Answer to Question #257810 in Molecular Physics | Thermodynamics for Mwalimu

Question

A refrigeration system of 10.5 tonnes capacity at an evaporator tem￾perature of – 12°C and a condenser temperature of 27°C is needed in a food storage locker. The refrigerant ammonia is sub-cooled by 6°C before entering the expansion valve. The vapour is 0.95 dry as it leaves the evaporator coil. The compression in the compressor is of adiabatic type. Using p-h chart find : (i) Condition of volume at outlet of the compressor (ii) Condition of vapour at entrance to evaporator (iii) C.O.P. (iv) Power required, in kW. Neglect valve throttling and clearance effect

Answer 1

$_TE=T_{satPe}=−12℃,T_C=T_{satPc}=28℃$

Capacity=10 TR, x₁=0.95

$h_1=h_{f−12℃}+x_1×h_{fg−12℃}$

$h_{fg−12℃}=h_g−h_f=1447.74−144.929 =1302.811~\frac{kJ} {kg}$

(from psychrometric chart booklet for N₃)

$h_1=144.929+0.95×1302.811 =1382.6~\frac{kJ} {kg}$

Condition of refrigerant at outlet of compressor

$T_2=67°C,h_2=1598~\frac{kJ}{kg},v_2=0.15~\frac{m^3}{kg},p_2=10.5~ bar$

Condition of refrigerant at inlet to evaporator

$T_4=−12°C,h_4=310~\frac{kJ}{kg},p_4=2.65 ~bar$

COP of given system = $\frac{R.E.}{W_c}=\frac{h_1−h_4}{h_2−h_1}=4.98$

Also, 

$Q̇_a=ṁ_R×(h_1−h_4)$

For 10 TR capacity, $10x3.5=ṁ_R×(1382.6−310)$

mass flow rate, $ṁ_R=0.0326~\frac{kg}{sec}$

Now, power required = $ṁ_R×(h_2−h_1)=7.0287 ~kW.$