Engineering, 2009, 1, 188-195
doi:10.4236/eng.2009.13022 Published Online November 2009 (http://www.scirp.org/journal/eng).
Energy Analysis of Pid Controlled Heat Pump Dryer
Ilhan CEYLAN
Mechanical Education Department, Technical Education Faculty, Karabuk University,
Karabuk, Turkey
Received September 2, 2009; revised September 18, 2009; accepted September 23, 2009
Abstract
In this experimental study, a heat pump dryer was designed and manufactured, in which drying air tempera-
ture was controlled PID. Manufactured heat pump dryer was tested in drying kiwi, avocado and banana from
among tropical fruits and energy and exergy analyses were made. Drying air temperature changed between
40 oC - 40.2 oC while drying the tropical fruits. Before the drying process in heat pump dryer, initial moisture
contents were determined as 4.31 g water / g dry matter for kiwi, 1.51 g water / g dry matter for avocado and
4.71 g water / g dry matter for banana. Then tropical fruits were dried separately in heat pump dryer. Drying
air temperature was kept unchanged with the error of +0.2 oC. Drying air velocity changed between 0.3 and
0.4 m/s in a period of 310 min. COPws of the heat pump dryer was calculated as 2.49 for kiwi, 2.47 for ba-
nana and 2.41 for avocado during the experiments. EUR changed between 13 % and 28 % for kiwi, 18% and
33% for avocado and 13% and 42% for banana in heat pump dryer.
Keywords: Tropical Fruit, Drying, Heat Pump Dryer, PID Control
1. Introduction
Drying is extracting liquids in a matter. In technical dry-
ing, external interference is applied to the drying process
and the moisture in the matter is extracted through vari-
ous methods. Thus, drying is described as the reduction of
product moisture to the required dryness values at a defi-
nite process. All of the units that enable the product to
reach the drying values at the definite process and which
consist of various units (heating, dehumidifying) are de-
scribed as the drying system [1].
The systems used at the drying process are applied at
many industrial branches (such as food, paper, cement,
timber and chemistry). The drying applied to the food-
stuff serves a number of aims, the most important of
which is to prevent the product from breaking down dur-
ing the long storage. During the long storage, the drying
process helps product remain without breaking down by
reducing the moisture of the product to the level, which is
enough to limit microbial development or other reactions.
Besides, with the reduction of the moisture content, the
conservation of the characterizations of quality such as
the value of aroma and food is realized. The other aim of
drying process is to reduce the product volume, thus in-
creasing the efficiency during the storage and transporta-
tion of the essential components of the foodstuff.
In the literature, there are a lot of studies about heat
pump drying systems. However, there have been no stud-
ies interested in PID controlled heat pump dryer. In this
study, the energy balance of PID controlled heat pump
dryer has been achieved successfully. The purpose of this
paper is an understanding of energy and exergy analysis
of PID controlled heat pump dryer. With the PID control
over drying air temperature in the dryer the tropical fruits
such as kiwi, avocado and banana were dried.
Fatouh et al. dried herbs using a heat pump dryer [2].
Ogura et al. made energy and cost estimation for applica-
tion of chemical heat pump dryer [3]. Queiroz et al. de-
termined the drying kinetics of tomato by using electric
resistance and heat pump dryers [4]. Chua and Chou
made performance analysis two stage heat pump system
for drying [5]. Achariyaviriya et al. presented mathe-
matical model development and simulation of heat pump
fruit dryer [6]. Chua et al. investigated recent develop-
ments and future trends for heat pump drying [7]. Haw-
lader et al. used a different drying method by using a heat
pump dryer for the drying of guava and papaya [8].

I. CEYLAN189
1. Evaporator 2. Condensated water 3. Capillary tube 4. Dryer filter 5.
Condenser 6. Axial fan 7. Compressor 8. Power supply 9. Process
control equipment 10. Invertor (AC variable speed drive) 11. Thermo-
couple (T, pt-100) 12. Lid 13. Sliced 14. Shelf 15. Manometer
Figure 1. Schematic diagram of the experimental setup.
2. Experimental Setup
Heat pump dryer, which was analyzed in the experiment-
tal drying of tropical fruits, was shown in Figure 1. Dryer
consists of the heat pump system, axial fan, thermocou-
ple, process control equipment, invertor and drying
chamber. Heat delivered in condenser is re-extracted
from evaporators at the exit of the drying chamber. In
this way, thermal balance of the heat pump system is
achieved. PID controlled heat pump dryer adjusts the
cycle of the axial fan according to the temperature value
which is set in process control device. If the set value is
higher than the temperature which is measured with the
thermocouple, the flow of the air which is blown from
the axial fan decreases. Thus, lower flow outer air is
passed through the condenser so as to ensure that the
temperature reaches the set value. If the set value is less
than the temperature which is measured with the ther-
mocouple, air velocity of the air blown from the axial fan
will increase. Thus, fresh air with a bigger flow is passed
through the condenser so that the temperature which is
measured with the thermocouple reaches the set value.
When the temperature, which is measured by the
thermocouple, reaches the set value; in other words, dry-
ing air temperature is equated with the set value, fan ad-
justs the air velocity by means of the invertor according
to the measured temperature value. In heat pump dryer
process, temperature control device is set to 40 oC and
aims to keep the drying air temperature at the set value.
3. Experimental Procedure
Before the experiments launched in the heat pump dryer,
the tropical fruits namely, kiwi, avocado and banana
were peeled off and the following preparations were
made.
1) Peeled off fruits were sliced at the thickness of 5 mm.
2) The fruits sliced at the thickness of 5 mm were
dried in a drying oven at 70 3 oC. 
3) During the drying period of 5 hours, weight meas-
urement was made once an hour. At the end of two con-
secutive measurements, absolute dry weight was consid-
ered to be achieved on the condition that the weight
changed less than 1%. 1% accurate digital weight meas-
urement instrument (METTLER TOLEDO) was used for
weight measurement.
Initial moisture content of the fruits was calculated
from Equation (1).
100
id
db
d
MM
MC M


 (1)
Tropical fruits were placed in the heat pump dryer
which was on the shelf in the drying chamber and drying
process started. During the drying process, drying air
temperature was determined to be 40 oC and it was set
on process control device. PID control flow diagram for
heat pump dryer is presented in Figure 2.
4. Energy Analysis
In first and second law analyses of thermodynamics, the
drying process was considered as a steady flow process.
Figure 2. The systematic diagram of PID control system
and air flow.

I. CEYLAN 190
o
The SMER can be defined as the energy required to
remove 1 kg of water and may be related to the power
input to the compressor (SMERhp) or to the total power
to the dryer including the fan power and the efficiencies
of the electrical devices (SMERws), as given by Jia et al.
[11], Schmidt et al. [13], Hawlader et al. at Table 1 and
Equation 12 [14].
The main basis of these analyses is the phenomena of
thermodynamics of humid air. Within the scope of the
first law of thermodynamics, energy analysis of heat
pump dryer of tropical fruits is performed to find out
more about the energy aspects and behaviour of drying
air throughout the heat pump dryer. Actually, the air
conditioning processes can be modeled as steady flow
processes which are analyzed by employing steady flow
conservation of mass (for both dry air and moisture) and
conservation of energy principles [9].
5. The Second Law Analysis: Exergy
Analysis
For energy and exergy analyses of the single-layer
drying process, the following equations are generally
employed to compute the mass conservation of drying air
and moisture, energy conservation, and the exergy bal-
ance rate of the process [9,10]:
In the scope of the second law analysis of thermody-
namics, total exergy inflow, outflow and losses of the
heat pump dryer were estimated. The basic procedure for
exergy analysis of the chamber is to determine the ex-
ergy values at steady-state points and the reason of ex-
ergy variation for the process [9,15]. The exergy values
are calculated by using the characteristics of the working
medium from a first law energy balance at Table 1 equa-
tion 13 [16].
The overall performance of a HPD may be character-
ized by several criteria. Among them, the coefficient of
performance (COP) and the specific moisture extraction
rate (SMER) have been used by Jia et al. [11]. For an
ideal refrigeration system operating between a condenser
temperature of TC and an evaporator temperature of TE,
the maximum heating coefficient of performance, COPc,h
was obtained from the Carnot cycle as Table 1 and Equa-
tion 10 [12].
There are variations of this general exergy equation. In
the analyses of many systems, some, but not all, of the
terms shown in Equation (13) are used. Since exergy is
energy available from any source, the terms can be de-
Table 1. The equations of energy and heat pump dryer performance.
General equation of mass conservation of drying air i
mm



(2)
General equation of mass conservation of moisture


wi mpwo
mm m


 
(3)
General equation of mass conservation of moisture


ia impoa o
mm m



 


 
(4)
General equation of energy conservation
22
2
oi
Cdiaoa ia
VV
QW mhh


 



 (5)
Heat used during moisture extraction in drying cham-
ber

D
ciaiaoa
Qmhh
 (6)
The heat delivered in the condenser () was esti-
mated using the experimental values [11].
Cd
Q
,(
Cdiap airiaaai
QmC TT 
)
V
(7)
iaia i
m 


 (8)
Energy utilization ratios of chamber



,
iaia oa
dc
iap airiaaai
mhh
EUR mCT T



 (9)
The coefficient of performance ,
C
ch
CE
T
COP TT  (10)
The system COP Cd ws
F
C
Q
COP ww 


(11)
Specific moisture extraction rate (SMER) d
hpd
F
C
m
SMER ww 


(12)

I. CEYLAN 191
veloped using electrical current flow, magnetic fields,
and diffusional flow of materials. One common simpli-
fication is to substitute enthalpy for the internal energy
and PV terms that are applicable for steady-flow systems.
Equation (13) is often used under conditions where the
gravitational and momentum terms are neglected. In ad-
dition to these, the pressure changes in the system are
also neglected because of .

VV
In this case, Equation (13) is derived Equation (14).
The inflow and outflow of exergy can be found de-
pending on the inlet and outlet temperatures of the shelf
and the HPD chamber.
Applying Equations (17-21), the inflow, and outflow
of exergy can be found depending on the inlet and outlet
temperatures of the drying chamber. Hence, the exergy
loss is determined by Table 2 and Equation (19).
The quantity of the exergy loss is calculated by apply-
ing Equations (14-21). The exergetic efficiency can be
defined as the ratio of the product exergy to exergy in-
flow for the dryer chamber. However, it is explained as
the ratio of exergy outflow to exergy inflow for the
chamber. Thus, the general form of exergetic efficiency
is written as Table 2 and Equation 21 [16,17].
Figure 3. Variation of drying air temperature with drying
time.
Figure 4. Variation in moisture content as a function of
drying time.
Figure 5. Variation in energy utilization as a function of
drying time for the tropical fruits.
Figure 6. Variation in energy utilization ratio as a function
of drying time at for the tropical fruits.
Figure 7. Variation in exergy loss with drying time for the
drying chamber and the tropical fruit.
Figure 8. Variation in exergetic efficiency as a function of
drying time in the drying chamber for the tropical fruit.
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I. CEYLAN 192
Table 2. The equations of exergy analysis.
The exergy values of the working medium (13)
Exergy inlet to the drying chamber
 


iaiaiaaai aaiaaiia iaaai aai
ehThT TSTST


 

 (14)
In the equation;






ia iaaaiaaip iaaai
hTh TCT T (15)
 
ln ia
ia iaaai aaip
aai
T
STS TCT





(16)
Re-organized in accordance with the Equations
(15-16)

ln ia
iapiaaaiaai
aai
T
eCTT TT

 


 




 (17)
The average specific heat (p
C)of drying air
ppaiap
CC C 
v

 (18)
The exergy loss xL xi xo
EEE


(19)
The exergy outflow
  


oaoa oav aaiaaioaoav aai
ehThTTSTST

 

 (20)
The general form of exergetic efficiency inf
exergy outflow
exergy efficiencyexergy low  oa
ex
ia
e
e
  (21)
6. The Results of Experiment
Drying air temperature was attempted to be maintained at
40 oC in the heat pump dryer. The change of the drying air
temperature according to the drying time during the dry-
ing process of the tropical fruits in heat pump dryer was
given in Figure 3. As can be seen in Figure 3, drying air
temperature changed between 40 oC and 40.2 oC. Drying
air temperature in heat pump dryer was attempted to be
kept the same with the accuracy of + 0.2 oC. Drying air
velocity in heat pump dryer changed between 0.3 and 0.4
m/s. In the measurement of drying air velocity, air veloc-
ity measurement instrument (TESTO) with heated wire,
NTC sensor, and 0.01 m/s accuracy was used. The mean
value of dynamic drying air velocity between 0.3 m/s and
0.4 m/s during the drying period was 0.37 m/s. Drying air
velocity is obtained as 0.37 m/s for energy and exergy
analyses.
Before the drying process in heat pump dryer, initial-
moisture content calculated from Equation (1) in the
drying oven was 4.31 g water / g dry matter for kiwi,
1.51 g water / g dry matter for avocado and 4.71 g water/
g dry matter for banana. Initial moisture content of
tropical fruits was determined. Then tropical fruits were
dried separately in heat pump dryer. The change of their
moisture contents according to the drying period during
the drying in heat pump dryer was calculated from Equa-
tion (1) and given in Figure 4. Drying ratio of kiwi and
banana whose initial moisture content was high was
faster when compared to avocado whose initial moisture
content was lower.
Energy utilization in heat pump dryer was calculated
from Equation (5) and the change according to the drying
time was given in Figure 5. As can be seen in Figure 5,
the energy utilization increased at the onset of the drying
process. As the drying process went on, utilized energy
decreased. The increase in utilized energy at the onset of
the drying process was a result of the energy made use of
in heating drying chamber. Energy utilization in heat
pump dryer together with heating the drying chamber
was for evaporating the moisture in tropical fruits.
Energy utilization in drying chamber decreased as the
moisture content in fruits decreased. Energy utilization-
ratio of heat pump dryer in drying chamber was cal-
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I. CEYLAN 193
Table 3. Evaluation of heat pump dryer performance.
Fruit COPws
SMERws
(g/kWh)
Drying time
(min)
Initial and final
moisture content
(g water/ g dry matter)
Mean air
velocity
(m/s)
Drying air temperature
(oC)
Kiwi 2.49 81.5 360 4.31 - 0.59 0.37 40
Avocado 2.41 58.8 360 1.51 - 0.24 0.37 40
Banana 2.47 87.9 360 4.71 - 0.39 0.37 40
culated from Equation (9) and given in Figure 6. Energy
utilization ratio of banana was high, whose moisture
content was also high. Energy utilization ratio in drying
chamber decreased as the moisture content in fruits de-
creased, similar to the utilized energy.
COPws was calculated from Equation (11) for whole
system of heat pump dryer and SMER was calculated
from Equation (12) and given in Table 3.
The outlet temperature of the drying air from the dry-
ing chamber was low due to the energy utilization for
heating the drying chamber and the fruits at the onset of
the drying process. Therefore, both energy utilization and
exergy loss increased at the onset of the drying process.
Exergy loss in the drying chamber was calculated
from Equation (19) and given in Figure 7. At the onset of
the drying process, exergy efficiency decreased due to
the exergy loss. Therefore, the exergy efficiency, which
was low at the onset of the drying process, increased as
the drying process continued. Energy utilization in dry-
ing chamber decreased as the moisture content of the
fruits decreased and exergy efficiency increased. The
change of the exergy efficiency according to the drying
period was calculated from Equation (21) and given in
Figure 8.
7. Conclusions
PID controlled heat pump dryer was analysed experi-
mentally in the drying of the tropical fruits such as kiwi,
avocado and banana. The study carried out on the ob-
tained experimental results is as follows:
1) An energy source other than heat pump dryer sys-
tem condenser can be used in dryer.
2) As a little amount of fruits were dried in heat pump
dryer, SMER was low. SMER will increase as the
amount of dried fruits or the moisture contents of the
fruits to be dried are increased.
3) Some by-pass air can be used in heat pump dryer
instead of fully using fresh air. This may also decrease
the drying air velocity
4) The temperature value set in process control
equipment was 40 oC. The air velocity may be increased
by decreasing of set temperature.
5) It was experimentally shown that PID controlled
heat pump dryer, which was studied herein, can be used
for drying the materials which were adversely affected
from the temperature changes during the drying process.
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Nomenclature
Mi initial weight
Md exact dry weight
p
C specific heat, kJ kg
-1 K -1
p
C mean specific heat, kJ kg
-1 K -1
m
 mass flow rate, kg s
-1
Cd
Q
 heat delivered in condenser, kJ s -1
T temperature, K
W
 energy utilization, kJ s
-1

specific humidity, g g -1
V velocity, m s -1

density of air , kg m
-3
C
w
 power input to compressor (kW)
F
H enthalpy, kJ kg -1
w
 power input to fan (kW)
hc
COP ,
heating coefficient of performance of Carnot
cycle
ws
COP heating coefficient of performance of heat
pump
d
m
 drying rate, kg h
-1
e exergy, kJ kg-1
S specific entropy, kJ kg
-1 K-1
i
V
 volumetric flow rate of air, m3 s-1
EUR energy utilization ratio, %
PID proportional integral derivative
Subscripts
wi water inlet
we water evaporation
wo water outlet
i inlet
oa outlet air

surrounding or ambient
ci condenser inlet
ws whole system
HPD heat pump dryer
v vapour
ia inlet air
aai ambient air inlet
hpd
SMER action rate for whole
system, kg kJ-1 s h-1
specific moisture extr
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