This configuration is the evolution of uncoupled systems with constant flow in primary and variable in secondary, and seeks to minimize pumping consumption, minimizing the flow that circulates through the bypass.
CONTENTS
1 - TECHNICAL SHEET
2 - GENERAL
It is a hydraulic configuration in trend, an evolution of the uncoupled configuration with variable flow in secondary in which it is intended to maximize energy savings in pumping.
The designer must take into consideration:
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It is mandatory that the secondary be variable flow
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The maximum AT that can be achieved in primary will always be slightly lower than in secondary
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For the regulation of the primary flow, the whole must be taken into account, and therefore the regulation that is included from the factory in the chillers, will surely not be valid when the system is made up of several chillers.
This hydraulic configuration is normally projected in a system with 2 or more chillers and variable load variation, so as to partialize production by stopping chillers, andAdditionally, there is a lot of flow circulating through the bypass for a long time. For example, with large chillers that work very well at part loads and therefore it is not necessary to install a pivoting chiller.
It's a very versatile setup setup-wise, not like theconstant flow coupled systemsand especially those ofvariable flow, VPF
The main objective of this configuration is to hydraulically DISCOUPLE the primary from the secondary, that is, that what happens in the secondary does not affect the primary. However, in most cases this does not happen because a shut-off cock or non-return valve is installed in the bypass COUPLING the primary and secondary.
In a decoupled type configuration pay attention to themanifold connection order, is essential, but to make a variable flow in primary it is even more important. To what was described in the previous configurations, an additional detail now comes into play, the order of connections of the returns if the AT of the secondaries is not the design in all operating scenarios. .
3 - COMBINATION OF CHILLERS
As previously mentioned, this is a very versatile combination in terms of configuration:
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Flow compatibilities between chillers, here it is not essential that all the chillers that make up the plant are compatible in minimum and maximum flow as a system VPF
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asymmetric configuration; From a design and control system point of view, an asymmetric configuration does not complicate the system at all. Furthermore, an asymmetric configuration is recommended if it is the best option to match production to demand. In an asymmetric configuration you can have:
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Base chiller, which will have priority in operation. It could be one with very high efficiency, or heat recovery, or any other reason.
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Peak chiller, the one that will enter the system last, only when it is really necessary, for example the one with the worst efficiency, or the most deteriorated, etc.
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Pivoting chiller, the one that is used in the sequencing of chillers both in addition and in subtraction to make the transitions between chillers of greater power.
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Any of the chillers within the system can take a different role, for example a base chiller with heat recovery and lower efficiency can go to peak when recovery is not needed, or a chiller with integrated freecooling.
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Symmetrical configuration, in which all the chillers are the same. At this point it is recommended to take into account that the minimum system load is not below the minimum capacity step._d04a07d8-9cd1-3239-8139-6b7d8
4 - CAPACITY MANAGEMENT
In the production control system, in the first place each type of chiller must be considered what role it will play in the order of the sequence, base, peak, pivoting or let's say normal.
If the collector is correctly made and the location of the probes is correct, then an optimized production management can be done, otherwise the system will only be able to do a "simple" sequencing that can present problems in production management.
The control cannot solve the problems that a bad hydraulic design causes, just maybe mask it. Control cannot work miracles.
Addition of chillers:
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By flow temperature in the collector, under the premise of fewer chillers in operation possible
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By % load, if you want to take a strategy based on partial load curves
Subtraction of chillers, as long as the delivery setpoint condition is met,
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By AT on the collector, as long as the order of connections is correct
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By % load in chillers, check before the subtraction that when removing the next chiller in the sequencing that the rest can be done with the demand.
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By bypass flow, either measured by flowmeter or by collector probes.
Other functions such as Chiller Water Reset may not have an impact on overall energy savings.
The control system must also manage the variable frequency drives of the primary pumps but as a consequence or adaptation of the flow to the number of chillers running.See detail below.
Adjusting production to demand only in terms of thermal power is a concept that is very fashionable, but by itself it presents deficiencies and might not work if other factors are not taken into account.
Plant control methods:
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delivery probe
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return probe
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bypass probe
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bypass flow
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% chiller load
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general return probe
5 - EXCESS FLOW IN PRIMARY AND AT MAXIMUM PRIMARY
In the section5 Excess flow in primaryFrom the decoupled configuration variable flow in secondary, the conditions, the need and connotations that the secondary must have to be considered variable flow and how it affects the sequencing of chillers are detailed.
However, in order to carry out the flow variation in primary, there must ALWAYS be excess flow in primary and for this, the designer must be clear that:
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The variable flow configuration in primary and constant in secondary is not compatible.
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The maximum AT that can be achieved in primary will always be less than the AT in secondary; logically if there is a current of cold water that circulates through the collector that mixes with the return of the installation.
In this configuration, not having excess flow in primary, not only does not make sense, but also from the point of view of efficiency de , which would be the objective, not only saves energy but also consumes more in the chillers, since in order to compensate the hot water mixture in the collector, a strategy ofChiller Water Reset.
6 - FORMS OF CONTROL VARIABLE PRIMARY FLOW
In the first place, it should be emphasized that the control must be taken into account as a whole, and therefore it is very likely that the individual control that is loaded in the chiller controller is not valid.See more.
There are several ways to carry out the Hz control in the pumping, which will depend on various factors, such as the order of connections in the collector, length of the bypass, AT of the secondaries, new project or renovation, etc.
How NOT to control the flow variation:
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Individual chiller control, normally there are two options, constant AT or constant AP. Neither of the 2 are valid.
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AP in the primary circuit, is also not valid. Remember that the function of the collector is to decouple the primary and secondary and that it is a point of almost zero differential pressure, so there is no pressure variation depending on the demand.
How to control the flow rate variation:
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If all of the following circumstances apply:
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The order of connections in the manifold is correct
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All secondaries work with a correct AT in the entire flow range
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All secondaries work with the same AT in design
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The flow could be controlled in such a way that the collector return temperature probe is X ºC lower than the circuit return.
This method is not recommended as the control system is blind to any disturbance.
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Measure the bypass flow and modify the rotation speed to maintain a slight excess primary flow. The less the better. 136bad5cf58d_ _cc781905-5cde- 3194-bb3b-136bad5cf58d_ _cc781905-5cde-3194 -bb3b-136bad5cf58d_ _cc781905-5cd e-3194-bb3b-136bad5cf58d_ _cc781905-5cde-3194-bb3b -136bad5cf58d_ _cc781905-5cde -3194-bb3b-136bad5cf58d_ _cc781905-5cde-3194-bb3b- 136bad5cf58d_
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✅ Advantages, it is not necessary that the above conditions are met
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❌ Disadvantages, it is necessary to have enough space in the bypass so that themeasurement is correct, thecollector diameter, and the cost of the flowmeter.
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Gathering all the returns at a single point before entering the general collector, and with a general return temperature probe, it is a matter of maintaining a differential of XºC with the collector return probe. _cc781905 -5cde-3194-bb3b-136bad5cf58d_ _cc781905-5cde-3194- bb3b-136bad5cf58d_ _cc781905- 5cde-3194-bb3b-136bad5cf58d_ _cc781905-5cde-3194-bb3b -136bad5cf58d_ _cc7 81905-5cde-3194-bb3b-136bad5cf58d_ _cc781905-5cde-3194 -bb3b-136bad5cf58d_ _cc781905 -5cde-3194-bb3b-136bad5cf58d_ _cc781905-5cde-3194- bb3b-136bad5cf58d_ _cc781905- 5cde-3194-bb3b-136bad5cf58d_ _cc781905-5cde-3194-bb3b-136b ad5cf58d_
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✅ Advantages: The first 3 conditions and the investment cost do not have to be met, especially in new construction.
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❌ Disadvantages: In an existing installation there may not be space available in the machine room or it may not be possible to empty it completely.
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7 - ENERGY SAVING POTENTIAL
The objective of this configuration is to reduce energy consumption in the primary pumps.
The graph represents the relationship between flow and power in a decoupled configuration with constant flow in primary and variable in secondary, in green the secondary and in blue the primary:
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Secondary: The relationship is linear and proportional if the AT remains constant.See more
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Primary: The relationship is not linear, but by steps. Starts chiller 1 and its primary pump, pumps a fixed amount of flow regardless of the delivered capacity.
The unused flow, area shaded in blue, is the overflow pumped in the primary and diverted to the bypass,
The graph represents the relationship between flow and power in a decoupled configuration with variable flow in primary and variable in secondary, in green the secondary and in blue the primary:
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Primary: In this graph, the relationship of pumped flow and capacity is proportional, except in the zones of minimum flow of chillers (1) and addition (3)
Note that the blue line is represented slightly apart from the green line. This area would be the slight excess flow that is necessary to maintain between primary and secondary.
AND HOW MUCH IS THE ENERGY SAVING?
To calculate the energy savings, it should be taken into account that the control of the pump drive is not carried out by pressure, but rather the hz is reduced based on the bypass flow, either by temperature probes or flowmeter, and therefore it is follows the characteristic curve of the primary circuit (Kv) and the relationship between flow and HP is quadratic:
Kv = Q / √ΔP
where
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Kv: Kv value [m³/h]
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Q: Flow rate [m³/h]
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ΔP: Pressure loss across the regulating valve [bar]
Applying the pump affinity laws, we have:
1.- The flow is proportional to the speed of the shaft
2.- The static pressure is proportional to the square of the shaft speed:
3.- The electrical power absorbed by the pump motor is proportional to the cube of the shaft speed
Therefore, the reduction in consumption is cubic proportional to the reduction in flow
In the primary pump selection graph of the examples, the nominal duty point (1), the pump curve and the circuit characteristic curve are shown.
The point 178 m3/h and 21.97 mca and 14.77 KW
If the flow is reduced to 140 m3/h (2), the pressure drop offered by the circuit is 11.24 mca and the consumption is 6.4 KW
If the flow is reduced to 81 m3/h (3), the pressure drop offered by the circuit is 4.3 mca and the consumption is 1.5 KW
8 - DESIGN RECOMMENDATIONS
In a configuration of the decoupled type primary variable flow and secondary variable flow, attention must be paid to the following points:
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Selection of chillers
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Nothing to note, those that best suit the circumstances of the project
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In energy studies, take into account thatchillers in parallel work at the same %of load.
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Chillers with greater flow variation possible
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Chiller Sequencing
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Addition by flow temperature in collector
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Addition by % of load if additionally a strategy to work at partial loads is required
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Subtraction by % load
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Subtraction by AT collector
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Bypass flow subtraction
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Primary pump speed control (hydraulic design and control)
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plant configuration
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Symmetrical configuration (all chillers are the same) or asymmetrical configuration
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Peak, base, pivoting chillers
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collector design
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Excess flow in primary
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Control of terminal units
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Pay attention to the selection of V2V to ensure good flow control and therefore an AT similar to the design.This element is vital for the proper functioning of the entire plant.
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