Use of pressure independent control valves in CW units

In a previous blog (CW standby management), we already discussed the fact that liquid cooling systems with centralized chilled water supply and so-called CW (= chilled water) precision air conditioning units are the most popular choice for cooling larger data centers. This is primarily because of their good scalability and comparably simple hydraulics.

Along with the chilled water heat exchanger and fans, the chilled water control valve is the other principal mechanical component of a CW precision air conditioning unit. In the past, either 3 or 2-way control valves were used, depending on the type of hydraulic system and pump used (variable or constant speed). However, for some time now so-called "pressure independent control valves" or "PICVs" have also been frequently used as 2-way control valves or ball valves.

In order to better understand the method of operation and advantages of the PICV, we would do well to recall some fundamental hydraulic principles:

A control valve ensures that a heat exchanger is always supplied with the correct quantity of water for the current operating point or cooling needs (full load or partial load). The appropriate valve position or degree of opening is determined by an external control signal.
The valve size (keywords: Kvs value, Valve Authority) must be based on the required quantity of water (full load operation) and the water-side pressure drop at the heat exchanger.
The pressure drop at the control valve resulting from the valve calculation is also referred to as "differential pressure". This differential pressure and the pressure drop at the heat exchanger must be correctly harmonized with one another. When the differential pressure is too low (valve too large), the valve has only a small stroke range, with adverse effects on control quality and unstable control behavior (fluctuations) as possible consequences. With a too high differential pressure (valve too small) there is major noise and cavitation possible and superfluous pump energy consumed.
In every hydraulic system, pressure through the valves, heat exchanger and pipes vary depending on the type of system, installation location and distance from the pump, as well as on changing load conditions.
The definitive factor when determining the size and settings of the pump is to make sure that the last consumer in the system is always supplied with the necessary quantity of water at full load, and that the associated differential pressure can be surmounted.
The closer a consumer (e.g. a CW precision air conditioning unit) is situated to the pump, the greater the flow rate and, without so-called "hydraulic balancing", the differential pressure through the control valve of this consumer will also rise. Hydraulic balancing makes sure that each consumer in the system always receives the required quantity of water, and that the water does not take the path of least resistance.

But what is the role of the pressure independent control valve here?

A modern electronic pressure independent control valve basically always combines four functions in one valve unit – pressure independent control, measurement of the water flow, a shut-off function, and automatic hydraulic balancing. These functions are performed by the control ball valve, valve drive and flow sensor.

This means that in a pressure independent control valve, the setpoint is always the required water flow rate. Since the current flow is measured continuously, the valve adapts the required quantity of water in line with the load, and the valve's pressure drop (differential pressure) is therefore the result of the flow rate, not defined by valve size or the kvs value. Consequently, any difference between the setpoint and the current flow due to a change in differential pressure is compensated fully automatically by the opening angle of the control ball valve.

So "pressure independent" (or more accurately, "independent from differential pressure") means that the correct amount of water is always supplied to the consumer, and the control quality is dependent neither on the valve's position in the hydraulic system nor the prevailing pressure conditions.

Advantages of a pressure independent control valve of this kind

1. Planning/design:

Fast and simple valve design based only on the required quantity of water – kvs values, Valve Authority and varying differential pressures can basically be ignored
No balancing valves or circuit control valves needed – lower investment and installation costs

2. Start-up/operation:

No balancing valves or circuit control valves needed – therefore lower water-side pressure drops and the possibility of reduced pump power consumption.
No time-consuming, labor intensive hydraulic balancing required – the pressure independent control valve performs the task of hydraulic balancing; the required quantity of water is adjusted easily
Stable and precise control in all load states thanks to the defined quantity of water, regardless of the type of hydraulic system chosen
Water quantity can be flexibly adjusted in the event of extensions, conversions and/or modernization
Water quantity can be easily read – more in-depth analysis (e.g. cooling capacity) is possible

It is clear that the use of so-called pressure independent control valves makes sense in most cases, as investment and operating costs can be lowered, and stable control is guaranteed irrespective of the chosen hydraulic system and current load conditions.

About the author

Norbert Wenk is head of Product Management. Following a degree in Mechanical Engineering, he joined STULZ in 1999. He started his career at STULZ in the R&D department and later moved to the Export (Sales) department, where he worked as an Area Sales Manager. He therefore now has over 20 years of experience in the Data Center cooling industry.