Development of Indigenous Psychrometer for Estimating Psychrometric Properties during Preservation of Vegetables in Evaporative Cooler in Akure, Nigeria

Development of Indigenous Hygrometer for Measuring PsychrometricProperties During Preservation of Vegetables Using Evaporative

Cooler in Akure, Nigeria

By

Simon O. Odey1, Seth I. Manuwa2 and Raphael S. Bello3

1 Department of Agronomy, Cross River University ofTechnology, Obubra Campus, Nigeria

2 Department of Agricultural Engineering, The FederalUniversity of Technology, Akure, Nigeria

3 Department of Agricultural Engineering, Federal College ofAgriculture, Ishiagu, Nigeria

Correspondence: Simon O. Odey, [email protected],07034575615

Abstract

Locally developed psychrometers were used to measure the

environmental condition of a medium during preservation of

vegetables using evaporative cooler. The constructed

psychrometers were made up of hard wood and two thermometers.

The bulb of one of the thermometers was fixed with wick and

inserted into a bottle containing distilled water to serve as

the wet bulb. The psychrometers were placed in the storage

medium. Wet bulb and dry bulb readings were recorded hourly

from 7.00 am to 6.00 pm during preservation of vegetables.

Temperature drop, relative humidity, and evaporative

efficiency of the medium were determined using the

psychrometric tables. Results from the use of locally

developed psychrometer were compared with that from orthodox

psychrometer using correlation and graphs. The maximum

temperature drops of 7.9 oC and 8.0 oC were realized from the

use of orthodox and indigenously developed types respectively.

The mean relative humidities were 88.74 oC and 88.68 oC for

locally developed and orthodox psychrometers respectively.

While the mean evaporative efficiencies determined from

locally developed and orthodox psychrometers were 74.51 oC and

74.91 oC respectively. Relative humidity and evaporative

efficiency determined using both types of hygrometers revealed

their effectiveness. It is hereby suggested that local

development of tools and equipment for scientific and

engineering application should be highly encouraged as this

will at the long run enhance technological development.

KEY WORDS: Psychrometric Properties, Preservation, Evaporative Cooler

1.    INTRODUCTION 

Measurement and control of psychrometric properties is

very vital to the efficient storage of agricultural produce.

Relative humidity, temperature, equilibrium moisture content,

dew point, vapour pressure are very useful in adequately

designing a storage system for preservation of agricultural

products. Biological activity occurs only when moisture is

present. Therefore the moisture content of the product itself,

as well as the moisture content of the surrounding air is

important for safe storage. Each product has its own

characteristic balance (or equilibrium) between the moisture

it contains and the water vapour in the air surrounding it.

This equilibrium is known as the equilibrium moisture

content/relative humidity pattern (Kittas, et al., 2001, Xuan,

et al., 2012).

Psychrometer is an easy to use tool for measuring the

amount of humidity in the atmosphere. There are two main types

of Psychrometers – a dry and wet bulb psychrometer and a

mechanical hygrometer. Measurement of these properties is made

simple by installing psychrometers strategically inside (to

measure storage medium or cooler condition) and outside (to

measure ambient condition) the storage medium. Nigeria is one of the developing nations in tropical Africa

with more than 70% of its population engaging in agriculture.

Most of the people in rural communities depend mainly on

subsistence farming. Hayman (2003) revealed that between 25

and 40% of stored agricultural products is lost each year

because of inadequate farm and village-level storage. In the

field and during storage the products are threatened by

insects, rodents, birds and other pests. Moreover, the product

could be spoiled by infection from fungi, yeasts or bacteria.

In addition, for sowing seed it is important that the

viability (its capacity to germinate) is maintained (Acedo,

1997). The loss of food is not only wasteful; it poses food

security problems and loss of potential income for farmers who

are forced to sell immediately after harvest (Mohammed, 1995).

In order to minimize the losses during storage it is important

to identify and measure the optimum environmental conditions

for storage of the product, as well as the conditions under

which its attackers flourish (Olosunde et al, 2009).

Dowdy (1982), Manuwa (1989), Roka et al. (1991), Ndukwu

(2011), and Manuwa and Odey (2012) stated that the performance

of evaporative cooler is measured by the following parameters.

- Evaporative efficiency

- Total cooling capacity

- Relative humidity

- Dry bulb temperature of air

- Wet bulb temperature of air

Evaporative efficiency is defined by:

Nf = Tdb – To x 100%

Tdb – Twb

Where,

Nf = Evaporative efficiency

Tdb = Dry bulb temperature of air

To = Output temperature of the cooler (dry bulb)

Twb = Wet bulb temperate of air

Tdb – To = Temperature drop

Tdb – Twb = Wet bulb depression

Thus having the above parameters make it quite easy forknowing the psychrometric properties earlier mentioned.

In line with the above Otterbein (1996) illustrated the

use of psychrometer in measuring the efficiency of a storage

medium such as evaporative cooler. According to him if a

cooler has 21.1oC air leaving the pad during a 37.8oC day, and

has a wet bulb temperature of 15.6oC.

Evaporative efficiency,

Nf = Tdb – To x 100Tdb – Twb

= 37.8 - 21.1 X 10037.8 - 15.6

= 16.7 x 10022.2

= 75%

Thus, the efficiency of a storage medium, relative

humidity, equilibrium moisture content, dewpoint and other

properties can be easily measured provided the psychrometer is

available. In most research institutions and Nigerian

universities, most of the simple equipment for measuring

similar properties are lacking; this is why Nigerian

Universities are poorly rated (Olatunji, 2012). There is need

therefore for concerted effort by Nigerian Scientists and

Engineers to design and produce such equipment using locally

sourced and cost effective materials; rather than continuously

depending on imported laboratory tools.

1.2    Objectives of the Research Work

The objectives of this research work are: 

(a) To design, construct and test psychrometers, using

locally available materials for measuring pschrometric

properties of a medium,

(b) To compare its performance with the orthodox type, and

(c) To use the constructed psychrometers to measure the

relative humidity, temperature drop and evaporative

efficiencies of a storage medium.

2. Materials and Methods2.1 Study Area

The experiment was conducted behind the Agricultural Engineering

Laboratory Building, Federal University of Technology, Akure, Ondo

State (with geographical coordinate of 7o 10 North and 5o 05 East),

between 5th – 10th December, 2003.

2.2 Design Considerations of Psychrometer

The following considerations were given to the design of

the psychrometer:

The height, width and thickness of the components

were such that the equipment would easily be placed

in a storage medium such that it can readily sense

the temperature of the system.

The drilled holes on the wood were such that common

laboratory thermometers could fit into them without

slipping.

One side of the psychrometer is exposed so as to make

it quite sensitive to the temperature of the storage

medium.

The following dimensions were considered for the equipment:

- Six No 2.5cm x 1cm x 33cm hard wood 

- Nine No 2.5cm x 1cm x 6.5 x cm hard wood

- Three No  8.5cm x 0.5cm x 33cm plywood  

- Holes of 5mm diameter drilled 2.5cm apart.

2.3 Main Components of the psychrometer

Each of the three constructed psychrometers was made up

of wood, two thermometers, a wick and rubber can or bottle.

The constructed wood was made up of 2.5 cm by 6.5cm by 30cm in

dimension with one side covered by plywood. Two thermometers

were inserted into the four holes with each thermometer

entering two holes respectively. A wick was fixed into the

bulb of one of the thermometers as seen in plate 1 below. The

wick is then connected into a rubber or bottle can containing

distilled water.

Wet bulb thermometer

Dry bulb thermometer

Wooden case

Wick

Bottle containing water

Plate 1. One of the Locally Fabricated Psychrometers showing

its parts

2.4 Calibration and Testing of the Psychrometers

The psychrometers were calibrated by comparing their

readings at different conditions with orthodox psychrometer.

The dry and wet bulb temperatures were taken hourly from 7.00

hours to 18.00 hours. From the dry-bulb and wet-bulb

temperatures that were observed, wet-bulb depressions were

determined and the corresponding relative humilities were read

from standard psychrometric tables. The relative humidity

values determined were those of the ambient environment, the

coolers' inner space at hourly intervals.

The performance of the cooling system was based on the

saturation or cooling efficiency. The evaporative efficiency

was calculated using the model reported by Otterbein (1996)

and Haris (1997) :

ȵef = Tab - Tcool X 100%Tab - Twb

Where,

Tab - Tcool = temperature drop

Tab - Twb = wet bulb depression

ȵef = evaporative efficiency

Tcool = dry bulb temperature of air within cooler

Tab = dry bulb temperature of air entering pad (ambient temperature)

Twb = wet bulb temperature of air entering pad

2.5 Cost of Development of Each Psychrometer

The cost of development of each psychrometer is ……………………

3. Results and Discussion

3.1 Temperature Drop, Relative Humidity and Evaporative

Efficiency of Cooler

In other to know the performance of the fabricated

psychrometers, temperature drop, relative humidity and

evaporative efficiency as determined were compared with that

of the orthodox psychrometer.

Figure 1. Shows the temperature drop (TD) of cooler with

time as determined using orthodox and locally constructed

psychrometer. The maximum temperature drop as determined using

locally constructed hygrometer was 8.0oC while that determined

using the orthodox type was 7.9 oC. Thus the temperature drop

as determined using both hydrometers increased uniformly from

the early hours of the day towards the afternoon hours and

started dropping uniformly. This results compare favourably

with the result recorded by Ndukwu (2011) where temperature

reduction of 10.0 oC was realized using orthodox hygrometers.

Figure 2 depicts the variation in mean saturation

efficiency (Nf) as determined from readings for Locally

constructed and Orthodox psychrometers. While in Figure 3 the

variation in mean Relative Humidity (Rh) determined from

readings for locally constructed and orthodox psychrometers

were plotted. In both graphs, the Nf and Rh for orthodox and

locally developed psychromers were comparably related. Showing

that either of the measuring devices can be used during

measurement and determination of psychrometric properties of

an environment. Thus the results are related to the works of

Turner (1986), Karpiscak and Marion (1994), Harner et al.

(2007 and 2009), Ndukwu (2011); and Manuwa and Odey (2012)

where psychrometic equipment were used in the determination of

relative humidity, temperature drop and evaporative efficiency

during use of evaporative coolers.

Thus correlated results for temperature drop, relative

humidity as determined using orthodox and locally developed

hygrometers revealed 2.9 (lower) and 6.2 (upper) at 95%

confidence interval of the difference for both hygrometers

respectively. While the mean difference was 4.57 and 4.60 for

orthodox and locally constructed psychrometers respectively.

0

1

2

3

4

5

6

7

8

9

TD for orthodox PsychrometerTD for locally constructed Psychrometer

Time of day (Hrs)

Temp

erat

ure

drop

(0C

)

Figure 1. Variation of Mean Temperature Drop (TD) with time for orthodox and locally constructed psychrometers.

38.043.048.053.058.063.068.073.078.083.088.093.0

Nf for orthodox Psychrometer

Time (Hrs) Satu

rati

on E

ffic

ienc

y (%

)

Figure 2: Variation of Mean Saturation Efficiencies of cooler using usin Orthodox and Fabricated Psychrometer

7.00

9.00

11.0013.0015.0017.0019.00

82.084.086.088.090.092.094.096.0

Cooler Rh for orthodox PsychrometerCooler Rh for fabricated psychrometer (%)

Time of day (hrs)

Figure 3. Comparism of Relative humidities Determined using

Orthodox and Fabricated Psychrometer

Rela

tive H

umidit

y (%

)

4.0 Conclusion

Indigenous psychrometer was developed. This was used in

measuring dry bulb and wet bulb temperatures. Psychrometeric

properties such as relative humidity, temperature drop and

evaporative efficiency were determined during preservation of

vegetables using evaporative cooler. The results obtained were

compared to that when orthodox type was used. This shows a

favourable comparison as the results were not significantly

different from each other. Thus maximum temperature drops of

7.9 oC and 8.0 oC were realized from the use of orthodox and

indigenously developed types respectively. Relative humidity

and evaporative efficiency determined using both types of

hygrometers revealed that both worked effectively well as

shown in figures 1, 2 and 3.

It is therefore suggested that local designed and

fabricated tools and equipment for scientific and engineering

application should be highly encouraged as this will at the

long run enhance technological development.

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