Page 87 - Defrosting for Air Source Heat Pump

Modeling study on uneven defrosting 79

where P is the actual compressor discharge pressure. P c is the critical pressure for R22,
and a value of 4.99 MPa was used.
Because the boundary of the two-phase region was moving during defrosting,
h TPM , a mean heat transfer coefficient of refrigerant in Eq. (4.15), was evaluated
by Shah [24] as:

2:09
h TPM ¼ h r,L 0:55 + (4.20)
0:38
p re
In Eq. (4.13), A t is the total refrigerant tube surface area of each circuit,

A tube
A t ¼ (4.21)
3
where A tube is the total refrigerant tube surface area of the entire three-circuit
outdoor coil.
Moreover, in Eqs. (4.2) and (4.13), q j can also be evaluated by [22,26]:

ð
q j ¼ m r, j h r,in h r,e Þ j ¼ 1 3ð Þ (4.22)
where m r, j is the mass flow rate of refrigerant in Control Volume j. Enthalpies of the
refrigerant at both the inlet and outlet of each circuit, h r, in and h r, e , were evaluated
from the measured inlet and outlet refrigerant temperatures and the measured com-
pressor discharge pressure.

Second stage: Frost melting without water flowing away from a circuit As
shown in Fig. 4.2B, energy conservation in Control Volume j, taking into account
the convective heat transfer between the water layer and ambient air, required:

dM w, j T w, j
q j ¼ L sf m f, j + c p + h c,w T w, j T a A f a + q Me j ¼ 1 3ð Þ (4.23)
dt
where h c, w (T w, j T a )A f a is the heat transferred to the ambient air from the effective
airside surface area, A f a , which was covered by only the melted frost in Control Vol-
ume j. In the second and third stages, A f a , was evaluated [8,14]:

0 ð t 11:5
m f , j dt
B C
0
A f a ¼ A 0 B C ð j ¼ 1 3Þ (4.24)
M f, j
@ A
Because the outdoor fan was turned off during the entire defrosting process, natural
convection was the most important form to transfer the heat from the outdoor coil or