Carrier SUPRA 950Mt - Heat and Defrost Mode

2-33
2.1
1.2
Hea
t
and
defr
ost
mode
In
he
at
mo
de
,
two
tec
hn
ol
og
ie
s
ca
n
be
us
ed
:
Ho
t
ga
s
he
ati
ng
thr
ou
gh
the
ho
t
ga
s
so
le
no
id
va
lv
es
(HG
V1, H
GV2
& HGV3) or heating by electric heaters in the evaporator.Heating by hot gas is allowed only if no evaporator is in
cooling mode.
If
two
or
thr
ee
ev
ap
ora
tor
s
arein
he
at
mo
de
,
on
e
ev
ap
ora
tor
on
ly
wi
ll
be
in
ho
t
ga
s
he
ati
ng
.
Th
e
oth
er
ev
ap
or
ato
rs
will be given inductive heating using the electric heaters in the evaporator.
Both hot gas and electric heat are used for defrost.
a.
Hot G
as H
eating
When refrigerant vapor is compressed to a high pressure and temperature in a reciprocating compressor, the
mechanical energy necessary to operate the compressor is transferred to the gas as it is being compressed. This
energy is referred to as the “heat of compression” and can be used as the source of heat during the heating cycle.
Whe
n
in
the
he
at
mo
de
,
wi
th
no
ev
ap
ora
tor
s
ca
ll
in
g
for
co
ol
in
g,
thehot
ga
s
so
le
no
id
va
lv
es
HGS
V1,
HGS
V2
,
an
d
HGSV3 cou
ld be en
ergize
d. The mai
n heat val
ve (MHV)
will clo
se,
divertin
g the
refriger
ant to
HGSV1, HGSV2,
and
HGSV3. The normally closed liquid solenoid valves LSV1, LSV2, LSV3 will energize and open. The normally closed
receiver pressure valve (RPV), situated in the hot gas line to the receiver will open. This allows the receiver to be
pressu
rized and liquid refrigeran
t t
o f
low through the drier
and sight glass and
pass through any liquid line solenoid
valve
s which would be energized. The refrigeran
t passes through the expansion
valve into the evapo
rator
. At
the
sa
me
tim
e
hi
gh
tem
pe
ra
tur
e,
hi
gh
pr
es
su
re
ga
s
en
ter
s
theeva
po
ra
torvia
thesole
no
id
va
lv
es
HGS
V
(1
,2
an
d
3)
to
gi
ve
the required heating. The extra liquid purged from the receiver ensures maximum heating capacity in low ambient
conditions. The evaporator fan passes the air over the hot refrigerant pipes and distributes heated air into the cargo
space.
The hot gas travels through the suctio
n line check valve into the accum
ulator where it is drawn
back throug
h
the
compre
ssor pressure regulatin
g
valve (CPR)
to
begin the
proces
s
again
.
Whentemper
atu
re
is
ach
iev
ed
in
all
eva
pora
tors
,
the
y
wil
l
go
int
o
nul
l
mod
e.
Thecompa
rtme
nt
withthe
hig
hes
t
set
point will then take the lead and revert back to cool/heat cycles.
b.
Princi
ple Of Induction Heating (Electr
ic Heat)
A control box recuperates the self indicate
d current from the
electric motor when the unit is driven by the diesel
engine. This current energizes electrical heaters mounted inside the evaporator.
On
standb
y
operati
on the
heaters are directly
energi
zed by
the main
electric
al supply
.
The system includes a control box, connected with electrical cables to the refrigeration unit and to the heaters
inside the evaporator .