Engineering, 2010, 2, 461-465
doi:10.4236/eng.2010.26060 Published Online June 2010 (http://www.SciRP.org/journal/eng)
Hydraulic Domestic Heating by Throttling
Mohammad A. K. Alia, Tariq Younes, Hussein Sarhan
Al-Balqa Applied University, Faculty of Engineering Technolog, Al-Salt, Jordan
Received February 3, 2010; revised March 15, 2010; accepted March 25, 2010
Abstract
In this work an experimental investigation was carried out in order to explore the possibility of realizing a
domestic heating system by throttling hydraulic oil. Considering the continuous increasing price of diesel oil,
this work gains unique importance. Generating heat directly by throttling is realized using a simple environ-
ment friendly system which does not require oil transportation and storage, and eliminates the need for
chimneys and annual preventive maintenance, as it is the case with heating by utilizing oil burners, which is
prevailing in Jordan. Experimental results show that it is possible to raise the room temperature up to 70C
during 15 minutes which is not a limit value. Experimental results show that temperature rate could be in-
creased by selecting the appropriate pump power and by connecting a number of throttles in parallel.
Keywords: Throttling, Pressure Drop, Hydraulic Oil, Heating
1. Introduction
It is an established fact that heat is generated in a fixed
displacement hydraulic system whenever fluid is throt-
tled from high pressure to low pressure without doing
any work. This generated heat is normally taken as a
measure of system efficiency. Although many thermo
and fluid references [1,2], describe the generated heat as
“losses”, fluid flow and pressure drop may be converted
to work.
Nowadays, this phenomenon finds a practical applica-
tion in swimming pools in order to increase heating sys-
tem efficiency. For this particular case water circulates
through a hydraulic motor/turbine, via a heat exchanger,
where water is heated. Further it is directed through noz-
zles in order to be heated for the second time by admix-
ing it, by means of its turbulence with the pool water.
Another application is the adjustment of the temperature
of large area heating system by automatic adjustment of
the throttle degree, so that the temperature difference
between the feed-line and the return line of the heating
loop stays the same.
It is a common practice in our daily life to directly
convert electrical energy into heat energy by using elec-
tric wire coils. In such a case the wasted electrical power
remains a target. What we are suggesting is to make an
analogue process by converting hydraulic energy into
heat energy by utilizing a hydraulic resistance (throttle).
Considering the continuous increasing price of oil, it
becomes apparent that direct consumption of electrical
energy by throttling will be more economical then gen-
erating heat energy using separate individual domestic
oil burners, which is the prevailing heating technology in
Jordan. There will be no need for oil storage and oil
transportation. Add to this the suggested heating system
does not require chimneys and does not involve any
contamination. Moreover the system is very simple and
less complicated when compared with diesel burner equip-
ment. Building on the above, the target of this work is to
experimentally investigate the possibility of realizing a
hydraulic domestic heating system by throttling.
2. Why so Much Heat is Generated during
Throttling?
Heating by throttling is generated by friction due to
pressurized oil forced into restriction [3]. The higher
friction will be between the oil and the inner surfaces of
the throttling valve. By circulating the heated oil during a
specific interval of time a considerable temperature rise
will take place, and as a result of that surrounding envi-
ronment will be heated. Fluids in motion are subjected
by various resistances, which are due to friction. Friction
may occur between the fluid and the pipe work and
within the fluid as sliding between adjacent layers of the
fluid takes place, which is normally characteristic for
turbulent flow.

M. A. K. ALIA ET AL. 462
Although friction is an important factor, friction alone
does not seem to account for the heat generation in the
hydraulic throttling process. A reasonable explanation is
to refer to the long molecular chain which gives oil its
viscosity and lubricity. As the fluid stream passes though
the pipes, molecules at the edge of the stream tend to
adhere to the inner walls of pipes. This slows down the
outer surface of the flow, and causes stresses and tearing
of molecular chains. Heat is then generated when the
molecular bonds are broken and re-formed. This is the
reason for the addition of heat to any liquid in motion.
The lower the velocity of the liquid is, the smaller the
ratio of stressed molecular chains, as well as the amount
of heat build-up in the liquid. Now it is clear why turbu-
lent motion, which is associated with relatively high
speed, is accompanied with heat build-up. However, in a
throttling situation, energy from the prime mover is
transferred into liquid by the pump. Molecules of the
liquid get compressed and in turn it slows the molecular
activity. When we throttle a system, we allow some of
the compressed liquid to escape to the low pressure side,
where the liquid decompresses and increases at the same
time its molecular activity and releases energy into heat.
The only energy that oil can store is potential energy. If
it cannot release its energy by transferring it to kinetic
energy, sound, and force, it will always release it in the
form of heat.
3. Which is the Preferred Heating Medium?
Nowadays water is the dominating heating medium in
Jordan. This is because it is cheap. As water causes rust
and corrosion it must be chemically treated and heating
network must be flushed annually. The specific heat of
water is 4186 J/Kg.C [4]. Concerning mineral oil, its
specific heat is 1966 J/Kg.C and the heating network is
not subjected to rust or corrosion. This means that time
interval required to raise the room temperature to the
preset value is two times shorter than that required when
using water. Considering the working temperature range
for water, practically, it is limited to 90C in order to
avoid water evaporation. For oil it is limited to 82C in
order to keep oil viscosity not deteriorated [4]. So practi-
cally both water and oil have the same temperature work-
ing range. When applying hydraulic throttling, cavitation
of the throttling valve becomes an important issue. In
water hydraulic valve, cavitation is more likely to happen
than in oil hydraulic one. Water has a low viscosity and
high vapor pressure. Its vapor pressure increases rapidly
when temperature rises [5]. Because of high vapor pres-
sure of water cavitation is more likely to happen espe-
cially when the pressure drop across the valve orifice is
large. As a result of higher density of water than oil, the
pressure impact resulting from cavitation in water hy-
draulic system is more serious than in an oil system and
the flow coefficient of the throttle using water as a pres-
sure medium is larger than that using oil as a working
medium. Considering the above, oil is recommended as
the pressure medium.
4. Experiment Setup and Hydraulic Circuit
Block Diagram
The experiment setup and hydraulic circuit block dia-
gram are shown in Figures 1(a) and 1(b).
The circuit includes a gear pump, pressure relief valve,
pressure gauge, throttle valve and a tank. In addition to
that a manifold piping network may be added to the cir-
cuit in order to allow throttling through one valve only,
two throttling valves in series or in parallel, and three
throttling valves in series or in parallel. This is shown in
Figure 2.
(a)
(b)
Figure 1. (a) Experiment setup; (b) Hydraulic circuit block
diagram.

M. A. K. ALIA ET AL.463
The tank is insulated by fiber glass and includes an in-
ternal oil distributor. The tank dimensions are 50 × 50 ×
25 cm. Oil volume was 15 liters. The tank represents the
oil filled radiators in real domestic heating system. Cir-
culation pump is not shown in the experimental setup
because the distance between the output of the throttle
valve and the tank is short.
Temperature is measured every three minutes using a
glass thermometer. The pressure relief valve is installed
for safety to prevent pressurization and possible failure.
Throttle valve is a manually operated multiturn valve.
A 3 kW Power gear pump is selected in order to realize
high pressure. The rigid design of gears and housing of
the pump allow for high pressures and the ability to
pump highly viscous liquids. Gear pumps are ideally suit-
able for working pressure below 120 bars. It deserves to
note that gear pumps have high power dissipation and
deliver a continuous supply with no pulsations, which
aids the heating process and gives a uniform oil flow.
5. Experimental Procedure and
Experimental Results
The first experiment was carried out using one throttle
valve. In order to adjust the throttle opening it was closed
completely, and then it was opened manually by rotating
its wheel 2, 3, 4, 5 turns successively. The pump was
started manually and oil temperature was taken every
three minutes until oil temperature has reached a specific
value. Room temperature was 21C. After taking the
readings of the specific number of turns the system was
left to cool to room temperature. Experiment results are
given in Table 1 and Figure 3.
The second experiment was carried out using three
identical throttles with the same opening (5 turns). Table
Figure 2. Manifold piping network connected with the hy-
draulic circuit.
2 and Figure 4 show the experimental results when only
one throttle was used, then two throttles were used in
parallel connection, and at the end three throttles were
used in parallel connection.
Normally a real domestic heating system is controlled
automatically, using an ON-OFF controller with an ac-
ceptable hysteresis loop width and an appropriate tem-
perature transducer [6]. For a finer control a proportional
Table 1. Experimental results using single throttle with
different settings.
Time[min] Temp[℃]
0 21
3 36
6 49
9 57
12 65
2 turns
15 70
0 21
3 34
6 47
9 55
12 63
3 turns
15 67
0 21
3 32
6 42
9 51
12 59
4 turns
15 62
0 21
3 30
6 41
9 49
12 55
5 turns
15 60
Tank
Time [min]
Figure 3. Temperature versus time graph at single throttle.

M. A. K. ALIA ET AL. 464
Table 2. Experimental results using three parallel throttle
of the same settings.
Time [min] Temp[℃]
0 21
3 30
6 41
9 49
12 55
1 Throttle(5 t)
15 60
0 21
3 33
6 44
9 53
12 62
2 Throttles(5 t)
15 67
0 21
3 37
6 46
9 54
12 63
3 Throttle(5 t)
15 68
Figure 4. Temperature versus time graph at one, two and
three throttles in parallel.
throttle valve may be utilized in order to control the
valve internal plug position as a function of system tem-
perature.
6. Design and Practical Considerations
As a rule of thumb, total losses are estimated by 25% of
the input power of the most common hydraulic systems
[7]. For a heating by throttling system there is no work
done. The whole input power has to be converted into
heat power. This heating system is in essence on oil/air
heat exchanger. Such an exchanger may be rated ac-
cording to the amount of heat dissipated for each degree
of difference between the maximum oil temperature and
the ambient temperature at a given flow of oil through
the throttle. The heat load of the hydraulic system, which
corresponds to pump power, can be determined by meas-
uring the radiators temperature at start up and again after
the heating system has been in operation for a measured
time interval by using the formula (Cooling and Heating,
2006),
32.4
VT
Pt


 (1)
where
P ―Power (heat load) in kW
V ―tank volume in liters
T ―temperature change (T2 – T1 ) in C
t ―time change (t2 – t1) in minutes
If T is equal to (75 – 15 = 60)C, which is a real
practical value
t is equal to 15 minutes
then ,
60 0.123 kW
32.5 15



V
PV
If the temperature controller works in an OFF-ON
mode with a hysteresis bandwidth equal to 6C and it is
required that the controller must clear the temperature
deviation within (3) minutes, then the required power for
each working cycle shall be equal to:
30.03 kW
32.5 3



V
PV
This relationship may be utilized in order to estimate
the consumed power for any time period.
7. Conclusions
 Experimental results show that it is practically pos-
sible to make use of the throttling effect in hydraulic
systems in order to realize commercial industrial
domestic heating systems.
Time [min]
 Oil is the preferred pressure medium. It has a low
specific heat capacity, it does not cause rust or corro-
sion and its working temperature range is practically
the same as that of water. In addition to that valve
cavitation is severely eliminated.
 Temperature rise-time may be controlled utilizing an
adjustable or controllable one throttle or more than
one throttle connected in parallel.
 Given a simple method for heating load evaluation
and for the determination of the consumed power.
8. References
[1] Y. A. Cengel and M. A. Boles, “Thermodynamics an
Engineering Approach,” McGraw-Hill, USA, 2006.

M. A. K. ALIA ET AL. 465
[2] Hydraulic Supermarket, “Cooling and Heating,” 2006. http://
www.industrialhydrauliccontrol.com
[3] Hydraulic Secrets Revealed, http://www.insidersecretstohy-
draulic.com
[4] P. Frank, “Fundamentals of Heat and Mass Transfer,”
John Wiley and Sons, UK, 2004.
[5] J. Michael, “Industrial Control Electronics Applications
and Design,” Prentice-Hall International Inc., USA, 1990.
[6] M. Prasad, “Refrigeration and Airconditioning Data Book,”
New Age International, India, 2009.
[7] R. L. Mott, “Applied Fluid Mechanics,” Prentice Hall,
USA, 2009.