Functional Design of the mCCHP-RES System - Design for Micro-Combined Cooling, Heating and Power Systems - page 271

where:

# p entry þ# p exit
2

# p ¼ is the average temperature of the thermal fluid for primary

circuit ( o C);
# s ¼

# s entry þ# s exit
2

is the average temperature of the thermal fluid for secondary

circuit ( o C);
q p , q s —

fluid for primary and secondary circuits (kg/m 3 ); density of the thermal

c p , c s —

speci c heat of the thermal fluid for primary and secondary circuits

(kWh/kg ° C);

V p , V s —

volume flow of the thermal fluid for primary and secondary circuits

(m 3 /s);
k cient of the heat exchanger (kW/m 2

— °

overall heat transmission coef C);

heat exchanger area (m 2 );

—

A

Q

— heat flow transmitted by the heat exchanger (kW).

In heat exchanger selection, the required characteristics are k and A, while the
rest of variables which appear in mathematical model are the entry data.
The heat exchanger may be of plate type that consists of exchanger package and
casing. The exchanger package consists of aluminum or stainless steel plates with
pressed-in spacers; condensate drainage is possible in every direction. The plates
are connected by a fold, which gives a several fold material thickness at fluid entry

and exit (Fig. 16 ).
Plate heat exchangers operate on the cross- ow principle. Heat is transmitted via

the plates from the warm to the cold stream.
The heat flow transmitted is directly dependent on the exchanger surface area A.
If the heat

flow to be transmitted is known, then the area A is determined with

relations:

Fig. 16 Plate heat exchangers