Energy Analysis of Pid Controlled Heat Pump Dryer

doi:10.4236/eng.2009.13022

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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

E-mail: [email protected]

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

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

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

Cdiaoa ia

QW mhh





 







 (5)

Heat used during moisture extraction in drying cham-

ber



ciaiaoa

Qmhh

 (6)

The heat delivered in the condenser () was esti-

mated using the experimental values [11].

,(

Cdiap airiaaai

QmC TT 

)

(7)

iaia i







 (8)

Energy utilization ratios of chamber







iaia oa

iap airiaaai

mhh

EUR mCT T







 (9)

The coefficient of performance ,

COP TT

 (10)

The system COP Cd

COP ww







(11)

Specific moisture extraction rate (SMER) d

hpd

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.

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

STS TCT











(16)

Re-organized in accordance with the Equations

(15-16)



ln ia

iapiaaaiaai

aai

eCTT TT























 (17)

The average specific heat (p

C)of drying air

ppaiap

CC C





 (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



 (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-

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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I. CEYLAN

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I. CEYLAN 195

Nomenclature

Mi initial weight

Md exact dry weight

C specific heat, kJ kg

-1 K -1

C mean specific heat, kJ kg

-1 K -1

 mass flow rate, kg s

-1

 heat delivered in condenser, kJ s -1

T temperature, K

 energy utilization, kJ s

-1



specific humidity, g g -1

V velocity, m s -1



density of air , kg m

-3

 power input to compressor (kW)

H enthalpy, kJ kg -1

 power input to fan (kW)

COP ,

heating coefficient of performance of Carnot

cycle

COP heating coefficient of performance of heat

pump

 drying rate, kg h

-1

e exergy, kJ kg-1

S specific entropy, kJ kg

-1 K-1

 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