET, or SCHEDULED). In
the latter case, the controller for the cool ing coil has a
throttl ing range. As the temperature of the air leaving the coil
rises and falls within the controller's throttling range, the
controller varies the operation of the coil, increasing or
decreasing the output of the- coil. The action of the controller
must be simulated because it will have an effect on the coil
surface conditions and thus also on moisture removal by the coil.
Because this controller responds to the temperature of the air
stream, the sensible part load ratio of the coil or unit will be
calculated. This value will then be used the following hour to
modify the coil exit air temperature as a function of controller
set point, coil capacity, and controller throttling range
(see DKTEMP algorithm description).

First, the program calculates the total and sensible cool ing
equipment capacities (QCT and QCS)

where

QCT = COOLING-CAPACITY * QCMl (IV.178)

and

QCMl = CVAL(COOL-CAP-FT,EWB,EDB) [COOL-CAP-FT is a correction
function to the total cooling capacity to adjust for off-rated
entering wet-bulb and entering dry-bulb temperatures]

and

QCS = COOL-SH-CAP * QCM2 {however, for direct expansion units, QCS
=
(COOL-SH-CAP * QCM2) + [CONS{l) * CFM * (1.0 - CBF) *
(TM - 80.0)]}

or

QCS = QCT, whichever is smaller,

where

QCM2 = CVAL{COOL-SH-FT,EWB,EDB) [COOL-SH-FT is a correction function,
similar to QCMl, but to the sensible portion of the total
cooling capacity].

The equipment cooling part load ratio (PLRC) and the sensible part
ratio ?PASTPLRC? are then calculated,

PLRC = QC/QCT

and

= QC - QCLAT / QCS

For direct expansion equipment, the electric power consumed by the
heating
element in the crankcase is calculated as CRANKCASE-HEAT * (1.0 -
PLRC).

2011/2/2 Nick Caton

--

Arpan Bakshi

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Hi John,

The entering wet bulb temperature for the outside air. You will normalize
the curve around whatever that is to meet the ARI design conditions, 67F
maybe, I can't remember off the top of my head. There is a full set of
equations for equipment under Curve-fit for HVAC Equipment, but I'

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Sorry, hit the wrong button. I'm not sure which volume the equations are in
but I'll see if I can find and send to you.

Carol

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To clarify the ongoing discussion and reinforce my earlier response to John's query:

Standard ARI rating conditions (*) for packaged AC/heatpump equipment are 95?F DB entering at the outdoor section (outside air), and 67?F WB entering at the indoor section (the mixed air conditions entering the coil). If I'm not mistaken, those are the corresponding drybulb and wetbulb temperatures to be referenced when normalizing to and creating any of the DX performance curves that use temperatures as variables.

(*) See Table 3 on page 10 of the AHRI document Maria Karpman linked recently on bldg-sim (copied below).

~Nick

NICK CATON, E.I.T.

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