A flow control valve reduces the pressure
Elsewhere (in the blog "CW-Standby-Management") it has already been mentioned that cold water systems with a central cold water supply and so-called circulating air air conditioning units in CW design (CW = Chilled Water) are used to cool larger data centers . The reasons for this are mostly in the good scalability and a comparatively simple hydraulic system.
In addition to the cold water heat exchanger and the fans, the cold water control valve is the other main mechanical component of a circulating air air conditioner in CW design. Depending on the hydraulic system and the type of pump used (speed-controlled or with constant speed), either 3-way or 2-way control valves have been used in the past. For some time, however, so-called "pressure-independent" or "differential pressure-independent" 2-way control valves or ball valves have also been used more frequently.
In order to better understand the functionality and advantages of this differential pressure-independent control valve, we have to briefly recall a few hydraulic basics:
- A control valve ensures that a heat exchanger is always supplied with the correct amount of water for the current operating point or cooling requirement (full load or part load). The associated valve position or the valve opening degree is specified via an external control signal.
- The valve size (keywords: kvs value, valve authority) must be designed depending on the amount of water required (full load operation) and the water-side pressure loss across the heat exchanger.
- The pressure loss across the control valve resulting from the valve calculation is also referred to as "differential pressure". This differential pressure and the pressure loss of the heat exchanger must be correctly matched to one another:
- Differential pressure too small (= valve too large): Valve only works in a small stroke range, impairment of the control quality and unstable control behavior (vibrations) possible
- Differential pressure too large (= valve too small): strong noise development and cavitation possible, consumption of unnecessary pump energy
- In every hydraulic system, the pressure ratios across the valves, heat exchangers and pipelines vary depending on the type of network, installation location and distance to the pump as well as depending on the load conditions.
- The decisive factor for the size and setting of the pump is to ensure that the last consumer in the system is always provided with the required full load water quantity and that the associated differential pressure can be overcome.
- The closer a consumer (e.g. a CW air conditioning unit) is to the pump, the greater the volume flow, and without a so-called "hydraulic balance", the differential pressure across the control valve of this consumer also rises. Hydraulic balancing ensures that every consumer in the system always receives the required amount of water and that the water does not take the path of least resistance.
How does the differential pressure-independent control valve come into play here?
A modern electronic pressure-independent control valve always combines 4 functions in one valve unit - pressure-independent control, measurement of the water volume flow, shut-off function and automatic hydraulic balancing. This is ensured by the control ball valve, valve drive and volume flow sensor components.
This means that with a "pressure-independent" or "differential pressure-independent" control valve, the required full-load water volume flow is always set as the setpoint. Since the current flow rate is measured permanently, the valve adjusts the required amount of water depending on the load, and the pressure loss of the valve (differential pressure) is then the result of the volume flow and not defined by a valve size or the kvs value. A difference between the set target value and the current flow rate due to a change in the differential pressure is thus compensated fully automatically via the opening angle of the control valve.
“Independent of the differential pressure” thus means that the correct amount of water is always made available to the consumer and that the control quality does not depend on the position of the valve in the hydraulic system and the prevailing pressure conditions.
Advantages of a differential pressure-independent control valve
1. Planning / execution:
- Simple and quick valve design based only on the required amount of water - kvs values, valve authority and different differential pressures are in principle negligible
- No balancing throttles and / or double regulating and commissioning valves required - lower investment and installation costs
2. Commissioning / operation:
- No balancing throttles and / or double regulating and commissioning valves required - this means lower water-side pressure losses and thus a reduction in pump performance
- No lengthy and labor-intensive hydraulic balancing necessary - the differential pressure-independent control valve takes over the hydraulic balancing, simply setting the required amount of water
- Stable and precise control in all load conditions thanks to the fixed amount of water, regardless of the selected hydraulic system
- Flexibly adjustable water volume for extensions, conversions and / or modernizations
- Readability of the amount of water - further evaluations (e.g. cooling capacity) possible
The use of so-called differential pressure-independent control valves therefore makes sense in most cases, since investment and operating costs can be reduced and stable control is guaranteed regardless of the hydraulic system selected and the load conditions.
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