Research Background

The Abu Dhabi Future School Program was launched by the government of Abu Dhabi (UAE) and aims to build (100 ) new schools and refurbish (50) old ones by 2020. The programme is based on three key pillars: understanding educational needs and community expectations, achieving the highest international standards and producing designs that are safe, sustainable, well built, easy to maintain and appropriate for learning.

From previous studies on power consumption of the total energy used in the building sector, as much as 45% goes toward heating, ventilating, and airconditioning (HVAC) systems (Kreider et. al 2002). . For the Gulf area reached up to 60% .Therefore the Executive council of Abu Dhabi government through their Engineering consultancy conducted a pilot study confirmed the actual consumption is by HVAC equipment. They recommend optimization of all HVAC equipment’s installed in Abu Dhabi government buildings .Feedback from HVAC maintenance expertise, facility managers confirmed that different faults on HVAC equipment lead to raise the power consumption by those faulty equipment. The existence of faults in (HVAC) systems plays a significant role in the degradation of comfort levels for building occupants on one scale, and the degradation of quality of life on earth. Analyses of the faults in HVAC systems indicate that the functioning of some components is not in accordance with the design intent. The faults in the HVAC systems can be classified into two types of component failure:
abrupt failure and degradation (Cook et al., 2012).

Degradation faults typically manifest themselves after certain initial operation periods and gradually become worse with the passage of time (Soyguder & Alli, 2009). If these faults are not noticed in the early phases of their occurrence, they can lead to serious problems, for example, fouling of the tubes of cooling coils. However, until the degradation exceeds the critical level, it is difficult to identify these faults. On the other hand, abrupt failure corresponds to a sudden breakdown of equipment (Kok et al., 2011). This type of failure causes more devastating impacts as compared to degradation failures.

One study from the Institute for Building Efficiency addressed two categories: analysis of failures in individual HVAC components and the energy savings possible by correcting those failures. The study was based on interviews with HVAC maintenance experts. The study found that regularly-scheduled maintenance of HVAC systems can increase the energy efficiency. The HVAC industry can develop better tools to help building owners and facility managers evaluate the relationship between maintenance and energy costs and support investment in the appropriate maintenance approach (Institute for Building Efficiency, 2012).

The aim of the study is to develop Fault Detection and Diagnosis (FDD) Methodology for HVAC sub system to restore HVAC system performance and efficiency in order to reduce power consumption, maintain system design performance, and reduce HVAC system maintenance costs for Abu Dhabi future schools

Figure 1? .1: Monthly Average Temperature in Abu Dhabi from 2003-2012
(NCMS, 2012)

1.2 Statement of the Problem
Previous studies showed a massive amounts of energy is consumed by HVAC systems, around 40% globally. In UAE the energy consumed by the HVAC sometimes exceeds 60% of the total energy usage. From the literature review, technical reports and preliminary study conducted with some expertise showed that this high energy consumption is due to HVAC equipment inefficient performance. This equipment inefficient performance is related to maintenance activities and malfunctioning of control system. Lack of standard maintenance key performance indicators.

1.3 Research Objectives

The primary goal of this research is to reduce power consumed by HVAC systems in UAE, and this goal can be achieved by the following objectives:
1. Identify the factors affecting the power consumption in the HVAC system in UAE.
2. Determine HVAC maintenance performance indicators (MPI).
3. Optimizing of HVAC system performance to reduce the power consumption.
4. Develop improved MPI and energy efficient model.
5. To evaluate capital operation cost for HVAC system and variable refrigerant flow(VRF) system
6. Validation the proposed model experimentally

Figure: 1.2: Typical Energy Consumption Breakdown in a HVAC System
(Lasath et al, 2012)

1.4 Research Questions

The research questions listed below will form the focus of the research:-
1. What are the factors affecting HVAC systems maintenance
2. How HVAC systems operation can be optimized to minimize overall energy consumption.
3. How can performance indicators for HVAC systems in the Abu Dhabi Future Schools Program can be developed?
1.5 Conceptual Framework

Almost half of the total energy used in modern buildings is consumed by HVAC systems according to EIA (2012) statistics. Among the various factors driving the energy performance of buildings is the maintenance of HVAC systems which make a significant contribution to the operating costs and down times, particularly in the light of today’s growing levels of awareness of the importance of energy efficiency (Grainge, 2007).

In general, best practice in HVAC system maintenance focuses on three factors: cleaning, monitoring and tuning the system and monitoring and repairing components (Penny, 2013). The primary aim of such practices is to ensure effective preventive maintenance that avoids system failures. Such failures not only degrade the quality of the service offered, but also tend to strain the system causing higher running costs (Boyer, 2010). Therefore, proper maintenance ensures stable operation of the HVAC system in the long run and minimizes the cost and disruption (Grainge, 2007).

The primary need for research into the characteristics of HVAC maintenance stems from the technology gap between newly-developed energyefficient systems and older system components. For example, Elkhuizen et al. (2003) noted that it is important to consider the specific heating and cooling characteristics of HVAC systems while integrating newly developed technologies into the building, both in the design and operating stages.

Current performance indicators generally focus on three areas: the maintenance of system components; the detection of elements that are in failing conditions and performance status relative to benchmarks or a datum (Boyer,
2010). Therefore, the study intends to run a field experiment on faults to develop risk-fault indicators for the HVAC systems for the Abu Dhabi Future Schools Program

Figure 1? .3: The Required maintenance for HVAC equipment during the Operational period (Salonen & Deleryd, 2011).

1.6 Significance of the Research

This research is anticipated to expand the existing knowledge on fault modelling and simulation methods in maintenance applications. The study aims at introducing a set of performance indicators for assessing the performance of maintenance procedures. It can be utilized by maintenance personnel and building managers as a tool to assess the impacts of common maintenance issues on HVAC performance, and improve the integration of HVAC systems operations into the overall building maintainability (BM) strategies.

It has been noticed that few maintenance operators apply predictive and preventive maintenance strategies on the HVAC systems of their buildings, although research has verified that good maintenance can cut the energy cost of heating, ventilation and air-conditioning systems (Toor & Ogunlana, 2010).

Performance indicators are viewed as important tools for translating and delivering concise, scientifically credible information in a manner that can be readily understood and utilized by various individuals to evaluate the performance in relation to the accomplishment of goals and objectives.

Conducting the current study is essential, since it is observed that maintenance personnel and facility managers in the UAE lack an adequate understanding of the significance of energy-efficient HVAC systems and performance indicators which can reduce maintenance costs. The study proposes various benefits of developing performance indicators with respect to maximizing the effectiveness and efficiency of the specific buildings (Abu Dhabi Future Schools Program).

The study intends to develop risk-of-failure indicators for HVAC systems in the Abu Dhabi Future Schools Program and this will improve the HVAC system maintenance procedures to maintain and reduce maintenance costs.

1.7 Scope and Limitations of the Study

The aim of the study is to develop the risk-of-failure indicators and maintain system design performance in order to reduce maintenance costs of HVAC systems and increase their performance and efficiency in terms of energy consumption. To achieve this target:-
Five new school buildings will be selected and their energy and operation model will be developed. Field experiments on faults will be run, validation data will be collected, a model will be validated and, lastly, different faults will be run using the model to arrive at a representative set of risk-fault indicators.

The limitations associated with the current research will be highlighted in this section of the chapter One expected potential limitation to be confronted by the researcher during the primary research arises from the busy schedules of the respondents. Given that the respondents of the survey are the active employees of different organizations, it will be difficult for some of the participants to take time out from their busy schedules to respond to the survey questions. .

Another potential limitation is the budget for the research. Therefore, it is not possible to target all the facility managers of buildings in the UAE. Moreover, the sample of the survey is limited to participants from the UAE only, consequently their views cannot be considered as being representative of maintenance personnel across the globe. Time was another constraint of the study, which has limited a thorough and complete analysis of the findings of the study.

1.8 Organization of the Thesis

Chapter 1 (Introduction), introduces the issues under consideration and provides a background for the study which presents an insight for the topic under research.
Chapter 2 (Literature Review), provides the previously conducted similar studies and research from various authentic and reliable resources.

Chapter 3 (Research Methodology) highlights the methodology selected by the researcher to acquire data that meets the research requirements. That involves selecting building samples, developing an energy and operation model, running a field experiment on faults, collecting validation data, validating a model, running different faults using the model, developing risk-fault indicators and the writing of maintenance procedures.

Chapter 4 (Model development and data collection) focuses on the development of modelling and simulation methods for maintenance issues and assesses the impacts of common maintenance issues on building performance in order to develop the HVAC system maintenance procedures.

Chapter 5 (Analysis and Discussion) presents the responses of the participants in the form of bar charts and frequency tables. In the same chapter, the researcher interprets the responses and provides a discussion of the findings of the study.

Finally, Chapter 6 (Conclusion and Recommendations) concludes the outcomes and discusses the improvements that can be applied based on this study.

1.9 Summary

This chapter has introduced the background of the study along with a synopsis of the statement of the problem. Therefore, this research aims to identify the problems that are confronted during the maintenance of HVAC systems in residential and commercial buildings in Abu Dhabi since they affect the indoor air quality. The aim of the research is to improve the efficiency of the HVAC maintenance procedures with special reference to those used in Abu Dhabi.

CHAPTER 2

LTERATURE REVIEW

2.1 Introduction

For completeness, this literature review addresses local as well as international maintenance practice. These areas include sources of faults for HVAC systems, HVAC operation maintenance models, energy efficiency related to HVAC maintenance procedures and fault indicator models.

2.2 Over view on HVAC systems

This section addressed the famous two types of air-conditioning system the first system is DX systems (including; window type, split unit, package unit and Variable refrigerant flow VRF) and the second system is Central Air Conditioning System which is including chiller system and district cooling(Saleh,2013).

2.2.1 Overview of DX systems

DX systems includes; window type, split Air-Conditioning Systems, package unit and Variable refrigerant flow VRF (Saleh, 2013).

I. Window Type air conditioner

Window type air conditioner including an indoor heat exchanger for heat exchange with room air, an outdoor heat exchanger for heat exchange with external air, an air guide for partitioning a space for fitting the indoor heat exchanger and a space for fitting the outdoor heat exchanger(Figure 2.1). A turbo fan fitted inside of the air guide directly, for discharging the room air heat exchanged as the room air passes through the indoor heat exchanger into the room, again, and a fan fitted to outside of the air guide for discharging the external air heat exchanged as the external air passes through the outdoor heat exchanger again, thereby permitting uniform discharge of cooled air from an outlet while a structure is made simple, improving a strength as the structure is simple, and preventing air leakage between indoor side components as assembly between the orifice and the air guides are made accurate(Kang,2002).

Figure 2.1: Window type air conditioner(Kang,2002)
II. Split Air-Conditioning Systems

Split type air conditioning systems are one-to- one systems consisting of one evaporator (fan coil) unit connected to an external condensing unit. Both the indoor and outdoor units are connected through copper tubing and electrical cabling as shown in Figure 2.2. And multi-type air conditioning system is same as split type air- conditioning system however in this case there are ‘multiple’ evaporator units connected to one external condensing unit. The indoor part (evaporator) pulls heat out from the surrounding air while the outdoor condensing unit transfers the heat into the environment (Goetzler, 2007).

Figure 2.2: Split Air-Conditioning Systems (Goetzler, 2007)

According to Goetzler ( 2007) the advantages of using Split Air-conditioners are:
• Low initial cost, less noise and ease of installation;
• Good alternative to ducted systems;
• Each system is totally independent and has its own control.
Disadvantages
• There is limitation on the distance between the indoor and outdoor unit i.e.
refrigerant piping can’t exceed the limits stipulated by the manufacturer (usually 100 to 150 ft) otherwise the performance will suffer;
• Maintenance (cleaning/change of filters) is within the occupied space;
• Limited air throws which can lead to possible hot/cold spots;
• Impact on building aesthetics of large building because too many outdoor units will spoil the appearance of the building.

A multi-type air conditioning system operates on the same principles as a split type air- conditioning system however in this case there are ‘multiple’ evaporator units connected to one external condensing unit. These simple systems were designed mainly for small to medium commercial applications where the installation of ductwork was either too expensive, or aesthetically unacceptable. The small-bore refrigerant piping, which connects the indoor and outdoor units requires much lower space and is easier to install than the metal ducting (Goetzler, 2007). Each indoor unit has its own set of refrigerant pipe work connecting it to the outdoor unit as shown in Figure 2.3.

Figure 2.3: Multi-Type Air Conditioning System (Goetzler, 2007)

According to Goetzler ( 2007), the advantage of Multi-splits includes:
• The fact that one large condenser can be connected to multiple evaporators within the building reduces and/or eliminates the need for ductwork installation completely.
• Multi-splits are suitable for single thermal zone (defined below) applications with very similar heat gains/losses.
The disadvantage of Multi-splits includes:
• Multi-split systems turn OFF or ON completely in response to a single thermostat/control station which operates the whole system. These systems are therefore not suitable for areas/rooms with variable heat gain/loss characteristics.
III. Package Unit
Package air conditioner and heat pump units generally comprise both indoor and outdoor coils, a compressor, and both indoor and outdoor fans. The outdoor coil and fan are disposed in communication with the outdoor ambient to circulate ambient air through the outdoor coils. The indoor coils and fan are disposed in communication with air ducts which are connected to the space being cooled or heated thereby circulating the indoor air through the indoor coils (Figure 2.4). For heat pump units, electric strip heating elements may also be disposed with the indoor fan to supplement the heating of the indoor coil when used as a condenser. Conventionally, the package units include a box shaped cabinet made of sheet metal in which the coils, fans, and compressor are located, with the cabinet including supply and return ports for connection with the corresponding supply and return air ducts of the structure being serviced( Katatani, 2004).

Figure 2.4: Single Package Roof Top Unit (Katatani, 2004).

IV. Variable Refrigerant Flow (VRF )Systems
VRF systems are similar to the multi-split systems which connect one outdoor section to several evaporators. However, multi-split systems turn OFF or ON completely in response to one master controller, whereas VRF systems continually adjust the flow of refrigerant to each indoor evaporator. The control is achieved by continually varying the flow of refrigerant through a pulse modulating valve (PMV) whose opening is determined by the microprocessor receiving information from the thermistor sensors in each indoor unit. The indoor units are linked by a control wire to the outdoor unit which responds to the demand from the indoor units by varying its compressor speed to match the total cooling and/or heating requirements as shown in Figure 2.5. VRF systems promise a more energyefficient strategy (estimates range from 11% to 17% less energy compared to conventional units) at a somewhat higher cost (Zhou, 2007).

Figure 2.5: Variable Refrigerant Flow or (VRF ) Systems(Zhou, 2007).

The speed to follow the variations in the total cooling/heating load as determined by the suction gas pressure measured on the condensing unit. The capacity control range can be as low as 6% to 100%.Refrigerant piping runs of more than 200 ft are possible, and outdoor units are available in sizes up to 240,000 B.t.u.h (Liu, 2010).

The installed cost of a VRF system is highly variable, project dependent, and difficult to pin down. Studies indicate that the total installed cost of a VRF system is estimated to be 5% to 20% higher than air or water cooled chilled water system, water source heat pump, or rooftop DX system providing equivalent capacity. This is mainly due to long refrigerant piping and multiple indoor evaporator exchanges with associated controls. Building owners often have no incentive to accept higher first costs, even if the claimed payback period is short, as the energy savings claims are highly unpredictable(Liu, 2010).

According to Aynur (2010). the main advantages of a variable refrigerant flow system are:
• Ability to respond individually to fluctuations in space load conditions. The user can set the ambient temperature of each room as per his/her requirements and the system will automatically adjust the refrigerant flow to suit the requirement; humidity regardless of outside conditions. Any area in the building will always be exactly at the right temperature and humidity, ensuring total comfort for their occupants;
• VRF systems are capable of simultaneous cooling and heating. Each individual indoor unit can be controlled by a programmable thermostat. Most VRF manufacturers offer a centralized control option, which enables the user to monitor and control the entire system from a single location or via the internet;
• VRF systems can generate separate billing that makes individualized billing easier;

• VRF systems use variable speed compressors (inverter technology) with 10 to 100% capacity range that provides unmatched flexibility for zoning to save energy. Use of inverter technology can maintain precise temperature control, generally within ±1°F.

2.2.2 Central Air-Conditioning System

Central air conditioning systems are known as Air cooled chillers, Water cooled chillers, centrifugal chillers .And are typically comprised of a compressors and heat exchangers. A gaseous refrigerant is compressed by the compressor to a high pressure superheated gas. The superheated refrigerant flows into the first heat exchanger, known as the condenser. Within the condenser, cold water passes over the tubing that carries the superheated refrigerant and removes heat there from. The refrigerant leaves the condenser as a saturated liquid under high pressure. It flows through an expansion valve, which reduces the pressure, and then flows to the evaporator, the second heat exchanger. A fan forces warm air over the evaporator tubes and the refrigerant expands, removing heat from the air. The refrigerant leaves the evaporator as a saturated gas under low pressure, flows back into the compressor, and the cycle is repeated( Nanami, 2013).

Figure 2.6: Central Air-Conditioning System ( Nanami, 2013)

2.2.3 District Cooling

District cooling system (DCS) is a massive cooling energy production scheme that serves a group of buildings. The system performance can often be improved by the incorporation of a cool-storage system, in that part of the cooling demand is shifted from peak hours to non-peak hours. This brings mutual benefits to the power supplier and the consumers (Chan, 2006).

District cooling technology is advantageous in warm and hot climatic regions, in that chilled water from a central refrigeration plant is delivered through a distribution network to groups of buildings (Figure 2.7). The technology is most suitable for new urban development where system design and construction receive much freedom. The process involves a series of building design load computation, dynamic simulation, and plant energy consumption analyses for different phases of development (chow, 2004).

Figure 2.7: District Cooling System (chow, 2004).

2.3Refrigeration System Components

The refrigeration system components are compressors, evaporators, condenser and expansion devices and auxiliaries as shown in Figure 2.4.

Figure 2.4: Refrigeration System Components

2.3.1 Compressors

Compressors are the heart of the air conditioning systems, their functions are to raise the pressure of the refrigerant vapor from Evaporator pressure to Condensing pressure. Refrigerant condensates and evaporates in Condenser and Evaporator at the temperature of the air or other Liquid used for condensing and Evaporating Pressures and temperatures during Condensation and Evaporation processes are always constant(Chandra,2010).

2.3.1.1 Compressor Types.

There are four types of compressors used in the air conditioning industry: Reciprocating (Figure 2.5), Scroll(Figure 2.6), Screw(Figure2.7) and Centrifugal Compressors(Figure 2.8). Three Types of Reciprocating, Scroll and Screw work on the principle of trapping the refrigerant vapor and compressing it by gradually shrinking the volume of refrigerant. They Call Positive Displacement Compressors Centrifugal compressors use the principle of Dynamic Compression which involves converting energy from one form to another in order to increase the pressure and temperature of refrigerant (Bloch, 2010).

Figure 2.5: Reciprocating Compressors (Bloch, 2010)

Figure 2.6: Scroll Compressors (Bloch, 2010)

Figure 2.7: Screw Compressors (Bloch, 2010)

Figure 2.8: Centrifugal Compressors (Bloch, 2010)

2.3.2 Evaporators

Type of evaporators includes Bare Tube Evaporator, Finned Tube Evaporator, Plate heat exchanger Evaporator, Shell and Tube Evaporators (Ayub, 2003).

2.3.2.1Bare Tube Evaporator

The bare tube evaporators are made up of copper tubing or steel pipes. The copper tubing is used for small evaporators where the refrigerant other than ammonia is used, while the steel pipes are used with the large evaporators where ammonia is used as the refrigerant (Figure 2.9). The bare tube evaporator comprises of several turns of the tubing, though most commonly flat zigzag and oval are the most common shapes. The bare tube evaporators are usually used for liquid chilling. In the blast cooling and the freezing operations as the atmospheric air flows over the bare tube evaporator and the chilled air leaving it used for the cooling purposes(Ayub,2003).

Figure 2.9: Bare Tube Evaporators (Ayub, 2003)

2.3.2.2 Finned Tube Evaporator

Multiple circuits can be used in finned evaporators as distributor lines are used to feed the liquid refrigerants flow rate in parallels circuits(Figure 2.10). In large capacities, it needs to use more than one distributors Vapor refrigerant will leave the coil and collects in Suction headers. The saturated vapor must be superheated at the end of evaporators where it is height temperature difference (between air and refrigerant) before entering to compressor (Domanski, 2005).

Figure 2.10: Finned Tube Evaporator (Domanski, 2005)

2.3.2.3 Plate Heat Exchanger Evaporator

Number of plates that are assembled in such a way that when these plates are bolted together one of the fluids flows between two of the plates and the other fluid between the pairs of adjacent plates (Figure 2.11). The plates are corrugated with a pattern that physically strengthens the plates and also promotes turbulence of the fluids, providing excellent convection heat-transfer coefficients. the challenge of how to seal the refrigerant passages. This goal is achieved by a construction pairs of plates forming the refrigerant passages are brazed or welded (Kakac, 2012).

Figure 2.11: Plate Heat Exchanger Evaporator (Kakac, 2012)

2.3.2.4 Shell and Tube Evaporators

The shell-and-Tube cooler is usually made up of one or more spiral-shaped, bare-tube coils enclosed in a shell. Baffles within the shell are to create turbulence in direction of water within the shell to increase and improve the heat transfer. It is used chilled water system, drinking water, beverages, and others industrial applications. Liquid refrigerant flow through the tubes and water fills the shell space surrounding the rubes (Figure 2.12). Heat transferred from warm water to cold liquid refrigerant caused boiling of refrigerant inside the tubs and changed state to vapor sucking to compressor . Water temperature in shell will be reduced as a result of heat transfer with liquid refrigerant inside the tubes to the design
condition and will be pumper to chillier water units (FCU’s, AHU’s,….etc)(Chen,2006).

Figure 2.12: Shell and Tube Evaporators (chen, 2006)

2.3.3Expansion Devices

According to Garcia (2002),expansion devices are used to achieve:
• Maintain difference between the high pressure (condenser) and low pressure ( evaporator) sides. This pressure difference allows the evaporator temperature to be low enough to absorb the heat from air or water and reduce the temperature to be cooled one.
• Control the refrigerant flow into the evaporator
2.3.3.1 Type of Expansion Devices

There are several types of expansion devices: Capillary Metering Device,
Orifice Metering Device andThermal Expansion Valves (Garcia, 2002)

I. Capillary Metering Device

Capillary tube is one of the most commonly used throttling devices in the refrigeration and the air conditioning systems (Figure 2.13). The capillary tube is a copper tube of very small internal diameter. It is of very long length and it is coiled to several turns so that it takes less space. The internal diameter of the capillary tube used for the refrigeration and air conditioning applications varies from 0.5 to 2.28 mm (1/16 to 1/8 ) inch. When the refrigerant leaves the condenser and enters the capillary tube its pressure drops down suddenly due to very small diameter of the capillary. In capillary the fall in pressure of the refrigerant takes place not due to the orifice but due to the small opening of the capillary(Whitman, 2012).

Figure 2.13: Capillary Metering Device (Whitman, 2012)

According to Whitman (2012), here are some of the advantages of using capillary tube as the throttling device in the refrigeration and the air conditioning systems:
• The capillary tube is a very simple device that can be manufactured easily and it is not very costly.
• The capillary tube limits the maximum amount of the refrigerant that can be charged in the refrigeration system due to which the receiver is not required in these systems.
• When the plant is stopped the pressure at the suction and discharge side of the compressor are same. Thus compressor is restarted there with low load on it since it does not have to overcome very high pressures. Due to this the compressor motor of smaller torque can be selected for driving the compressor, thus reducing the cost of the compressor.

II. Orifice Metering Device

Designed for dual flow operation, commonly used in reverse cycle residential and light commercial air conditioning applications. Two valves can be used on reverse cycle air conditioning units and their orifices matched to the load of the indoor and outdoor coil (heat pump systems) Orifice Metering Device As liquid refrigerant flows into the valve, it drives the orifice onto a seat which only allows refrigerant to flow through the orifice restrictor. The refrigerant is then metered into the evaporator. When the refrigerant flow is reversed (reverse cycle air conditioning) the orifice moves to the other end of the valve – in this position it offers no restriction to the flow of refrigerant (Figure 2.14)(Camp, 2000).

Figure 2.13: Orifice Metering Device (Camp, 2000)

III. Thermal Expansion Valves

A very common type of metering device is called a TX Valve (Thermostatic Expansion Valve)(Figure 2.14). This valve has the capability of controlling the refrigerant flow. If the load on the evaporator changes, the valve can respond to the change and increase or decrease the flow accordingly.
The TXV has a sensing bulb attached to the outlet of the evaporator. This bulb senses the suction line temperature and sends a signal to the TXV allowing it to adjust the flow rate. This is important because, if not all, the refrigerant in the evaporator changes state into a gas, there could be liquid refrigerant content returning to the compressor. This can be fatal to the compressor. Liquid can not be compressed and when a compressor tries to compress a liquid, mechanical failing can happen. The compressor can suffer
mechanical damage in the valves and bearings. This is called” liquid slugging”. Normally TXV’s are set to maintain 10 degrees of superheat. That means that the gas returning to the compressor is at least 10 degrees away from the risk of having any liquid (Yano, 2003).

Figure 2.14: Thermal Expansion Valves (Yano, 2003)

2.3.4 Condensers

Condenser is a heat exchanger that rejects heat from the refrigerant to air, water or some other fluids. There are three famous and common t