Specifications

Part Load Efficiency
Chillers usually spend 99% of their operating
hours under part load conditions and most of this
time at less than 60% of design capacity. One
compressor of a dual chiller operates with the full
heat transfer surface of the entire unit. For
example, one 75-ton compressor on a 150-ton dual
chiller utilizes 150 tons of evaporator and
condenser surface. This results in very high unit
efficiency and also increases the compressor’s
capacity.
The inclusion of VFDs, as standard, to the dual
compressor chiller can produce astonishing ARI
Certified IPLVs, in the range of 0.375 kW/ton.
Specific selections can vary up or down from this
example.
Compliance with ASHRAE Std. 90.1
With the WMC capacity range of 110 to 175 tons,
it falls into two ASHRAE Std. 90.1 efficiency
requirements – below 150 tons and 150 tons &
larger. Below 150 tons, the standard requires a full
load and IPLV of 0.703 kW/ton (5.0 C.O.P.). The
WMC chiller will easily meet these requirements.
For 150 tons and larger, some care may be
required in the system design to meet the full load
requirement. The standard requires 0.633 kW/ton
(5.55 C.O.P.) for both full load and the IPLV. The
WMC’s excellent part load performance easily
meets the part load requirements with IPLVs as
low as 0.365 kW/ton (approximately 40% lower
than required). The unit full load efficiency at the
ARI standard rating point could slightly exceed
the standard.
To resolve this dilemma, the ASHRAE Std. 90.1
provides full load performance requirements for
reduced condenser water flow. This allows the
WMC to meet the requirements. As the condenser
water flow drops below 3.0 gpm/ton, the WMC
can meet the standard’s requirements. See the
example below. It is wise to concentrate on the
outstanding part load performance provided by
WMC, rather than only full load, because nearly
all of the operating hours for a typical comfort
cooling application are at part load conditions.
Example
Project requirements are 160 tons at the standard
ARI temperatures of 44 degrees leaving
evaporator water temperature and 85 degrees
entering condenser water temperature.
ASHRAE Standard 90.1 requirements for full load
and IPLV efficiencies are 0.633 kW/ton (5.55
C.O.P.).
At 3.0 GPM/ton for the condenser, the WMC
performance is 0.640 kW/ton at full load and
0.367 kW/ton IPLV. Although the part load is
vastly lower than the requirements, the full load is
a little higher (one percent).
To meet ASHRAE 90.1 requirements, consider
lowering the condenser water flow to 2.5
GPM/ton. With all other conditions unchanged,
the WMC full load performance becomes 0.653
kW/ton. Because ASHRAE 90.1 requires 0.660
kW/ton at these conditions, the WMC now meets
the requirements.
ARI Certification
McQuay International has an on-going
commitment to supply chillers that perform as
specified. To this extent, McQuay centrifugal
chillers are part of the ARI Certification Program.
On-going performance verification of chiller
capacity and power input plus ARI certified
computerized selection output provide the owner
with specified performance in accordance with the
latest version of ARI Standard 550/590.
All chillers that fall within the scope of the
certification program have an ARI certification
label at no cost to the owner. Equipment covered
by the ARI certification program includes all
water-cooled centrifugal and screw water chilling
packages rated up to 2000 tons (7,000 kW) for 60
hertz service at ARI standard rating conditions,
hermetic or open drive, with electric driven motor
not exceeding 5000 volts, and cooling water (not
glycol). For 50 hertz applications the capacity
range covered is 200 to 1,000 tons (700 to 3500
kW).
Published certified ratings verified through testing
by ARI include:
• Capacity, tons (kW)
• Power, kW/ton (COP)
• Pressure drops, ft. of water (kPa)
• Integrated Part Load Value (IPLV) or
Non-Standard Part Load Value (NPLV)
Catalog WMC-4 17