By Michael Young, Pr. Eng.

Part 5: Operating conditions within a data centre for a DX system

Things to know…

The integration of variable speed compressors have improved the overall energy efficiency of a DX system when used within a data centre type of application.

When considering a DX type of cooling system, operating air conditions have become the starting point of the design. Some end users require a specific entering air condition to the server and HVAC designers often need to reverse engineer to meet these requirements.

MICHAEL YOUNG Michael Young is a trainer, coach and a pre-sales engineer in the HVAC industry. He graduated from the University of the Witwatersrand in the field of Mechanical Engineering (B.Sc Mech Eng) in 2008 and qualified as a Professional Engineer (Pr.Eng) in 2013. Michael is passionate about promoting knowledge and helping other young engineers grow within the industry through his training workshops and coaching sessions.  

Let’s imagine that a specific client requires the supply air condition to the servers to be 24°C and states that there will be a 12°C temperature rise as the air passes over the server. This means that the return air condition to the computer room air conditioning (CRAC) unit will be 24°C + 12 °C = 36°C.

Designers now consider the electrical consumption of the IT load and specify that the power drawn by the IT servers is the sensible cooling requirement of the CRAC unit.

The correct selection of the DX CRAC unit involves more than just selecting a unit that meets the required net sensible cooling – the unit must also meet the required operating air conditions.

So how is a CRAC unit correctly selected and sized? The first parameter to be kept constant is the return air condition. The user will now adjust the fan and compressor speed to satisfy the supply air temperature as well as the required net sensible cooling capacity.

Let’s take a look into the thermodynamics that is occurring. If return air temperature is to remain constant while supply air temperatures are increased, airflow needs to be increased as per the sensible equation. To accomplish this, the fan speed will be set to higher values so more sensible cooling is delivered by CRAC unit. The compressor speed now needs to either increase or decrease in speed to meet the required net sensible cooling effect.

It’s also important to note that the supply air fans introduce some inefficiency within the system as the air leaving the coil picks up heat as it flows over the fan motors. Therefore, the compressor speed is increased to provide a higher gross cooling effect to offset this inefficiency.

Based upon this analogy, the relationship that exists is that a higher supply air condition requires more airflow at a lower compressor speed. According to ASHRAE, design conditions are based upon the inlet rack temperature.

So, when supply air or return air condition is increased, the physical size of the unit may also increase. It important to also note that the compressor may also begin to operate on the limit of the compressor envelope with these changes. Therefore, when sizing a CRAC unit, both the required cooling and psychrometrics need to be satisfied and this requires a balance between airflow and acceptable compressor operating limits.

On the surface this seems simple, but the operations of a DX system become complicated when the unit is required to operate at part loads.

The cooling unit is often selected at the maximum required cooling load, but in reality, the unit is often required to operate at a part load. Now many would say, “so what, who cares about part loads?” as the compressor will just modulate down – but there is more to this statement which we will discuss in next month’s publication. 

Wishing you a successful month ahead.