BALANCING OF DISTRIBUTION CIRCUITS

TECHNICAL FOCUSBalancing part 1

Balancing is undoubtedly important and relevant for system engineering.Balancing a system means flowing through its terminals the right amount of medium and therefore obtaining the right heat emission. Balancing involves not only the system installation and management phase but also, and above all, the design phase.All this becomes even more relevant in older plants where the generator, circulating pumps are replaced and where electronic adjustments or control systems that reduce the flow rate are added, in an effort to reduce thermal waste.In these situations, there may be discomfort in the ambient thermal comfort (some rooms reach the well-being temperature with great difficulty while others are subject to overheating conditions), in the heating bodies efficiency (under low heat demand, ambient temperatures tend to oscillate) and in the overall system efficiency (especially during the most demanding transient thermal periods such as morning start and night shut-down).

LIMITING CIRCUITIt is equipped with a two-way adjustment valve that limits the flow rate according to the thermal requirement of the terminal. The flow rate passing through the circuits can therefore be variable both on the primary circuit (distribution circuit) and on the secondary circuit (terminal circuit). The temperature of the thermal medium on the return line is as low as possible (in heating systems) in both circuits, so it is a distribution system that is well suited to condensing boilers.

Specifications:a) The flow rate is variable both on the primary circuit

and on the secondary circuit.b) Low return temperatures on the primary circuit.c) With the fully closed valve, problems related to

the pump may occur (use variable displacement circulating pumps).

Specifications:a) The flow rate is variable on the secondary circuit

while it is constant on the primary circuit.b) Medium-high return temperatures on the primary

circuit under partial load conditions.

DEVIATING CIRCUITThe required medium amount is sent to the terminal through a three-way valve that deviates the excess medium on the by-pass line. The flow rate passing through the secondary circuit is variable and equal to the actual requirement, while the flow rate passing through the primary circuit is constant. In some partial load conditions, the return temperatures may be medium-high (problem with condensing boilers).

FLOW RATE CONTROL

Air conditioning systems must ensure the right thermal comfort for people who are in the air conditioned ambient. To ensure these conditions, the system must generate a quantity of thermal energy that must be correctly distributed to the emission terminals.

The thermal output emitted to the environment where the terminal is inserted depends on the volumetric flow rate, on the thermal head and on the characteristics of the thermal medium (specific heat "c" and density "ρ") that passes it, according to the formula:

Considering constant specific heat and density, the thermal power is proportional only to the flow rate and the thermal head:

To modulate the power emitted from the terminal, it is therefore possible to distinguish between flow rate modulating circuits with a constant flow temperature (P ∝ Gvar ⋅ ΔT) or between circuits controlling the flow temperature at constant flow rate (P ∝ G ⋅ ΔTvar).

G P ΔT

T1

T2

P = ρ ⋅ c ⋅ G ⋅ ΔT P ∝ G ⋅ ΔT P = ρ ⋅ c ⋅ G ⋅ ΔT P ∝ G ⋅ ΔT

THERMAL MEDIUM DISTRIBUTION SYSTEMS

BALANCING OF A DISTRIBUTION SYSTEM

MIXING CIRCUITThe flow rate through the secondary circuit is constant; the temperature of the thermal medium is changed by mixing together an amount of medium from the primary circuit with an amount of secondary circuit return. In this configuration the flow rate flowing through the primary circuit is variable.

Specifications:a) The flow rate is constant on the secondary circuit

while it is variable on the primary circuit.b) Lower returns on the primary circuit.

Specifications:a) The flow rate is constant both on the primary

circuit and on the secondary circuit.b) Medium-high return temperatures on the primary

circuit under partial load conditions.

INJECTION CIRCUITIn the injection circuit, the pump in the primary circuit "injects" into the secondary circuit a quantity of thermal medium regulated by the three-way valve that deviates its excess in the by-pass. The pump on the secondary works at constant flow rate and the system modulates the temperature mixing the "injected" medium amount from the primary circuit pump with an amount from the secondary circuit return. In this type of system, unlike the mixing circuits, the flow rate is constant both on the primary and on the secondary circuits.

TEMPERATURE CONTROL

A terminal circuit (identified as a secondary circuit) is always inserted into a wider primary circuit. To ensure proper operation, the secondary circuit must always be fed with the appropriate flow rate and the correct flow rate and head conditions regardless of the operating conditions of all other circuits that form the system in its entirety.For this reason, it is always good practice to use balancing valves in the thermal medium distribution systems.

The four distribution systems presented illustrate a general picture of the types most widely used in recent years. However, with the establishment of condensing boilers and the obligation to use environmental thermoregulation systems (e.g. Thermostatic radiator valves), the most common distribution system is becoming the limiting circuit for flow rate control (two-way system with variable flow rate).In the next Technical Focus, attention will be given on balancing this type of system.

Why balancing?

1. To avoid operating malfunctions such as over-feeding and/or under-feeding in certain zones and noise phenomena.

2. To reduce pump power consumption by circulating only the required flow rates.

3. To minimize return temperatures in order to ensure maximum thermal efficiency of condensing boilers.

4. To achieve high thermal comfort conditions, for example avoiding inadequate feeding to the terminals and overheating caused by external sources.

5. To operate the plant with the correct flow rates, and therefore with the correct thermal emissions, both with total load and with partial load.

BALANCING METHODS

FLOW COEFFICIENT OF A VALVE

The flow coefficient of a valve, commonly known as Kv, defines the flow rate of water (between 5°C and 40°C), expressed in m³/h, passing through a valve with a pressure drop of 1 bar at the ends of the valve.

The Kv value depends on the valve internal passage cross section. If a valve does not have moving parts, therefore it always appears in the same configuration (e.g. a filter), the Kv value is univocal. If a valve is used to perform an adjustment/setting of a circuit, it will be equipped with an internal adjustment element.Balancing valves, whether static or dynamic, operate by varying their Kv value to establish a balancing condition in the circuit in which they are inserted.

FLOW RATE STATIC BALANCINGIt is carried out using the manual balancing valves. To achieve the design flow rate value, the valve introduces a load loss in the circuit which depends on its adjustment point (design condition) and therefore on the consequent Kv value. The nominal flow rate thus obtained depends on conditions outside the circuit in the instant in which the head (H) to which the circuit is subjected may vary, the passing flow rate would be subject to a variation not fully compensated by the balancing valve; since the latter is a static device, in the new conditions it would always maintain the same Kv value causing a load loss which is no longer adequate to balance the circuit.

FLOW RATE DYNAMIC BALANCINGIn the case of secondary circuits subject to unstable heads due to frequent operating conditions at partial loads of the system, it is preferable to use components such as automatic flow rate regulators (AUTOFLOW®) or self-balancing regulators (FLOWMATIC®).These dynamic components mechanically change their Kv value adapting to the changing load conditions to maintain the flow rate constant to the design value.

Constant G

Constant G

Constant G Constant G Variable G

Constant G

Variable G

Cons

tant

H

varia

ble

Hva

riabl

e H

G = Kv ⋅ √ΔP

Cons

tant

H

variable depending on HKv variable depending on HKv

constant

(design conditions)

Kv constantKv

varia

ble

H

pump

constant

Kv

Kv

Δp

cons

tant

Cons

tant

H

pumpKv

Δp

cons

tant

varia

ble

H

pump

pump

Kv

Kv

Δp

cons

tant

Constant G

Constant G

Constant G Constant G Variable G

Constant G

Variable G

Cons

tant

H

varia

ble

Hva

riabl

e H

G = Kv ⋅ √ΔP

Cons

tant

H

variable depending on HKv variable depending on HKv

constant

(design conditions)

Kv constantKv

varia

ble

H

pump

constant

Kv

Kv

Δp

cons

tant

Cons

tant

H

pumpKv

Δp

cons

tant

varia

ble

H

pump

pump

Kv

Kv

Δp

cons

tant

Constant G

Constant G

Constant G Constant G Variable G

Constant G

Variable G

Cons

tant

H

varia

ble

Hva

riabl

e H

G = Kv ⋅ √ΔP

Cons

tant

H

variable depending on HKv variable depending on HKv

constant

(design conditions)

Kv constantKv

varia

ble

H

pump

constant

Kv

Kv

Δp

cons

tant

Cons

tant

H

pumpKv

Δp

cons

tant

varia

ble

H

pump

pump

Kv

Kv

Δp

cons

tant

Constant G

Constant G

Constant G Constant G Variable G

Constant G

Variable G

Cons

tant

H

varia

ble

Hva

riabl

e H

G = Kv ⋅ √ΔP

Cons

tant

H

variable depending on HKv variable depending on HKv

constant

(design conditions)

Kv constantKv

varia

ble

H

pump

constant

Kv

Kv

Δp

cons

tant

Cons

tant

H

pumpKv

Δp

cons

tant

varia

ble

H

pump

pump

Kv

Kv

Δp

cons

tant

Δp

KvG G = Kv ⋅ √Δp

WE RESERVE THE RIGHT TO MAKE CHANGES AND IMPROVEMENTS TO THE PRODUCTS AND RELATED DATA IN THIS PUBLICATION, AT ANY TIME AND

WITHOUT PRIOR NOTICE.

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The hydraulic circuit balancing components can therefore be divided into three macro-families according to the functionality they are associated with: static flow rate balancing, dynamic flow rate balancing, differential pressure adjustment.

FLOW RATE STATIC

BALANCING

Manual valvewith fixed orifice

Automatic flow rate regulator Pressure independent adjustment

Differential pressure regulating valve Differential by-pass valve

Manual valve with flow meter

Manual valve with variable orifice

Pre-settable radiator valve

FLOW RATE DYNAMIC

BALANCING

DIFFERENTIAL PRESSURE

ADJUSTMENT

DIFFERENTIAL PRESSURE CONTROL

If it is preferable to adjust the differential pressure (Δp) at the secondary circuit ends to allow a correct operation of the modulating adjustment valve contained in it (e.g. a valve with thermostatic control head), it is preferable to use the Δp regulators in way as to maintain constant the differential pressure value at the ends of the circuit itself.

Constant G

Constant G

Constant G Constant G Variable G

Constant G

Variable G

Cons

tant

H

varia

ble

Hva

riabl

e H

G = Kv ⋅ √ΔP

Cons

tant

H

variable depending on HKv variable depending on HKv

constant

(design conditions)

Kv constantKv

varia

ble

H

pump

constant

Kv

Kv

Δp

cons

tant

Cons

tant

H

pumpKv

Δp

cons

tant

varia

ble

H

pump

pump

Kv

Kv

Δp

cons

tant

Constant G

Constant G

Constant G Constant G Variable G

Constant G

Variable G

Cons

tant

H

varia

ble

Hva

riabl

e H

G = Kv ⋅ √ΔP

Cons

tant

H

variable depending on HKv variable depending on HKv

constant

(design conditions)

Kv constantKv

varia

ble

H

pump

constant

Kv

Kv

Δp

cons

tant

Cons

tant

H

pumpKv

Δp

cons

tant

varia

ble

Hpump

pump

Kv

Kv

Δp

cons

tant

Depending on the available head (H) but with fixed adjustment valve, the flow rate remains constant.

The adjustment valve intervenes by varying the flow rate in the secondary circuit; the value of this variation depends only on the Kv of the adjustment valve as:

Gpump= Kv ⋅ √Δpconstant