Danfoss TGH Series Applications And Installation Manual Download

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M-AP-001-EN Rev. Q

Chapter 10.0 Control Logic Guidelines for Multiple Compressors

Due to the nature of centrifugal compression, special control logic must be implemented for proper
staging of multiple Turbocor compressors when installed on a common circuit. This section is intended
only as a guide without going into details. Control details are specific to each OEM’s individual control
strategy. The Turbocor centrifugal compressors can be controlled by staging compressors and running
the on line compressors in parallel.

Staging valve:

The Staging valve is piped in upstream of the check valve to provide a low pressure

bypass path and is used to reduce the pressure ratio in the system to assist in startup and shutdown of
a compressor. Staging valves are mandatory for all Danfoss LLC compressors.

Load balancing valve (hot gas bypass):

The Load Balance Valve (LBV) is piped downstream of the

check valve and is primarily used in low load conditions to keep the compressor operational instead of
cycling off. It is possible to use a staging valve as a load balance valve (HGBP) but sizing and control
can be a little more challenging and for that reason they are both frequently installed in many systems.

For additional details related to starting, stopping, staging, for all aspects of single and multiple
compressor control, please consult the OEM programming manual.

NOTE

The hot gas evacuated by the staging valve must be injected downstream of the main EXV in order to de-superheat the gas prior
to entering the suction of the compressor.

• • • CAUTION • • •

If the staging valve is used as load balancing valve (HGBP) there are two major risks:
• Check valve chattering if the bypassed flow is too high, this chattering will lead to bearing faults due to vibrations (Please
contact Danfoss LLC for further information)
• High suction superheat which may lead to compressor trip

Staging valve sizing method:

Using CPR, rate a compressor at the maximum pressure ratio for

which the chiller is designed to operate. Use the maximum Evaporator Mass Flow Rate to select an
appropriately sized staging valve.

Load Balancing Valve Sizing:

Using CPR, rate the compressor at the minimum load and conditions

at which the compressor is expected to operate. Use the Evaporator Mass Flow Rate to determine
the required mass flow which must be bypassed through the load balance valve to reach the desired
minimum load.

NOTE

Danfoss LLC highly recommends the use of a solenoid valve as a staging valve as it is faster to provide pressure relief to the
compressor in case of emergency shutdown. We recommend to command the Solenoid valve with the "compressor status"
contact on the I/O board. Staging valve solenoid valve can be selected from Danfoss portfolio e.g., EVR32 for TTS300 & TTS350
and EVR40 for TTS400 & TTS700. These valves should provide satisfactory starting and shutdown functionality, however Danfoss
LLC strongly recommends testing with supervision of an Application Engineer to ensure proper operation during all start and
stop conditions.

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