Development of a Bench-Top Air-to-Water Heat Pump Experimental Apparatus

Abstract

A bench-top air-to-water heat pump experimental apparatus was designed,developed, and constructed for instructional and demonstrative purposes. Thisair-to-water heat pump experimental apparatus is capable of demonstratingthermodynamics and heat transfer concepts and principles. This heat pumpexperimental setup was designed around the vapor compression refrigerationcycle. This experimental apparatus has an intuitive user interface, reliable, safefor student use, and portable. The interface is capable of allowing dataacquisition by a computer. A PC-based control system which consists ofLabVIEW and data acquisition unit is employed to monitor and control thisexperimental laboratory apparatus. This paper provides details about thedevelopment of this unit and the integration of the electrical/electronic componentand the control system.

Full text

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H. I. Abu-Mulaweh
International Journal of Engineering (IJE), Volume (3) : Issue (3) 359
Development of a Bench-Top Air-to-Water Heat Pump
Experimental Apparatus


H. I. Abu-Mulaweh
Mechanical Engineering Department
Purdue University at Fort Wayne
Fort Wayne, IN 46805, USA
mulaweh@engr.ipfw.edu

Abstract

A bench-top air-to-water heat pump experimental apparatus was designed,
developed, and constructed for instructional and demonstrative purposes. This
air-to-water heat pump experimental apparatus is capable of demonstrating
thermodynamics and heat transfer concepts and principles. This heat pump
experimental setup was designed around the vapor compression refrigeration
cycle. This experimental apparatus has an intuitive user interface, reliable, safe
for student use, and portable. The interface is capable of allowing data
acquisition by a computer. A PC-based control system which consists of
LabVIEW and data acquisition unit is employed to monitor and control this
experimental laboratory apparatus. This paper provides details about the
development of this unit and the integration of the electrical/electronic component
and the control system.

Keywords: Heat pump, laboratory apparatus. water heater


1. INTRODUCTION
One of the important applications of the subjects of thermodynamics and heat transfer is heat
pump systems. Exposing thermal engineering students to heat pump will enhance their
understanding of the principles and concepts of thermodynamics and heat transfer.

There are many are many heating, ventilating, and air-conditioning (HVAC) systems, but very few
of them are appropriate for undergraduate education [1, 2]. Some of these systems have
computer data logging equipment. However, the computer data logging applications are
exclusively standalone and not compatible with PC based data processing. In addition, these
systems tend to be rather large and expensive. Recently, Abu-Mulaweh [3] has designed,
developed, and constructed a bench-top air-conditioning experimental apparatus for instructional
and demonstrative purposes.

Acquiring new instructional laboratory apparatus is a challenge due to typical budgetary
limitations. In addition, the apparatus designed by companies specializing in education equipment
may not exactly reflect the educational objective intended by the faculty. These obstacles had
forced us to seek and search different venues to acquire “high tech” experimental laboratory
apparatus for demonstrating heating and refrigeration processes. It was decided to develop and
build a cost effective system that can be employed to demonstrate and monitor refrigeration
cycle, as well as some fundamental concepts in heat transfer, thermodynamics, and heat
exchangers.

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H. I. Abu-Mulaweh
International Journal of Engineering (IJE), Volume (3) : Issue (3) 360
Hot water heaters come in various sizes and either gas fired or electric. Using a heat pump water
heater to supply hot water for residential and commercial usage, is a much more efficient and
energy conservative method. Heat pump water heaters can be either water source or air source.
The heat pump is an electrically powered mechanical device that transfers heat from a lower-
temperature source to a higher-temperature body, such as an air conditioner. The feasibility and
the effectiveness of heat pump water heaters have been examined in the past [4-7].

The coefficient of performance (COP) of heat pump water heaters is typically in the order of three.
This implies that the energy consumption can theoretically be reduced by two-thirds when
resistance heaters are replaced by heat pumps. The installed electrical capacity is also reduced
by almost two-thirds due the COP. To the building owner, this would mean a reduction in both the
direct cost of units of energy consumed and the monthly peak demand charges.

The replacement of resistance heaters with heat pumps water heaters will also result in a
reduction in the peak electrical demand imposed on the national electricity supply grid. To the
utility, this could mean a reduction in the marginal cost of supplying each new unit of power since
the need to build new power stations may be deferred.

2. SYSTEM SPECIFICATIONS
The design process that the students follow in the capstone senior design project is the one
outlined by Bejan et al. [8] and Jaluria [9]. The first essential and basic feature of this process is
the formulation of the problem statement. The formulation of the design problem statement
involves determining the requirements of the system, the given parameters, the design variables,
any limitations or constraints, and any additional considerations arising from safety, financial,
environmental, or other concerns.

In order for the bench-top air-to-water heat pump to function as a useful piece of lab equipment,
the following requirements and specifications need to be met. These include requirements that
will make the heat pump useful for demonstrating thermal science and fluid dynamics principles
as well as ensure the heat pump will operate safely.

• Construction – The air-to-water heat pump is to be designed to operate based on a vapor-
compression cycle.

• Instrumentation – The instrumentation requirements have two distinct sets of necessary
specifications.

1. The heat pump must be fully instrumented with autonomous gages so that it may
demonstrate its principles without needing to be hooked up to an outside computer.
2. Although it may operate without an external computer, the heat pump must also be
outfitted with a data acquisition (DAQ) bus that can be connected to an external DAQ
board or software system. This bus must be able to supply to the external DAQ system
the measurements that will be shown on the onboard instrumentation. In addition, the
measurements must be logged by the DAQ.

• Safety – The safety considerations deal primarily with the fact that the design requires both
large amounts of electrical equipment and liquids to be in close proximity. For this reason the
following are required of the electrical design scheme:

1. Residual current circuit breaker.
2. Combined double pole main switch and overload cut-out.
3. All components connected to common earth conductor.

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H. I. Abu-Mulaweh
International Journal of Engineering (IJE), Volume (3) : Issue (3) 361
3. EXPERIMENTAL APARATUS
A bench-top air-to-water heat pump, shown in Figure 1, was designed, developed, and
constructed for instructional and demonstrative purposes. This heat pump was designed around
the vapor compression refrigeration cycle. The bench-top air-to-water heat pump has an intuitive
user interface, reliable, safe for student use, and portable. The interface is capable of allowing
data acquisition by an existing laboratory computer. The unit is capable of warming the water
10ºC in an open or closed configuration, and cool it back down if desired. The system fully
controls and monitors the fluid properties at key points in the refrigerant and water loops.



FIGURE 1: Experimental apparatus

3.1
To select properly sized components for the heat pump, parametric studies were completed using
the EES model and plots of the important operating variables were generated. Because the entire
heat pump apparatus needs to operate off of the power provided by a single 115V, 15A wall
circuit, the amount of steady-state power required by the compressor was limited to 800 W
(2732.14 BTU/hr). This power constraint is imposed in order to leave sufficient reserve power for
start-up energy requirements as well as sufficient power to run the rest of the powered devices
that will be used on the heat pump. For the heat pump to meet its water heating performance
requirements the water flowing through the condenser must experience at least a 10 ° C (18 ° F)
temperature rise between the inlet and exit.

3.2 Refrigeration Cycle Components
Condensing Unit: A Tecumseh model AEA4440YXAXW water cooled condensing unit was
chosen as the basis for the heat pump. At the nominal operating conditions anticipated, this
Mechanical Concepts Selection

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H. I. Abu-Mulaweh
International Journal of Engineering (IJE), Volume (3) : Issue (3) 362
condensing unit will deliver 0.53 GPM (3.34E-5 m3/s) of water raised 13.5° C (24.3° F) when paired
with a 4600 BTU/hr (1346.93 W) evaporator.

Evaporator & Fan: Knowing the requirements of the evaporator in terms of heat, a model 012-
0850 evaporator was chosen from www.Rparts.com. This evaporator is rated for 7500 BTU/hr
(2196.08 W) at an evaporating temperature difference of 15° C (27° F) and airflow of 200 CFM
(0.09 m3/s). The evaporating temperature difference is defined as the difference in temperature
between the air entering the evaporator and the saturation temperature of the refrigerant within
the evaporator. The evaporator is oversized to make sure that the compressor is never starved
for refrigerant and oil, which is emulsified in the refrigerant. Excess evaporator heat can be
reduced and adjusted for by decreasing the air flow across the coils or by blocking of a portion of
the heat exchange surface with dampers. The selected fan for the evaporator is the Falcon
model from Lytron. It is capable of providing up to 300 CFM (0.14 m3/s) at 0.36 amps of running
current.

Expansion Valve: The selection of the expansion valve, located right before the evaporator, is
based on the capacity of the system. The target capacity of the evaporator is approximately 4600
BTU/hr (1346.93 W) which corresponds to 0.383 tons of refrigeration. Based on this value, a
Danfoss expansion valve model 095-0200 (orifice part No. 095-0003) was selected. This model
is externally equalized and the orifice is rated for up to 0.5 tons (6000 BTU/hr) of refrigeration.

Filter Dryer: To remove any moisture or contaminants that could potentially damage the system, a
filter dryer from Sporlan was chosen (# 020-0052, Mod.C-082). This model will also neutralize
any acid that might initially form from the interaction between the refrigerant and coils.

Sight Glass: A Sporlan sight glass (#077-0100, Mod. SA12) was chosen to provide a visual to
warn of and diagnose excessively high moisture levels.

Tubing: Copper tubing of 3/8 inch (0.95 cm) inner diameter was selected for the refrigerant.

3.3 Water Cycle Components
Water Pump: The pumping force for the circulation of the water loop will come from a Delphi –
Laing DC Pump, (Mod. DDC-1TPMP). This pump can supply water at a maximum pressure of 22
psi (151.68 kPa), and a maximum flow of 1.75 GPM (1.10E-4 m3/s) at 13.2 VDC. It operates up to
a temperature of 60ºC (140ºF).

Water Reservoir and Tubing: An insulated water cooler was used as a water reservoir. Bulkhead
fittings were mounted on the cooler for pipes connections. The water piping was constructed from
copper tubing.

Radiator & Fans: To cool the water back down, an appropriate radiator was selected to fit the
requirements of this cycle. The radiator needs to move 6500 BTU/hr (1903.27 W) to cool the
water down close to room temperature from 113ºF (45ºC), with a flow rate of 1 GPM (6.31E-5
m3/s). The component is made by Lytron (Mod. M14-240). It has capacity of 373 BTU/hr ºF at the
specified flow rate, with an air flow of 550 CFM (0.26 m3/s). The radiator is equipped with two
fans with a capability of 450 CFM (0.21 m3/s) each, at a running current of 0.5 amps per fan.

4. INTEGRATION OF THE ELECTRICAL/ELECTRONIC COMPONENT
All of the low voltage electrical components were tested for functionality before assembly into
their respective circuits. This was done by applying voltages to each chip and checking output for
correctness. After these individual components were tested, each of the circuits were temporarily
built on a breadboard and tested to verify that they worked as expected. This was proven by
applying inputs of voltages or frequencies from lab equipment and measuring the output voltages
with a handheld meter. The microprocessors were also programmed and tested.

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H. I. Abu-Mulaweh
International Journal of Engineering (IJE), Volume (3) : Issue (3) 363

In the second phase of the building process, the final assembly of the low voltage electrical
system was performed. All of the circuits were laid out, built, and soldered together on a
perforated proto-board. The circuits were then retested with lab equipment to verify that building
and soldering was completed correctly. Once the circuits were built and soldered, more testing
was done to determine the accuracy and percentage error associated with these circuits.

The fourth phase comprised the high voltage wiring of the mechanical system. All high voltage
components were wired with a common ground, the ground fault interrupter (GFI) was placed on
the high voltage wire coming directly from the wall outlet to protect against large surges of
current, and the double pole single throw (DPST) circuit breaker was placed directly following the
GFI. In addition to these original specifications, two lights were added to show the status of the
high voltage power. The first light demonstrates that the system is connected to the high voltage
power and the second light confirms the system is on and operational.

The final integration of the electrical/electronic system with the mechanical system consisted of
running wires from the pressure circuits to the pressure transducers, connecting the
thermocouple wires to the temperature circuits, and applying low voltage power to the water
pump. The final step in the electrical/electronic building process was testing the DAQ
functionality. Wires were run from the control box on the heat pump unit connecting to the DAQ
board via a printer cable.

After the entire system was built and operational it was then connected to the SCB-68 break out
box and then to the DAQ card, via the on-board DB-25 connector and modified printer cable. It
was observed that the sensor signals dropped due to the voltage divider circuit created when the
DAQ was applied to the system. To remedy this problem, fifteen LM741 operational amplifiers
were employed as a non inverting amplifier and voltage followers. The non-inverting amplifier
was used to amplify (magnitude of ten) the water flow signal, as it was observed that the DAQ
was having difficulty accurately collecting the small signal. This signal isolation system was
added to the cable that connects the control box to the DAQ.

5. INTERFACE AND CONTROL SYSTEM
The control requirements of this system consist of monitoring two of the pressures (the condenser
pressure and the evaporator pressure) and switching the compressor and the evaporator fan on
and off, depending on the levels of the pressure transducers. In order to control this system there
are a few integral components; 1.) A PIC16F687, the microprocessor that will read run the control
program. 2.) Two Solid State Relays (P/N 4062RL), used for switching on and off the motors. 3.)
Two pressure transducer outputs (P/N 2CP5-50-1). The microcontroller runs the program that is
flowcharted in Figure 2. This program utilizes the onboard 10-bit analog to digital converters to
convert the pressure transducers’ voltages into a usable digital value. The microprocessor
compares the values that are produced from the pressure transducers to some preset values and
then output control signals to the solid state relays to control the motors.

The microcontroller is programmed using the flow chart shown in Figure 2. This program starts
both of the relays at zero output logic and initially checks the condenser pressure. If the
condenser pressure is above the set point, both of the motors will remain at zero. The relays will
remain at zero until the condenser pressure is below another preset value. Once this pressure
drops to the set point the microprocessor will check the evaporator pressure, if this pressure is
below a certain value the compressor will remain at zero and the evaporator pressure will
continue to be polled until it rises to the specified suitable temperature. Once this temperature is
achieved the compressor will be turned on and the evaporator pressure will be checked. If this
pressure becomes too large the evaporator fans will remain at zero and the system will poll the
evaporator pressure until it falls below the set point. Once the set point is achieved the