Product Manual PM WSC/WDC 13
SurgeGard
Protects The Compressor From Surge Damage
As centrifugal compressors operate at part load, the volume of refrigerant gas entering the impeller is reduced.
At the reduced flow, the impeller’s capacity to develop the peak load head is also reduced. When inadequate
maintenance of condenser tube cleanliness or a cooling tower or control malfunction occurs, artificially
elevating the compressor head, a rotating stall or surge condition can occur. Under normal operating
conditions, all WSC chillers will operate to 10% capacity without surge and WDC dual compressor chillers to
5% capacity without surge. For abnormal conditions, McQuay compressor designers have developed a
protective control system that senses the occurrence of a surge and stops the compressor before any damage is
sustained. This protection, called SurgeGard, is provided as a standard on all McQuay centrifugal compressors.
Quiet, stable capacity from 10% to 100% without hot gas bypass
Compressor capacity on McQuay chillers is maximized at full load and modulated to 10% load by interlocked
inlet guide vanes and the movable discharge diffuser. This seemingly esoteric and unimportant design detail,
like many other McQuay innovations, has real owner benefits. Compressors that do not unload this well, and
most don’t, waste energy at low load conditions by unnecessary cycling or use of hot gas bypass.
No leakage at the capacity control mechanism
An oil pressure operated guide vane activating piston is internally mounted and powered to eliminate external
linkage and seals. The vanes are positioned in response to variation in leaving chiller water temperature. A
built-in compensating control allows automatic override of normal operation to close the vanes for low suction
pressure or current limiting duty.
Pumpdown
Pumpout systems provide a means to collect and contain the refrigerant charge without loss, when the access
to internal chiller components is required for service.
McQuay condensers are sized to hold the entire unit refrigerant charge when not more than 90% full at 90°F
(32°C) ambient temperature. They are equipped with a tight-seating check valve at the hot gas inlet and a
manual shutoff valve in the liquid outlet. These valves, coupled with the condenser design, satisfy the
stringent requirements of the U.S. Department of Transportation for refrigerant shipping containers, as well as
ASME vessel codes. When service is required, the refrigerant charge may be pumped down into the condenser
by compressor operation and use of a refrigerant transfer unit. All dual compressor units and single compressor
units equipped with an optional suction shutoff valve can also be pumped down to the evaporator. Elimination
of the cost and space requirements of an external pumpout system is a major McQuay advantage.
Thermal Expansion Valves
Controlled refrigerant flow over the entire capacity range saves energy and dollars
Cooling loads and condenser water temperatures change daily. Refrigerant float valves and orifices on
competitive chillers are selected for peak load and peak condenser water temperatures and offer only partial
control of refrigerant flow at operating conditions experienced over 95% of the time.
On McQuay chillers a pilot operated thermostatic expansion valve meters refrigerant flow in direct response to
the suction superheat, regardless of changing load or condensing temperatures. In doing so, full utilization of
compressor, evaporator, and condenser efficiency over the entire operating range is achieved. Intermittent
refrigerant flood-back and excessive superheat characteristic of orifices and floats are eliminated.
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