3B Scientific Heat Pump D (115 V, 60 Hz) - Manual (Page 5)

5
8.2 Mollier diagram
An ideal cycle can be represented by a Mollier
diagram by measuring pressures p(3) and p(4)
before and after the expansion valve and the
temperature T(1) before the compressor:
T
(1) and p(4) determine point 1 of the Mollier
diagram (see Fig. 5). The intersection of the
corresponding isentropes with the horizontal line
p
(3) = constant defines point 2. The intersection
of the horizontal with the line representing the
boiling point gives point 3, then a perpendicular
down to the horizontal p(4) = const. provides
point 4.
Additionally, measuring temperatures T(2), T(3)
and T(4) provides an extra insight into the proc-
esses taking place inside the heat pump:
The temperature T(4) measured externally is in
agreement with the overall temperature read from
the temperature scales of the corresponding ma-
nometer to within the precision of the equipment.
This temperature scale is based on the curve rep-
resenting work done by the refrigerant. The meas-
urement therefore shows that the refrigerant be-
yond the expansion valve is in a mixture of liquid
and gaseous states.
The externally measured temperature T(3),
however, differs from the temperature read from
the manometer on the high-pressure side. The
refrigerant at this point contains no gas content.
It is entirely liquid.
The following equipment is recommended for
taking external measurements (see section 4.
accessories):
Pt100 thermocouple
with measurement terminal
1009922
3B NETlog (230 V, 50/60 Hz) 1000540 or
3B NETlog (115 V, 50/60 Hz) 1000539
8.3 Theoretical efficiency
The theoretical efficiency of the ideal cycle can
be calculated from specific enthalpies h
1
, h
2
and
h
3
, which can be read directly from the Mollier
diagram:
1 2
3 2 2
th
h h
h h
w
q
−
−
=
Δ
Δ
= η
8.4 Mass flow rate of refrigerant
Once the enthalpies h
2
and h
3
for the ideal cycle
are known as well as the amount of heat ΔQ
2
supplied to the water reservoir in a time interval
Δt, then it is possible to estimate the mass flow
rate of the refrigerant.
3 2
2
1
h h t
Q
t
m
−
⋅
Δ
Δ
=
Δ
Δ
9. Energy monitor
Key Function
Voltage Display of mains volt-
age
(unit V)
Time
(alternates with each
key press) Display of time
(unit mins)
or display of compres-
sor operating duration
ED
(unit h or mins)
Power Display of current
power consumption
(unit W or kW)
Energy Display of energy con-
sumption since last
reset
(unit Wh or kWh)
To reset hold down the “Time” key for about 3 s
until “RES” is displayed. The displays of time,
compressor operating duration, power and en-
ergy are all then set back to 0.
10. Mollier diagram
Mollier diagrams for a refrigerant are often use
to demonstrate the operating cycle for a com-
pression heat pump (see appendix). They plot
the pressure p against the specific enthalpy h for
the refrigerant (enthalpy is a measure of heat
content in the refrigerant and always increases
with increasing pressure and gas content).
In addition the isotherms (T = const.) and isen-
tropes (S = const) are given as well as the rela-
tive mass content of the liquid phase. Left of the
so-called boiling line, the refrigerant is fully liq-
uefied. To the right of the so-called saturated
vapour line, the refrigerant will exist as over-
heated vapour. Between the lines the refrigerant
will be in a mixture of liquid and gas states. Both
lines intersect at the critical point.
See Fig.6 on page 6.