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Indirect free cooling of a DX cooling unit (Part 2)

Indirect free cooling of a DX cooling unit (Part 2)

By Michael Young

How does the control philosophy work for a DX cooling system and how do we accomplish energy savings?

(If you missed part 1click here)

In the previous publication, we discussed the next advancement of the DX cooling system that integrated the use of a pumped refrigerant loop. We mentioned that the system is able to switch between a compression operation to a pumped refrigerant operation depending on the ambient temperature and load within the data centre.

So, how does the control philosophy work for this type of system and how do we accomplish energy savings? This particular type of cooling system is able to operate in three different modes, depending on the outdoor temperature and load within the data centre.

The first operating mode is compressor mode. In this cooling mode, all available compressors are used to maintain the required operating temperature of the data centre. The system will typically operate in this mode when the load and ambient temperatures are at their extreme limits.

The second operating mode is the refrigerant pumping mode. This mode of cooling is activated when the outdoor temperature is low enough to provide the required temperature difference between the indoor and the outdoor air, as well as meet the required cooling load. In this mode of operation, the room temperature, refrigerant temperature, and pump differential pressure need to be maintained.


The integration of a pumped refrigerant loop allows us to operate at different cooling modes according to the variation of cooling requirements and ambient conditions.


The control of the room temperature during pump mode is accomplished by modulating the pump speed with a variable frequency drive. When the system switches to pumping mode, the pumps are immediately defaulted to operate at a set speed according to the call for cooling at the time of transition. Once this initial procedure is complete, the pumps can modulate between 45% to 100% capacity to finely control the room temperature.

The refrigerant temperature is controlled by the condenser fans during pump mode. When the system switches from compressor to pump mode, the condenser fans modulate according to the actual outlet temperature of the condensing refrigerant instead of pressure control.

The actual fan speed of the condenser fans will depend on the load and the outdoor temperature, as fan speed will be lower when outdoor temperatures are lower for the same cooling requirements.

The pump pressure differential must also be maintained above a specific minimum value to ensure that cooling of the pump motor and lubrication of the pump bearings are met.

When the system switches over to pump mode, the electronic expansion valve changes from superheat control to manual control, where the position of the valve is adjusted to maintain a set pump differential pressure.

The final operating mode is the mixed-mode operation, where both the pump on one refrigerant circuit and the compressor in the other refrigerant circuit are on.

When this occurs, the room temperature is controlled by operating the pump at 100% on one refrigerant circuit while modulating the compressor’s capacity on the second refrigerant circuit. The opposite is also possible where the pump is at a modulated state and the compressor is operating at 100%. The final modulated state of equipment is dependent on the current cooling load and ambient temperature.

So, as we see, the integration of a pumped refrigerant loop allows us to operate at different cooling modes according to the variation of cooling requirements and ambient conditions. Energy savings are accomplished when the compressor is at its lowest modulated state during the mixed mode or switched off during the pumping mode.

So, is the integration of the pumped refrigerant loop the future of a DX system and will this become the new industrial norm?

Find out in next month’s publication. Feel free to call us on +27 (0) 73 171 2311 or email us at michael@yourhvaccoach.com to learn about this new technology. We look forward to hearing from you.

Wishing you a successful month ahead.

Mike signature 14 8 2017


Click below to read the January 2018 issue of RACA Journal

RACA JAN2018

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06 Jun 2018
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FRIGAIR Expo 2018 - Day1
07 Jun 2018
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08 Jun 2018
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