Functional Design of the mCCHP-RES System - Design for Micro-Combined Cooling, Heating and Power Systems - page 268

F. Expansion tanks

If the overall hydraulic device and its component circuits work under pressure,
the presence of under pressure tanks is necessary and compulsory. The functional
role is that of ensuring the pressure of the hydraulic circuits and the absorbing of the
thermal dilatation of

fluids along each circuit. The construction particularities of the
device make possible the existence of independent circuits and circuits which
couple while functioning. The criteria for the selection of expansion tanks are the
following:

Work pressure, [bar];

￿

Pressurization pressure in the expansion tank, [bar];

￿

Maximum work pressure (safety valve), [bar];

￿

Volume of accumulation tank, [ 1 ];

￿

Volume of adjoining pipes, [ 1 ];

￿

￿ Maximum/minimum work temperature, ( ° C).

The characteristics of expansion tanks are the following:

￿ Volume of expansion tank, [ 1 ];

°

￿ Maximum work pressure, ( C);

￿ Maximum/minimum expanded volume, [ 1 ];

Tank diameter and height, [mm].

￿

Dimensioning of the expansion tanks consists in determining their volume. To
this end, two procedures are used:
The analytical procedure, which uses different forms of the mathematical model
of the liquid volume dilation. One form is the following:

V et ¼ kV w v 1 = ½ ð v 0 Þ 1 = ½ 1 ð p 0 = p 1 Þ ð 47 Þ

where V et is the volume of the expansion tank, k = safety factor (approximately 2 is
common), V w = water volume in the system, [liter]; v 0 = speci c volume of water at

initial (cold) temperature, (m 3 /kg); v 1 = speci c volume of water at operating (hot)

temperature, (m 3 /kg); p 0 = system initial pressure

— cold pressure, (Pa); p 1 = system

operating pressure hot pressure, (Pa).

Another form of the mathematical model based on which the expansion tanks are
dimensioned is:

—

e V w

V ¼ ð 48 Þ

p c
p h

1

where: V w — volume of water in the circuit, [liter]; e — water dilatation coef cient;

p c — absolute pressure of the water in the system at initial temperature (cold), [bar];

p h — absolute pressure of the water in the system at maximum work temperature

(hot), (bar). The coef cient of water dilation at atmospheric pressure is given in the

Table 10 .