Danfoss TGH285 Applications And Installation Manual Download

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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 Danfoss LLC 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 Danfoss LLC

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 LBV 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 desuperheat the gas prior to

entering the suction of the compressor.

• • • CAUTION • • •

If the staging valve is used as a LBV, 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 fault

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 type) can be selected from Danfoss portfolio e.g., EVR32 for TTS300 & TTS350 and EVR40 for TTS400 & TTS700

compressors. These valves should provide satisfactory starting and shutdown functionality, however Danfoss LLC Applications strongly

recommends testing with supervision of an Application Engineer to ensure proper operation during all start and stop conditions.

M-AP-001-EN Rev. S-9/8/2021 Page 69 of 136