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Selecting pressure stabilizers

Relations:

 

1. Initial pressure in the expansion vessel:

p = pst+0,2

2. Increase in heating charge volume due to change in temperature from T₁ = 10ºC to T_Z (minimum vessel storage capacity):

Vu = V * ρ1 * ∆V

3. Minimum total expansion vessel capacity:

          pmax+1
 Vn = Vu ————————
          pmax-1

4. Minimum total expansion vessel capacity for TERMAT set:

        Vu
 Vn = ——————
       0,85

5. Minimum total expansion vessel capacity for TERMAX set:

        Vu
 Vn = ——————
       0,75

CAUTION: If the total expansion vessel capacity is higher than recommended or room dimensions does not allow use of high vessels, use more than one small vessels instead.

Quantities and units:

 

V       m³            – total water heating system capacity

V_u      dm³         – minimum storage expansion vessel capacity

V_n      dm³         – minimum total expansion vessel capacity:

ΔV     dm³/kg    – increase in water volume due to increase in temperature from T₁ = 10ºC to T_Z

T_Z      ºC            – design supply water temperature

p_st     bar           – static water heating system pressure at T₁ = 10ºC

p_max  bar           – maximum design water heating system pressure (safety valve opening pressure)

p       bar           – initial pressure in the expansion vessel:

ρ₁      kg/m³      – water density at initial temperature T₁ = 10ºC (ρ₁ = 999.7)

 

Increase in volume ΔV vs. supply water temperature T_Z.

EXAMPLE 1.

Select the expansion vessel for the central heating system based on the following data:

Data:

V = 17.5 m³             total water heating system capacity

p_st = 3.2 bar            static system pressure (building height 32 m)

p_max = 6 bar            safety valve opening pressure

T_Z = 90ºC                supply temperature

Procedure:

4. Expansion vessel designation:

2 x TerNWP 900              – expansion vessel (6 bar)

Comments:

1. Add additional 2 m column of water (0.2 bar) to the calculated static pressure to ensure heating medium supply to the highest heating system point. It also guarantees a minimum water reserve in the system.

2. The key factor when selecting the expansion vessel is to determine the increase in heating charge volume V_u due to heating from T₁ = 10ºC to T_Z (max. medium temperature).

3. Determine the expansion vessel size using a known increase in heating charge volume V_u,, initial pressure p and maximum system pressure p_max,. Determine the vessel designation after calculating the total vessel capacity V_n for required operating conditions. Select total capacity corresponding to the nearest higher calculated capacity from TerNWP data sheet, Specifications (6 bar expansion vessel) / Total capacity table.

4. Specify vessel type and total capacity, see Expansion vessel designation. Include pressure value in the symbol for 10 bar expansion vessels.

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EXAMPLE 2.

Select the pressure stabilizing system and top-up system based the following data:

1. Minimum total expansion vessel capacity:

For TERMAX system from TerNWP data sheet

Specifications (expansion vessel 6 bar) / Total capacity

– expansion vessel with total capacity: 900 dm³

3. Set designation:

TERMAX; 1 x TerNWP 900; p_st = 3.2 bar; water loss 0.1 m³/h

Comments:

1. Determine total expansion vessel capacity for the TERMAX system using the increase in heating charge volume V_u, calculated in example 1. Select total capacity corresponding to the nearest higher calculated capacity from TerNWP data sheet, Specifications (6 bar expansion vessel) / Total capacity table.

2. Specify equipment type, expansion vessel type (gravity vessel by default), static pressure and water loss, see Set designation.