For those of you who missed the post the other day regarding modeling of
water-cooled chillers, chilled water loops, and HVAC systems more
generally in IES (see "One Model" discussion
thread just a couple days ago), the following excerpt duplicates the
text from that post regarding water-cooled chillers, chilled water
loops. The discussion of currently offered capability along these lines
follows the description of the development that is soon to be released.

***

Regarding the modeling of water-cooled chillers, chilled water loops,
and HVAC systems in IES :

While not yet publically released or announced, in-depth capability for
modeling water-cooled chillers and chilled water loops within the IES
ApacheHVAC application has been developed and
implemented. This capability is presently undergoing testing and will be
available in the next release later this spring.

The new water-cooled chiller model is described by three performance
curves:

1) varies cooling capacity with entering condenser water
temperature

2) varies electric input ratio (EIR) with chilled water temperature

3) varies electric input ratio (EIR) with part-load fraction.

These scalable performance curves are pre-defined for common chiller
types: screw; centrifugal (hermetic, with or without VSD, and open); and
reciprocating (hermetic and open). Where water flow is variable,
performance curves are provided for pumps. These include
constant-speed/variable-flow "riding the pump curve" and electronically
commutated variable speed. For expert users, parameter values are
editable in the equations describing the performance curves for chillers
and pumps.

In addition to selecting appropriate performance curves, the user is
able to specify interconnected performance inputs for capacity, COP,
condenser and chilled water temperatures, flow rates, and loop delta-T
at both rated and design conditions. An input is provided for the load
fraction below which the chiller will cycle on and off. The user also
has control of operating parameters such as condenser-water setpoint and
variation of this setpoint via a formula profile (which may reference
outdoor or indoor conditions and other sensed variables), chilled supply
water temperature (SWT), and ramped SWT reset between high and low
outdoor temperature setpoints.

The cooling tower and condenser-water loop use essentially the same
cooling tower model that is provided for the waterside economizer
component in the current version of ApacheHVAC. The cooling tower model
presently includes a two-speed fan with flow fraction setting for each
speed, approach, range, and electric input ratio. This tower and
condenser-water loop model accounts for changes in outdoor conditions,
chiller heat rejection, and user-controlled tower operating parameters
as described above.

Primary and secondary chilled water loops are modeled for the first
release of this new facility in a PRM-compliant "common pipe"
configuration. For this configuration, the primary loop uses a
constant-speed pump that operates whenever the chiller is running and
the secondary loop with flow controlled to maintain the specified CHW
supply temperature and loop delta-T. Either a
constant-speed/variable-flow pump riding the pump curve or a
variable-speed pump with variable speed drive (VSD) can be specified for
the secondary (chilled-water distribution) loop. Cooling coils, chilled
ceilings, etc. are then assigned to the secondary loop associated with a
particular chiller.

The currently available release of the VE uses a simpler model that does
provide means of describing the performance of a water-cooled chiller,
cooling tower, and pumps in relation to both part-load fraction and
outdoor wet-bulb or dry-bulb temperatures. This is presently done via a
matrix of user input data points for the chiller COP, condenser-water
pump power, cooling tower fan power, and chilled-water pump power. With
appropriate inputs, the chiller COP in this simpler exiting model can be
thus be varied according to both load fraction and outdoor conditions,
while the power for the associated pumps and fans can separately be
varied according to load fraction. However, because this simpler model
does not use the type of curves that many EnergyPlus and DOE-2 users are
accustomed to and does not explicitly model the thermodynamics of the
CW, primary CHW, and secondary CHW loops or their controls, the new
facilities described above have been developed to do just this.

Hopefully this helps to clarify the current and near-term capabilities
of the VE for those concerned specifically with modeling HVAC systems.

Timothy Moore