Calculation of the temperatures generated by the separator

The hydraulic separator resolves the problem of interference among the pumps of the single circuitss by making them independent but effects the temperatures in that within them there are mixing pheonomena that can be quite significant. It can happen that the hot fluid coming from the generator of heat mixes with the cold fluid returning from the system circuits thus reducing the supply of the same.

In dimensioning the heating systems it is therefore important to keep in consideration the possibility of this variation in temperature in order to correctly calcolate the thermal output. The variations in temperature generated by the hydraulic separator depend on the configuration of the system and in particolar on the difference between the flow in the primary circuit and the secondary circuit.

By indicating the inlet and outlet temperature from the separator in the primary side with T_1P and T_2P, and the inlet and outlet temperature from the separator in the secondary side with T_2S and T_1S, with m_P and m_s the flows respectively in the primary side (supplied by the generator) and in the secondary side (required of the system) and Q being the thermal output requested by the system, the energy balance on both sides of the hydraulic separator is given by the following two relations.

Q = cP · mP · (TP1 - TP2) [3.7.4]

Q = cs · ms · (T^S1 - TS2) [3.7.5]

where c_p is the specific heat of the water equal to 4190 J/°C·kg.

a) Flow in the primary circuit equal to the flow in the secondary circuit Figure 3.7.38 Equal flows in the primary and secondary circuits.

TP1

TP2

TS2

TS1

© 2008 Valsir S.p.A.

If the flows present in the primary circuit and in the secondary circuit are the same mP = mS

then the effects of internal mixing are absent and therefore the temperatures have the following relationship:

T_P1 = T_S1

T_P2 = T_S2

With the supply temperature in the primary side T_P1 [°C], the flow in the primary m_P [kg/s] and the thermal output requested by the system Q [W] the return temperature in the primary and secondary sides can be determined by utilizing the equation [3.7.4]:

Q cp · mp

TS2 = TP2 = T^P1 -

101

TECHNICAL CHARACTERISTICS OF THE COMPONENTS

3

b) Flow in the primary circuit greater than the flow in the secondary circuit Figure 3.7.39 Flow in the primary circuit greater than in the secondary circuit.

TP1

TP2

TS2

TS1

© 2008 Valsir S.p.A.

Should the flow in the primary circuit be greater than the flow in the secondary circuit m_P > m_S

the water will begin to mix and will cause an increase in the return temperature of the primary circuit.

We have, therefore:

T_P1 = T_S1

T_P2 > T_S2

With the supply temperature in the primary side T_P1 [°C], the flow in the primary mP [kg/s] eand the thermal output requested by the system Q [W] it is possible to determine the return temperature in the primary side.

T_P2 = T_P1 - Q c_P · m_P

With the flow mP [kg/s] in the secondary side, the return temperature T_S2 can be determined:

Q Q

T_S2 = T_S1 - = T_P1 - c_P · m_S c_P · m_S

c) Flow in the primary circuit lower than the flow in the secondary circuit Figure 3.7.40 Flow in the primary circuit less than the flow in the secondary circuit.

TP1

TS1

TECHNICAL CHARACTERISTICS OF THE COMPONENTS

3

T_P2 = T_S2

the supply temperature on the primary side T_P1 [°C], the flow in the primary circuit mP [kg/s] and the required heat output of the system Q [W] the return temperature in the primary side can be determined.

T_P2 = T_P1 - Q c_p · m_P

With the flow m_s [kg/s] in the secondary side the supply temperature T_S1 can be determined, to be used for dimensioning the system (radiators, floor heating, etc.):

Q Q

T_S1 = T_S2 + = T_P2 + c_P · m_S c_P · m_S

3.7.4.3 Example 1

Consider a system such as then one shown in the figure and determine the supply temperature to the heating circuit.

Figure 3.7.41 System layout.

Radiator circuit Radiant floor 2Radiant floor 1

© 2008 Valsir S.p.A.

Radiator circuit - Flow _m1 = 400 kg/h - Heat output Q₁ = 9 kW Floor heating circuit 1

- Flow _m2 = 330 kg/h (boiler side) - Heat output Q₂ = 5 kW

Floor heating circuit 2

- Flow _m3 = 520 kg/h (boiler side) - Heat output Q₃ = 22 kW Generator

- Supply temperature T_P1=70°C - Flow m_P = 2500 kg/h - Heat output Q_G = 50 kW

103

TECHNICAL CHARACTERISTICS OF THE COMPONENTS

3

Total heat output required by the system

By summing the required outputs of each heating circuit we obtain:

Q = Q₁ + Q₂ - Q₃ = 9000 + 15000 + 22000 = 46000 W Flow of the secondary

The flow in the secondary circuit is obtained by summing the flows of the single heating circuits; with floor heating systems supplied by mixers, the flow to be considered is obviously the flow of the boiler side (diversion flow).

m_S = m₁ + m₂ + m₃ = 400 + 330 + 520 = 1250 kg/h = 0.3472 kg/s Flow of the primary

The flow in the primary circuit is the maximum flow supplied by the generator pump:

m_P = 2500 kg/h = 0.6944 kg/s

Supply temperature of the secondary circuit

In this case the flow of the primary circuit is greater than the flow in the secondary circuit and therefore the separator works with a supply temperature in the secondary circuit equal to the temperature in the primary circuit:

T_S1 = T_P1 = 70°C

Return temperature of the primary circuit The return temperature is:

Q 46000

T_P2 = T_P1 - = 70

-c_p · m_P 4190 · 0.6944 = 54°C

Return temperature of the secondary circuit

In these conditions the supply temperature of the secondary is equal to the supply temperature of the primary:

Q Q 46000

TS2 = TS1 - = TP1 - = 70 -

cp · mS c_p · m_S 4190 · 0.3472 =38°C Figure 3.7.42 System layout, flows and temperatures.

Radiator circuit Radiant floor 2Radiant floor 1

TECHNICAL CHARACTERISTICS OF THE COMPONENTS

3

3.7.4.4 Example 2

Consider a system like the one shown in the layout in the figure and determine the supply temperature of the heating system.

Figure 3.7.43 System layout.

circuito radiatori 1 circuito radiatori 2 circuito radiatori 3

© 2008 Valsir S.p.A.

Radiator circuit 1 - Flow m1 = 770 kg/h - Heat output Q₁ = 9 kW Radiator circuit 2 - Flow m2 = 1900 kg/h - Heat output Q₂ = 22 kW Radiator circuit 3 - Flow m3 = 1290 kg/h - Heat output Q₃ = 15 kW Generatore

- Supply temperature T_P1 = 75°C - Flow mP = 2500 kg/h

- Heat output Q_G = 50 kW

Total heat output required by the system

By summing the required outputs of each heating circuit, we obtain:

Q = Q₁ - Q₂ - Q₃ = 9000 + 15000 + 22000 = 46000 W Flow of the secondary circuit

The flow of the secondary circuit is obtained by summing the flows of the single heating circuits:

m_S = m₁ + m₂ + m₃ = 770 + 1900 + 1290 = 3960 kg/h = 1.1 kg/s Flow of the primary circuit

The flow in the primary circuit is the maximum flow supplied by the generator pump:

m_P = 2500 kg/h = 0.6944 kg/s

105

TECHNICAL CHARACTERISTICS OF THE COMPONENTS

3

Return temperature of the primary circuit The return temperature is:

Q 46000

TP2 = TP1 - = 75 -

c_p · m_P 4190 · 0.6944 = 59°C Return temperature of the secondary circuit

In this case the flow in the primary circuit is below the flow in the secondary circuit and therefore the return temperature in the secondary circuit is the same as the return in the primary circuit:

T_S2 = T_P2 = 59°C

Supply temperature of the secondary circuit

The supply temperature in the secondary circuit and that must be used for feeling the heating circuits is:

Q 46000

TS1 = TS2 + = 59 +

cp · mS 4190 · 1.1 = 69°C Figure 3.7.44 System layout, flows and temperatures.

75°C

69°C

59°C 59°C

2500 kg/h 3960 kg/h

Radiator circuit 1 Radiator circuit 2 Radiator circuit 3

© 2008 Valsir S.p.A.

TECHNICAL CHARACTERISTICS OF THE COMPONENTS

3

In document FLOOR HEATING VALSIR.pdf (Page 101-107)