Daikin WMC Instruction Manual download pdf (Page 9)

www.DaikinApplied.com 9 IOM 1210-1 • MAGNITUDE

MODEL WMC CHILLERS

Condenser Water

Temperature Control

Condenser water control is an important consideration in chiller

plant design since condenser water temperature will directly

impact chiller operation and efciency. When the ambient

wet bulb temperature is lower than peak design, the entering

condenser water temperature from the cooling tower can

be allowed to fall, improving chiller performance. However,

operational issues may occur when the condenser water

temperatures are either too high or too low. The WMC chiller

provides several options to assist the chiller plant designer in

providing the optimum control of condenser water temperature.

Cooling Tower Control

Control of the cooling tower is required to maintain stability and

avoid operational issues. This can be achieved through a BAS

or by using the MicroTech

II controller. For systems utilizing a

common condenser water loop for multiple purposes, the BAS

contractor must provide the control but use of the MicroTech

output signal is still recommended.

The preferred cooling tower control utilizes a variable speed

fan. MicroTech

II will provide a control signal to determine the

proper fan speed. It can also control up to four stages of fan

cycling. Note that fan cycling can cause cooling tower water

temperature to uctuate as fans stage on/off, potentially adding

instability to the system.

Special consideration must be given to starting the chiller when

cold condenser water is present, such as with inverted starts or

changeover from free (tower) cooling to mechanical cooling. It

is required that some method be used to control the condenser

water to maintain proper head pressure as indicated by the

MicroTech

II controller.

Acceptable methods include the following (Each of these

options can be controlled by the MicroTech

II or through

a BAS utilizing the MicroTech

II output signals.):

1. Three-Way Bypass Valve Operation

A traditional method for building condenser pressure at

startup with colder condenser water is with the use of

a three-way bypass valve. The device blends warmer

water leaving the condenser with cooler water from the

cooling tower at the condenser inlet. The bypass valve

position will change until full ow from the tower to the

condenser is obtained. The MicroTech

II provides only

the valve position control signal. Main power to drive the

valve’s actuator must be provided by the installer. The

three-way valve should be located close to the chiller

within the equipment room to minimize the volume of

water.

2. Two-Way Valve Operation

Another condenser control method is to use a modulating

two-way control valve located on the outlet connection of

the condenser. The valve will be nearly closed at startup

to restrict water ow, which keeps generated heat in

the condenser until an acceptable minimum condenser

pressure is reached. As heat builds, the valve will open

slowly until a full ow condition from the cooling tower

is established. A separate power source is required to

provide power to the valve actuator.

3. VFD Operating with a Condenser Water Pump

A third method of condenser control for startup is

utilizing a variable frequency drive with the condenser

water pump. The speed will change as directed by the

MicroTech

II output signal until design ow is reached.

Speed adjustments may be required during the initial

chiller startup as determined by the service technician.

NOTE: Not using the MicroTech

II logic to control valves

and variable frequency drives may result in system

instability, capacity reduction, and issues starting the

chiller with cold condenser water temperature.

Condenser Pump Sequencing

It is recommended to utilize the logic built into the MicroTech

controller to start the condenser pump and maintain condenser

head pressure control. MicroTech

II has the capability to

operate a primary pump and a secondary standby pump. The

condenser water ow should be stopped when the chiller shuts

off. This will conserve energy and prevent refrigerant from

migrating to the condenser.

Lenient Flow Operation

For chiller startup, the condenser control systems can reduce

the ow to very low rates, which can make operation of a ow

sensing device questionable. The MicroTech

II controller has

a “lenient ow” feature that acts as an override of the ow

sensor while protecting the chiller by monitoring a condenser

pressure setting that is below the high pressure cutout.

Water Side Economizer Cycle Operation

Water side economizers are commonly used for ASHRAE 90.1

compliance and energy savings. This system utilizes a heat

exchanger external to the chiller when cold cooling tower water

is available to provide cooling. The most common system

has a heat exchanger used in conjunction with the chiller’s

evaporator.

The BAS contractor will need to provide controls for the heat

exchanger including isolation valves and temperature control.

The BAS contractor will also need to control the isolation

valves for the chiller. It is important to use slow-acting type

valves to prevent rapid changes in system ows. Changeover

from economizer cooling to mechanical cooling requires one

of the methods previously mentioned to maintain suitable

condenser head pressure.

Contact your local McQuay International representative for

more information on this application.

InsTallaTIon

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