US20150362230A1

US20150362230A1 - Air conditioning system with pre-cooler

Info

Publication number: US20150362230A1
Application number: US14/580,140
Authority: US
Inventors: Abdulghani A. Al-Farayedhi; Palanichamy Gandhidasan; Nasiru I. Ibrahim
Original assignee: King Fahd University of Petroleum and Minerals
Current assignee: King Fahd University of Petroleum and Minerals
Priority date: 2014-06-16
Filing date: 2014-12-22
Publication date: 2015-12-17

Abstract

The air conditioning system with pre-cooler includes a compressor; a condenser; an evaporator; a condensate reservoir to collect condensed water discharged from the evaporator; and a condensate pump associated with the condensate reservoir. In one embodiment, the system includes a single pre-cooler that uses condensate to precool air before it reaches the evaporator. In a second embodiment, the system includes a sub-cooler between the condenser and an expansion valve that uses the condensate to pre-cool the refrigerant before it reaches the evaporator. In a third embodiment, the system includes a first pre-cooler that uses condensate to precool air before it reaches the evaporator; a sub-cooler between the condenser and an expansion valve that uses the condensate to pre-cool the refrigerant before it reaches the evaporator; and a second pre-cooler that uses the condensate to pre-cool air before it reaches the condenser.

Description

CROSS-REFERENCE TO RELATED APPLICATION

This application claims the benefit of U.S. Provisional Patent Application Ser. No. 62/012,955, filed Jun. 16, 2014.

BACKGROUND OF THE INVENTION

1. Field of the Invention
The present invention relates to air conditioning systems, and particularly to an air conditioning system that recirculates the condensate to precool air or to sub-cool refrigerant.
2. Description of the Related Art
An air conditioner (often referred to as “AC”) is an appliance, system, or mechanism designed to extract heat from an area. The cooling is done using a simple refrigeration cycle. Its purpose, in a building or an automobile, is to provide comfort during hot weather. In the refrigeration cycle, a heat pump transfers heat from a lower-temperature heat source into a higher-temperature heat sink. Heat would naturally flow in the opposite direction. This is the most common type of air conditioning. This cycle takes advantage of the way phase changes work, where latent heat is released at a constant temperature during a liquid/gas phase change, and where varying the pressure of a pure substance also varies its condensation/boiling point.
Most high occupancy buildings, such as schools, airports, office buildings, hotels and shopping malls, have high interior relative humidity and large amounts of air conditioner condensate. The condensed moisture can be considered as a byproduct of the air conditioning cooling process. The condensate production depends upon cooling load, humidity, and make-up air volumes. It is claimed that the reuse of the condensate reduces the need for desalinated water. Collected condensate temperature is usually between 10° C. and 15.6° C. The cold condensate typically drips from the evaporator surface into a pan and is discharged from the system through a drain.
One way to increase the efficiency of an AC unit is to lower the temperature of the air entering the evaporator and compressor units. That is, an air conditioning system can be made more efficient by sub-cooling the liquid refrigerant below the outdoor temperature thereby reducing the amount of flash vapor and allowing a much higher percentage of the refrigerant to be used as effective latent heat. This is beneficial because it will permit the use of less refrigerant or lower pressure, each of which will result in a more efficient unit.
Thus, an air conditioning system with pre-cooler solving the aforementioned problems is desired.

SUMMARY OF THE INVENTION

The air conditioning system with pre-cooler includes a compressor; a condenser having a condenser fan associated with the condenser that forces air to cool the condenser; an evaporator having a fan associated with the evaporator that moves cool air out of the air conditioning system; a plurality of refrigerant conduits; a condensate reservoir to collect condensed water discharged from the evaporator; and a condensate pump associated with the condensate reservoir. In one embodiment, the system includes a single pre-cooler that uses condensate to precool air before it reaches the evaporator. In a second embodiment, the system includes a sub-cooler between the condenser and an expansion valve that uses the condensate to pre-cool the refrigerant before it reaches the evaporator. In a third embodiment, the system includes a first pre-cooler that uses condensate to precool air before it reaches the evaporator; a sub-cooler between the condenser and an expansion valve that uses the condensate to pre-cool the refrigerant before it reaches the evaporator; and a second pre-cooler that uses the condensate to pre-cool air before it reaches the condenser.
These and other features of the present invention will become readily apparent upon further review of the following specification and drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a schematic diagram of the basic components of a conventional vapor compression air conditioner (AC) system of the prior art.

FIG. 2 is a pressure-enthalpy (P-h) diagram with additional sub-cooling from the sub-cooler arrangement.

FIG. 3 is a schematic diagram of an embodiment of an air conditioning system with pre-cooler according to the present invention.

Similar reference characters denote corresponding features consistently throughout the attached drawings.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The air conditioning system with pre-cooler utilizes the cold condensate that typically drips off the evaporator of a conventional air conditioner to pre-cool the incoming air and/or refrigerant in order to achieve greater efficiency. Reducing the air temperature before entering the evaporator and the condenser by incorporation of suitable pre-cooling technique and sub-cooling the refrigerant exiting the condenser will enhance the cooling capacity of the AC system and reduce the power consumption and increase the energy efficiency.
Referring to FIG. 1, a conventional vapor compression air conditioning system typically comprises four basic components: the evaporator 102, the compressor 107, the condenser 104, and the expansion valve 103. In a typical refrigeration cycle, the refrigerant fluid flows through the compressor 107 into the condenser 104, and into the expansion valve 103 and finally into the evaporator 102. An internal fan 105 moves the ambient air 106 through the condenser 104. Another internal fan 108 forces the incoming air 101 through the evaporator 102 and out of the AC unit into the room as cold, air conditioned air 109. When the refrigerant flows through the evaporator 102, moisture is removed from the air stream and a cold condensate forms on the surface of the evaporator coil 102. The condensate drips from the evaporator coil 102, which is collected into a condensate tray 110 and is discharged through a drain system. In hot, humid countries like Saudi Arabia, a great deal of condensate is collected from the humid air.
FIG. 2 illustrates the refrigeration cycle of a system of a pressure (P) versus enthalpy (h) diagram with additional sub-cooling that can be attained by adding a sub-cooler between the condenser 104 and the expansion valve 103 that uses the condensate to pre-cool the refrigerant before it reaches the expansion valve 103. The P-h diagram is useful in showing the amounts of energy transfer as heat and illustrates the thermodynamic characteristics of a typical refrigerant.
In a conventional AC, the refrigerant vapor enters the compressor at state 1 where it is compressed to higher temperature and pressure. The high pressure super-heated vapor then enters an air cooled condenser at state 2, where it is cooled by flowing air stream and exits as a liquid at state 3. Then the liquid refrigerant passes through the expansion valve 103, where it is expanded at constant enthalpy and the pressure is suddenly decreased, the refrigerant becoming a saturated mixture of liquid and vapor, and finally the mixture enters the evaporator at state 4. This cycle can be depicted as 1→2→3→4→1 in FIG. 2. Ambient air or mixed air is forced through the evaporator fan. Because the refrigerant is at a temperature below the dew point temperature of the air stream, it absorbs heat from the air and boils, changing phase from liquid to vapor. The air is now cooled due to heat exchange in the evaporator. During this transfer of energy, only latent heat is absorbed, resulting in the refrigerant remaining at constant temperature.
Additional sub-cooling can be achieved where the cycle commences from 1→2→3′→4′→1 as shown in FIG. 2. The process constitutes the refrigeration cycle with a sub-cooler for additional heat rejection at constant pressure and therefore sub-cooling of the refrigerant entering the expansion valve. In a conventional AC system, the cold condensate typically drips from the evaporator surface into a pan and is discharged from the system through a drain, However, this cold condensate can be utilized to sub-cool the liquid refrigerant exiting the condenser. The process constitutes the refrigeration cycle with a sub-cooler for additional heat rejection at constant pressure, thereby further sub-cooling of the refrigerant entering the expansion valve, shown schematically as: 1→2→3′→4′→1.
This sub-cooling can be achieved in different ways. The cold condensate which is collected from the cooling coil can be sprayed directly on the surface of the condenser coil to assist heat rejection from the refrigerant in the condenser and reduce the discharge pressure of the refrigerant. This system may not be that effective, since the heat rejected in the condenser is limited by the temperature of the outdoor ambient air. As the size of the condenser coil increases, the amount of heat rejected in the condenser coil does not increase proportionally. Therefore, the cold condensate has little cooling effect on the large condenser coils.
Instead of spraying the cold condensate on the refrigerant line exiting the condenser, it can be sprayed on the sub-cooling portion of the condenser to further sub-cool the refrigerant. It is also possible to locate the refrigerant line exiting the condenser coils in the condensate pan. Thus, the condenser rejects heat to the cold condensate in the condensate pan. However, it is difficult to adopt this type of sub-cooling technology.
Alternately, the sub-cooling technology with addition of a heat exchanger at the downstream of the condenser to reject heat from refrigerant can downsize the compressor and the condenser. In this method, the refrigerant flows through a counter-flow condensate heat exchanger positioned between the condenser and the expansion valve and is further sub-cooled by the cold condensate that is collected from the cooling coil. It is a lower temperature heat sink than the outside air. Reducing the air temperature before entering the evaporator and the condenser by incorporating suitable pre-cooling technique and sub-cooling the refrigerant exiting the condenser will greatly enhance the cooling capacity of the system and reduce the power consumption. This heat efficiency can be achieved by the newly designed AC with heat exchangers, as shown schematically in FIG. 3.
Referring to FIG. 3, the present air conditioning system includes additional heat exchangers, which are the sub-cooler and pre-coolers, to further lower the temperature of the air as compared to the conventional AC. In a typical refrigeration cycle, the refrigerant flows through the compressor 114 into the condenser 109 and into a sub-cooler 107, then into the expansion valve 106 and finally into the evaporator 115. The condensate discharged from the evaporator 115 is collected into a condensate reservoir 104, which is circulated using a condensate pump 103 to a first pre-cooler 102 and back into the condensate reservoir 104. The condensate is also circulated by the pump 103 to a second pre-cooler 110 via the control valve 111 and back to the condensate reservoir 104. The cold condensate is also circulated through the sub-cooler 107 located in between the condenser 109 and expansion valve 106, thereby further cooling the refrigerant before expansion.
As illustrated in FIG. 3, the ambient air 101 passes through the first pre-cooler 102, where it is pre-cooled by the circulating condensate. The air then enters the evaporator 115 and it is further cooled and dehumidified. The air temperature drops as it passes through the second pre-cooler 110 via the pump 103 before entering the condenser 109, thereby providing better cooling of the refrigerant in the condenser 109. Also, the condensate is circulated by the pump 103 to the sub-cooler 107 and back to the condensate reservoir 104. The condensate that is accumulated in the reservoir 104 can be used as water 119 for other purposes.
The heat exchange capacity of a suitable heat exchanger depends on the configuration of the heat exchanger, the heat exchange area, the fluids being heat exchanged, the materials of construction of the heat exchanger, and the flow rate of the fluids. The most important element in maintaining high efficiency is the flow rate of the fluids. To provide the highest efficiency, the condensate water must be present and sufficient to extract a reasonable quantity of heat during the limited period of contact. In instances where no condensate water in present, no sub-cooling will take place. Therefore, make up water must be periodically added in the system. In some conditions, the amount of condensate water will be in excess of that required for sub-cooling due to very high humidity levels, in which case some condensate water can be removed and used for other purposes.
Sub-cooling decreases the enthalpy of the refrigerant entering the cooling coil, resulting in an increase in the cooling capacity. The amount of sub-cooling is limited by the temperature of the condensate. An understanding of impacts of equipment, load and climate on the energy savings mechanism is essential to proper application of this technology. The sub-cooling technology requires some custom design and installation for proper operation and to obtain maximum savings potential. In some embodiments, the condensate is used to improve the performance of the air conditioner by sub-cooling the liquid refrigerant exiting from the condenser and pre-cooling the incoming air. Therefore, the cooling capacity of the air conditioner would be enhanced with the reduction of power consumption and increases the energy efficiency rating (EER). Thus, the sub-cooler can be used as a retrofit in an existing AC system.
Additionally, the amount of condensate collected from the cooling coils is estimated based on the thermodynamic analysis for different operating conditions and compared with the experimental results. After extracting the heat from the liquid refrigerant, the warm condensate can be collected and re-circulated back to the system after being cooled by mixing it with fresh cold condensate. The excess of the accumulated condensate can be used as source of water for non-drinking applications as well, such as irrigation, cooling towers make-up water and other uses. It can be used for drinking purposes after undergoing the required microbial processes to sanitize the water.
It will be understood that in a first embodiment, the air conditioning system of FIG. 3 includes only the first pre-cooler 102, but does not include the sub-cooler 107 or the second pre-cooler 110. This system was tested for thirty days in Dhahran, Saudi Arabia. It was found the pre-cooling the air with the condensate before it reaches the evaporator by 4° C. lowered the air conditioning load by 10%.
It will also be understood that in a second embodiment, the air conditioning system of FIG. 3 includes the sub-cooler 107, but not the first pre-cooler 102 or the second pre-cooler 110. In a third embodiment, the air conditioning system of FIG. 3 includes the first pre-cooler 102, the second pre-cooler 110, and the sub-cooler 107.
It is to be understood that the present invention is not limited to the embodiments described above, but encompasses any and all embodiments within the scope of the following claims.

Claims

1. An air conditioning system, comprising:

a compressor;

a condenser;

an expansion valve;

an air flow conduit having an air inlet and an air outlet;

an evaporator coil disposed in the air flow conduit;

refrigerant conduit connecting the compressor, the condenser, the expansion valve, and the evaporator coil;

refrigerant flowing through the refrigerant conduit in a continuous cycle from the compressor to the condenser to the expansion valve to the evaporator coil and back to the compressor;

a condensate tank disposed to receive water condensing from air onto the evaporator coil as hot, humid air flows through the air flow conduit;

a first pre-cooler, including:

a condensate pump connected to the condensate tank;

a first heat exchanger disposed in the air flow conduit between the air inlet and the evaporator coil; and

a pre-cooler conduit connected between the condensate pump and the heat exchanger, whereby cool condensate collected in the condensate tank is circulated through the first heat exchanger to pre-cool air before the air reaches the evaporator coil, thereby reducing cooling load on the air conditioning system;

a sub-cooler disposed between the condenser and the expansion valve, the sub-cooler having a counter-flow heat exchanger having a first conduit passing refrigerant flow from the condenser to the expansion valve and a counter-flow conduit passing condensate collected from the evaporator coil through the condensate pump, the sub-cooler, and back to the condensate tank in a direction opposite the refrigerant in order to cool the refrigerant; and

a second pre-cooler disposed adjacent the condenser, the second pre-cooler having a second heat exchanger receiving condensate from the condensate pump and conduit connecting the condensate pump with the second heat exchanger.

2-3. (canceled)