By:Amrita GhoshSupratik GhoshSubhasis Neogi

School of Energy Studies, Jadavpur University

Performance Evaluation of a Guarded Hot Box U-value Measurement Facility

under Different Software Based Temperature Control Strategies

IV th International Conference on Advances in Energy ResearchIndian Institute of Technology Bombay, Mumbai

Overview

• General Introduction• Setup Description• Experimental Details• Performance Evaluation of the System • Results & Discussions • Conclusions

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School of Energy StudiesJadavpur University

Introduction Overall Heat Transfer Coefficient or U-value gives

a quantitative measure of amount of heat flowing through unit area of any material for every degree differential temperature across the material.

The lower the U-Value, lower is the heat transfer through the material, better is the thermal performance of the building component.

In India energy consumption in the residential and the commercial and public services sectors: 40.84%

Method of determination of U-Value of a material: Guarded Hot Box Method.

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School of Energy StudiesJadavpur University

Guarded Hot Box U-value Measurement Facility

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Cold Box

Guard Box

Baffle

Plate heater

Baffle

FanFans

Tubular heaters

Heat ExchangerMetering

Box

Fans

Surround Panel

Specimen

School of Energy StudiesJadavpur University

Guarded Hot Box U-value Test Setup (complying with BS EN ISO 8990: 1996)

School of Energy StudiesJadavpur University

The Guarded Hot Box design is based on BS EN ISO 8990:1996.

It essentially consists of metering box which is enveloped by a guard Box and cold Box.

Walls of the guarded hot box is made of extruded polystyrene blocks.

Baffle plate is placed parallel to the surface of test element in both the boxes.

Purpose of guard box is to limit heat transfer via Metering Box wall.

Basic method involves measurement of heat flux through the test element at thermal equilibrium.

Metering Box Heat is supplied from a 100 W plate heater and

two 60 W tubular heaters.

It is highly insulated to minimize heat loss.

A baffle plate placed parallel to the surface of the test element provides a radiating surface of uniform temperature.

Circulating fans are installed inside the metering box to bring uniformity in the temperature.

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School of Energy StudiesJadavpur University

Location of heaters inside Metering Box

Plate Heater

Tubular Heater

Circulating Fans

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Guard Box

The guard box surrounds the metering box.

Heat is supplied by two 120 W tubular heaters. Circulating fans are provided.

The purpose of the guard box is to minimize the heat flow through the metering box walls.

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Location of heaters inside Guard Box

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Cold Box

The cold box provides a controlled environment at a constant low temperature.

A Chilled ethyl-glycol based storage system incorporating a Heat Exchanger-Fan coil unit is used for maintaining the desired environmental conditions.

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Experimental Details

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DAC HeatingUnits

Guard Box/ Metering Box

Thermocouple and Data Acquisition System

e

-

a mc

ym

Temperature Controller

+ ysp y

Block diagram of the system

School of Energy StudiesJadavpur University

Experimental Details (contd.)

A software based controller has been developed.

In the program, three air temperatures of the metering box and two air temperatures of the guard box are averaged out individually.

K-type thermocouples are used to measure the air temperatures at different locations of the metering box and the guard box.

The signals are logged into the Data Logger via (34901A 20-Channel Multiplexer) and then fed to the computer program via GPIB-USB interface.

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Experimental Details (contd.)

The average temperatures are used as the input signal parameter of the virtual temperature controllers.

The output signals of the controllers are fed to the DAC slot .

A 12V DC voltage is obtained from the DAC depending on the output value of each controller .

This DC voltage is used to operate control relays.

These relays in turn operate the heating units installed inside the metering box and the guard box.

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Experimental Details (contd.)

Three experiments were performed for evaluating the system performance using different control strategies.

The set point temperatures were kept constant at 40oC.

The AC circulating fans of the guard box were operated at a power of 30W to reduce the heat dissipation from the fans.

In these three cases the cold side of the Guarded Hot Box setup was maintained at certain low temperatures.

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GUI showing Real-Time Process Control

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Experimental Details (contd.)

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Case 1: The controller

outputs were evaluated with PID logic, and then on-off control was implemented.

The average ambient temperature for the test period was 28.5oC.

0 1 2 3 4 5 6 7T im e (h o u r)

2 5

3 0

3 5

4 0

4 5

Tem

pera

ture

(o C

)M e te r in g B o x T e m p e ra tu reG u a rd B o x T e m p e ra tu reS e t P o in t

School of Energy StudiesJadavpur University

Experimental Details (contd.)

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Case 2: The controller

outputs were evaluated using P logic, and then on-off control was implemented.

The average ambient temperature for the test period was 29oC.

0 1 2 3 4 5 6 7

T im e (h o u r)

2 5

3 0

3 5

4 0

4 5

Tem

pera

ture

(o C

)M e te r in g B o x T e m p e ra tu reG u a rd B o x T e m p e ra tu reS e t P o in t

School of Energy StudiesJadavpur University

Experimental Details (contd.)

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Case 3: The controller output

was calculated using the P term. This output was used to control the duty cycle of the heating units.

A proportional band of 5oC and a cycle time of 10 seconds were selected.

The average ambient temperature was 33oC.

0 1 2 3 4 5 6 7

T im e (h o u r)

2 5

3 0

3 5

4 0

4 5

Tem

pera

ture

(o C

)

M e te r in g B o x T e m p e ra tu reG u a rd B o x T e m p e ra tu reS e t P o in t

School of Energy StudiesJadavpur University

Performance Evaluation of the System

Performance Parameters PID (On-off control)

P (On-off control)

P logic (duty cycle

control)

Minimum metering box temperature after reaching set point (steady state for case 3) 39.95 oC 39.97 oC 39.49 oC

Maximum metering box temperature after reaching set point (steady state for case 3) 40.15 oC 40.14 oC 39.67 oC

Maximum fluctuation (offset for case 3) of the metering box temperature from the set point

0.3762 % 0.3572 % 1.27 %

Achieved metering box temperature range after reaching set point (steady state for case 3)

0.2044 oC 0.1772 oC0.1792 oC

Root Mean Square Deviation of the metering box temperature from the set point

0.0500 oC 0.0456 oC 0.4042 oC

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Performance Parameters PID (On-off control)

P (On-off control)

P logic (duty cycle

control)

Minimum guard box temperature after reaching set point 39.86 oC 39.88 oC 39.93 oC

Maximum guard box temperature after reaching set point 40.41 oC 40.41 oC 40.46 oC

Maximum fluctuation of the guard box temperature from the set point

1.03 % 1.04 % 1.15%

Achieved guard box temperature range after reaching set point 0.5495 oC 0.5359 oC 0.5281 oC

Root Mean Square Deviation of the guard box temperature from the set point

0.1149 oC 0.1015 oC 0.2027 oC

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Results and Discussions

Controllers with PID and P logic (case 1 and case 2) showed almost similar results.

In these cases the controller outputs were calculated using PID and P logics respectively. Based on these outputs on-off control was implemented.

Usually, the integral action is provided to eliminate the steady state offset. In the second case with P logic, this offset was eliminated since the heating units were operated at full power whenever the controller output was positive.

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Results and Discussions (contd.)

As the cold side was maintained at a temperature much below the ambient, there was continuous heat flow through the specimen to the cold side. This helped to bring the required stabilizing effect, which is generally offered by the Derivative control action.

Thus, the on-off controllers with PID and the P logic offered similar performance.

In case 3 the average offset for the metering box temperature during the last 3 hours of the experiment was found to be 0.403 oC.

In case of the guard box temperature, fluctuations were obtained. This may be due to the presence of low circulating air velocity in the guard box.

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Conclusions

The on-off control systems with PID and P logic showed almost similar performance.

The maximum fluctuations for the metering box temperature from the set point obtained in these two cases were 0.3762% and 0.3572% respectively, which are within the maximum limit of 1%.

For the guard box the maximum fluctuations were 1.03% and 1.04% respectively. However the fluctuations gradually decreased with time.

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Conclusions (contd.)

In the third case of duty cycle control the average offset obtained for the metering box temperature was 0.403 oC. The controller needs to be tuned in order to minimize the offset.

For the guard box temperature, fluctuations were obtained probably due to the low circulating air velocity.

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