3B Scientific Heat Pump D (230 V, 60 Hz) - Manual (Page 4)

Elwe Didactic GmbH • Steinfelsstr. 6 • 08248 Klingenthal • Germany • www.elwedidactic.com
3B Scientific GmbH • Rudorffweg 8 • 21031 Hamburg • Germany • www.3bscientific.com
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© Copyright 2008 3B Scientific GmbH
The low pressure in the evaporator allows the liquid
refrigerant to boil at a low temperature T
0
*. The heat
Q
a
required for this is absorbed from the surroundings
as long as they are at a temperature T
0
where T
0
> T
0
*.
The refrigerant vapour then returns to a pressure and
temperature of p
0
and T
0
* (state 1) and is once again
sucked into the compressor.
In the T, Q/T diagram the quantities of energy are
indicated by areas
T / Q T Q
Δ ⋅ = Δ
. The area beneath
the line between 4 and 1 where the refrigerant boils
(p
0
= const., T
0
* = const.), for example, represents the
heat of evaporation Q
a
transferred to the refrigerant.
If such a machine were to operate without losses, the
quantity of heat Q
z
emitted from the condenser would
be equal to the sum of the work W done by the
compressor and the heat Q
z
absorbed in the evapora-
tor.
4. Example experiment
Determining the efficiency
ε
Two thermometers are required to measure the rise
or fall in temperature in the water vessels.
• Take the beakers from the heat pump and fill
them with water.
• Put back the reservoirs as follows:
• Place the beakers carefully on the base plate and
move them with the low edge first under the
evaporator and the condenser.
• Turn the beakers in such a way that the high
edge points to the back wall.
• Lift up the beakers and mount them into the
retaining plates.
• Connect the heat pump to the mains supply.
• Allow the compressor to run for about 10 min-
utes before starting the experiment until it
reaches its operating temperature. Then replace
the water and start the experiment.
• Push the button „Zeit“ (time) at the energy
monitor for 6 seconds and at the same time
switch on the compressor. (toggle switch to the
right)
The energy monitor starts the time measurement
and records the power consumption of the compres-
sor.
• The water should be stirred thoroughly through-
out the experiment.
The efficiency
ε is given by the ratio of the change in
energy
ΔQ provided to the heat reservoir per unit
time
Δt, to the power P supplied to the compressor
to perform its work:
t P
Q
Δ ⋅
Δ
= ε
.
ΔQ is given by
T m c Q
Δ ⋅ ⋅ = Δ
where c = specific
heat capacity and m = mass of water.
The efficiency
ε decreases as the temperature differ-
ence between the evaporator and the condenser
increases.
To optimise efficiency the water beakers and the
pipes (but not the compressor) can be insulated with
foam strips, for example, in order to prevent radia-
tion losses.