Centralised air conditioners should ideally provide comfort and energy efficiency. Variable Primary Flow systems coupled with Pressure Independent Balancing and Control Valves deliver on both counts, says Ismail Serhan Ozten of Danfoss, who presents proof to support his assertion.

Infrastructure challenge

It is to state the obvious that most commercial buildings have very low occupancy after office hours or during weekends. This renders operating centralised air conditioning systems during these times wasteful. The challenge is to run them efficiently.

In example 1 the building has 10 floors and all of them are occupied from morning till late afternoon. During evening time, a single floor is open for business. Thus, the centralised air conditioning system that distributes air for the entire building has to work for the sake of cooling only a single floor.

Modern building outlook requirements do not allow an additional air conditioning equipment to be displayed outside the building. Therefore, running a separate smaller air conditioning system is not an option. In such a scenario, the Variable Primary Flow system could be the answer.

Variable Primary Flow system: Introduction

Variable Primary Flow (VPF) system was commonly published in the 1990s and is superior to Primary Secondary Flow system in terms of energy efficiency. In the growing interest of energy efficiency, some engineers have now come to think that the VPF system may be employed to work better at very low load condition.

In general, this system has lower initial and operation costs compared to primary secondary flow system.

The following are the saved initial costs:

1. Elimination of secondary pumps
2. No swing chiller needed
3. Fewer pipe connections
4. Fewer electrical connections
5. Less floor space required
6. Smaller bypass size

Saved operation costs are:

1. Varying primary pump speed
2. Varying compressor speed/quantity in compressors/quantity of chillers
3. Efficient chiller performance even during part load

Despite the merits stated above, the system has its limitations which are highly reliant on the hydronic balancing and control performance.

VPF system – implementation

Key to Figure 2

1. Motorised control valves to regulate room temperature
2. Motorised control valve to allow minimum flow in variable flow chillers
3. Digital flowmeter
4. Differential pressure sensor
5. Water immersed temperature sensor
6. Variable speed pumps
7. Motorised isolation valves to prevent inflow from other pumps during low load
8. Manual balancing valves/flow limiters or automatic balancing valves

Note: Flow limiters, also known as Automatic Balancing Valves, are not recommended to combine with item #1 modulating control valves, because their characters oppose one another, hence:

1. Modulating control valves = Regulate flow to maintain constant temperature
2. Flow limiters = Regulate orifice to maintain constant flow

Hydronic balancing and control

Liquid ΔP sensor across the variable speed pumps

This implementation is very commonly applied in most variable flow systems. Especially the hydronic configuration in the distribution circuits have manual balancing and motorised control valves. In large systems, there may be even manual balancing valves installed in the branches, risers and headers to enable proportional balancing method.

The ΔP transmitter located across the variable speed pumps has the following benefits:

1. Easy to troubleshoot
2. Shorter cable length
3. Less need for ΔP transmitter calibration

Note: The longer the cable length, the higher the voltages drop. When the analogue signal drops, the range of the control becomes smaller. It was intended to work from 0-10 VDC for a 0-100% pressure variation. For example, the incoming signal becomes 9V DC as maximum after travelling from a long cable distance. Therefore, a calibration must be done to recognise 0-9V DC as 0-100%.

Liquid ΔP sensor across the variable speed pumps

During partial load, even the highest resistance or furthest circuit ΔP becomes relatively close to the pump head pressure.

At this juncture, the control valve’s character is distorted from its desired authority. When a control valve loses its authority, the temperature response will be unstable and inaccurate. In most cases, it will constantly hunt for the desired temperature. While it is constantly hunting, the average flowrate pumped will be higher than needed. When a system has huge overflow, then the energy transfer across the coil will be inefficient. Most liquid will pass through the coil at high velocity, attributing to less heat transferred. This phenomenon is known as low ΔT syndrome.

Low ΔT syndrome will cause the incorrect leaving temperature and quantity of chillers.

ΔP transfer in relation to flowrate

When the cooling load is 70%, the required flow is 40%. When the flow is 40%, then pressure drop across the control valve is 84% with 16% left in the system fittings.

A small change of cooling load causes a large pressure transition from system fittings to control valves. Most of the time, the cooling load varies from 80% and below, depending on the size and efficiency of the coil. The reason it is not always 100% is because it is oversized for extreme circumstances.

Liquid ΔP sensor across the highest resistance unit

A ΔP sensor is installed across the circuit of the highest resistance unit in the system. This unit is sometimes perceived as:

1. Critical unit
2. Index unit
3. Reference unit

This practice will enable the speed of the pump to vary proportionally against the pump head pressure.

Liquid ΔP sensor across the highest resistance unit

Now that we know the pressure head will vary at the pumps against the system load, it is not possible for manual balancing valves to react to these variations.

For example, circuit 1 manual balancing valve is required to create a resistance of 230 kPa during full load. During part load, the available ΔP across the circuit is only 80 kPa. Hence, a manual balancing valve creates more resistance than needed when this terminal requires full flow. It is attributing under-capacity during part load.

For engineers, the biggest fear is under-capacity, which is in direct relation to their goals – comfort and efficiency.

In this scenario, the VPF system offers the most desirable implementation option.

Key to Figure 4:

1. PIBCV – Logarithmic character (water to air energy transfer)
2. PIBCV – Linear character (No energy transfer)
3. Digital flowmeter
4. Differential pressure sensor
5. Water Immersed temperature sensor
6. Variable speed pumps
7. Motorised isolation valves
8. Manual Balancing Valves / Flow limiters aka Automatic Balancing Valves

Note: Item (8) is no longer required because PIBCV is a self-balancing valve. Refer to Figure 2.

Unleashing an innovative technology

Figure 5-1 shows a construction of extremely innovative technology that performs balancing and control. This control valve has a built-in pressure controller that keeps the control valve character constant. Thus, the good valve authority is close to 1.

In combination with its actuator, it has the ability to change the curve of the character to match the coil’s non-linear character; the results of the control will be linear. Linearity signifies stability and accuracy.

Combining PIBCV with other components

Every separate entity in the system has its own characteristic. Combining each component correctly with a properly set and tuned controller elicits a good control response. Not only does it have a good response, but it is also energy efficient. Understanding the basis of the control algorithm helps optimise a chilled water system even further.

Example: Chiller leaving temperature setpoint: Decrease the temperature to decrease the overall flowrate. A load analysis of the chillers can be used to determine the leaving setpoint.

In a plant that has four identical chillers in parallel, the average load of three operating chillers is around 60% each. The chiller setpoint can be lowered to collect and consolidate the loading of the chillers to decrease the average flowrate. Having lower flowrates would require one set of the chiller deducted from the operation. Now, two chillers are operating with 90% load each. The objective is to fully load the active chillers and shut down some of the redundant ones.

 

Control valve authority

Most manufacturers produce typically two different valve characteristics based on Figure 6-1. The green colour character denotes the most desired character, which has no ΔP changes and, therefore, has no distortion. A stable and accurate control will require valve authority closer to 1.

If a valve authority is low, it is similar to filling a glass using a fire hose. As it opens under highly pressurised condition, water will spill over. If a valve authority is high, it is similar to filling a glass using a tap – a full range of control that precisely provides the exact flowrate that is needed.

Control valve authority in practice

The control valve authority is sized with respect to 0.5 minimum commonly as a guideline. Equation 1 has a common expression used, based on Figure 6-2:

Equation 2 is not a complete expression, because the system fittings have not been taken into account. Most of the time, chilled water systems operate two per cent full load and 98% part load.

Let us consider the worst-case scenario based on terminal unit 4 in the two diagrams: Figure 7-1 and Figure 7-2.

In essence, the denominator can be expanded, summarised and translated as in Equation 3.

PIBCV independent of pressure variations

Built-in pressure controller, DPC, takes away hydraulic influences from other parts of the installation. Regardless of different load conditions, it maintains a good valve authority.

Hence, it can be expressed, as in Equation 4.

PIBCV — a closer look

The primary task of a DP controller is to maintain constant ΔP across the control valve. When the pressure increases in P1, it inflates the bottom part (high side) of the DP controller’s chamber through capillary tubing. During inflation, the DP controller’s orifice becomes smaller to absorb pressure from P1, thus causing pressure in P2 to increase. For pressure transition chart refer to (Figure: 3-2-1).

The DP controller will shave off extra pressure applied to the control valve not only when removing hydraulic interference from other parts of the installation, but also when the control valve itself is closing. As long P1 increases, DP controller will instantly react to maintain pressure equilibrium.

We notice the DP controller’s chamber is connected to two capillary tubings that are taking both the high and low side pressures. The PIBCV is subjected to atmospheric pressure. Hence, it requires a low side capillary tubing to also achieve atmospheric pressure equilibrium.

It doesn’t matter if the DP controller is in front or behind the control valve, as long the capillary tubings are placed correctly across the control valve.

PIBCV made compact

Nothing is better than a happy union of these two components. However, an in-depth understanding will show that valve authority is not just about pressure independence. It is also subject to the actuator’s abilities to make a smarter move.

Full stroke weakness

Figure 11-1 Shows that stroke 2 was reduced from 20mm to 10mm. Stroke 1, control valve, remains 20mm as full stroke control. As shown in Figure 11-2. Stroke 1 is only effective when it is less than 50%.

If flow presetting is in a separate valve and not in the control valve itself, then the presetting will take away the control valve’s authority, reducing the control range and control authority almost to half, even though it is pressure independent.

Strokes 1 and 2 are a direct relation of (Figure 3-2-1: Pressure transition charts).

Full stroke versus stroke limitation

The diagrams shown here is to demonstrate the stroke limitation strength against full stroke. If the flow presetting is done within the control valve itself, then a good control authority can be achieved. The actuator maintains full control signal of 0-10VDC vs 0-100% flow control range after performing self-calibration

Matching the coil character

We now know that most heat exchanger’s character deviates from application to application. The next step is to determine the heat exchanger’s character and select the correct alpha value. There is a knob setting in the actuator that performs this function.

Variable primary system optimised with PIBCV

Unlike manual balancing valves (Page 8), it does not compensate pressure changes when pump speed is varying. Thus, the biggest fear of under-capacity may occur in such operations.

With PIBCV (Figure 14-2), we can rest assured that the DP controllers will perform self-balancing throughout different pump speeds. This installation will allow the system flowrate to vary in proportion to pump pressures.

ΔP sensor near to pump

ΔP sensor installed near the pumping source does not allow the pumps to run at the most optimum speed. In the diagram illustrated below, the pump pressure is constant even though system flowrate keeps varying.

ΔP sensor at critical unit

When we place the ΔP across the most critical unit, it allows the pump pressure to vary proportionally to the system curve as the flowrate reduces. Most engineers call this proportional pump control. It is illustrated in Figure 16.

PIBCV improves comfort

PIBCV eliminates low ΔT syndromes

It is evident that not only is the flowrate reduced without compromising on the cooling capacity, but there is also an improvement in the comfort level

Let’s count PIBCV’s blessings

• Increases comfort
• Saves energy
• Eliminates extra valves in the system
• Saves time
• Saves manpower
• Simplifies installation
• Increases the lifespan of the actuator
• Increases the lifespan of pumps, chillers, boilers and heat pumps
• No complicated conventional control valve sizing required

Disclaimer: Danfoss believes the facts and suggestions presented here to be accurate. However, final design and implementation decisions are your responsibility. Danfoss disclaims any responsibility for actions taken on the material presented.

Ismail Serhan Ozten is Area Sales Manager (MEA) at Danfoss Heating Solutions – District Energy. He can be contacted at serhan@danfoss.com