Specifications
Catalog WSC/WDC-4 11
Part Load Efficiency
Chillers usually spend 99% of their operating hours under part load conditions, and most of this
time at less that 60% of design capacity. One compressor of a dual chiller operates with the full
heat transfer surface of the entire unit. For example, one 500-ton (1,750 kW) compressor on a
1,000 ton (3,500 kW) dual chiller utilizes 1,000 tons (3500 kW) of evaporator and condenser
surface. This increases the compressor’s capacity and also results in very high efficiency.
Typical efficiencies for a dual compressor chiller, taken from a selection computer run, look like
this:
• Full load efficiency 0.550 kW per ton (6.5 COP)
• 60% load, one compressor 0.364 kW per ton (9.6 COP)
• IPLV 0.415 kW per ton (8.5COP)
The addition of VFDs to the dual compressor chiller produces an astonishing ARI certified IPLV
of 0.340 for the above case. Specific selections can vary up or down from this example. IPLV is
defined in the Selection section of this manual beginning on page 41.
Lower Installed Costs
The redundancy feature pays off in lower installed costs
An example of how to incorporate dual compressor chillers into a system requiring redundancy:
Job requirement: 1,200 tons (4200 kW), 50% Backup
Single Compressor Chillers
Dual Compressor Chillers
(2) 600 ton (2100 kW) On Line Units (2) 750 ton (2100 kW) Units with
+(1) 600
(2100 kW) ton Standby Unit 1,200 (4200 kW) On Line tons *
(3) @ 1,800 ton (6300 kW) Installed Capacity (2) @ 1500 ton (5250 kW) Installed Capacity
* One 750-ton (2100 kW) dual chiller running on two compressors for 750 tons (2100 kW), plus
one 750-ton (2100 kW) dual chiller running on one compressor for 60% of 750 tons (2100 kW) =
450 tons (1575 kW), for a total of 1200 tons (4200 kW) on any 3 of the 4 total compressors.
The elimination of the extra pumps, valves, piping, controls, rigging, and floor space can result in
as much as a 35% reduction in the installation cost for a chiller plant, plus the savings on the
chillers themselves.
The Replacement Market Advantage
• Bolt-together construction on single and dual compressor chillers
• Put 20% or more tons in the same footprint
• Add dual compressor redundancy
• Greatly reduce chiller energy consumption
• Install a refrigerant with no phase-out date
• Opens many options for multiple chiller plants