DIRECT EXPANSION AIR CONDITIONING SYSTEM

Abstract

An air conditioning system capable of treating a conditioned space by treating outdoor air from outside the conditioned space and treating air returned from inside the conditioned space, and mixing the outdoor air with the returned air to form supply air for the conditioned space, the air conditioning system including: •an outdoor air latent cooling treatment stage and a return air sensible cooling treatment stage, and •an air mixer for mixing outdoor air with return air to form the conditioned space supply air; wherein the outdoor air latent cooling treatment stage includes a dehumidification evaporator and the return air sensible cooling treatment stage includes a sensible evaporator, both evaporators being coupled to a direct expansion refrigeration circuit, and wherein the dehumidification evaporator thermal capacity is regulated in response to conditioned space humidity and the sensible evaporator thermal capacity is regulated in response to conditioned space dry bulb temperature.

Claims

1. An air conditioning system, the air conditioning system being capable of treating a conditioned space by treating outdoor air from outside the conditioned space and treating air returned from inside the conditioned space, and mixing the outdoor air with the returned air to form supply air for the conditioned space, the air conditioning system including: an outdoor air latent cooling treatment stage and a return air sensible cooling treatment stage, and an air mixer for mixing outdoor air with return air to form the conditioned space supply air; wherein the outdoor air latent cooling treatment stage includes a dehumidification evaporator and the return air sensible cooling treatment stage includes a sensible evaporator, both evaporators being coupled to a direct expansion refrigeration circuit, and wherein the dehumidification evaporator thermal capacity is regulated in response to conditioned space humidity and the sensible evaporator thermal capacity is regulated in response to conditioned space dry bulb temperature.

2. An air conditioning system according to claim 1, including a liquid refrigerant receiver that stores transitional refrigerant.

3. An air conditioning system according to claim 2, wherein the liquid refrigerant receiver provides storage and separation of liquid refrigerant and compressor oil for independent management through the system.

4. An air conditioning system according to claim 2, wherein the liquid refrigerant receiver incorporates liquid level sensing, an automated pressure equalization valve and automated oil return valves.

5. An air conditioning system according to claim 1, including a two stage refrigerant oil separation system.

6. An air conditioning system according to claim 5, including a first stage hot gas separator and a second stage cool liquid separator.

7. An air conditioning system according to claim 6, wherein the second stage cool liquid separator is provided by a liquid refrigerant receiver that stores transitional refrigerant.

8. An air conditioning system according to claim 1, including a sub-cool heat exchanger to reject heat into outdoor air to sub-cool liquid refrigerant.

9. An air conditioning system according to claim 8, wherein the sub-cool heat exchanger incorporates an adiabatic pre-cooling media or a progressive indirect adiabatic cooling process.

10. An air conditioning system according to claim 9, wherein the sub-cool heat exchanger also includes a fan to move air through the condensers, a refrigerant temperature sensor and a variable performance drive to regulate the mass flow rate of the fan.

11. An air conditioning system according to claim 1, including a super-cool heat exchanger to super cool liquid refrigerant utilizing condensate collected from the dehumidification evaporator.

12. An air conditioning system according to claim 1, including a three stage heat of rejection process that incorporates an ambient condenser, a sub-cool heat exchanger and a super-cool heat exchanger.

13. An air conditioning system according to claim 3, wherein the liquid refrigerant receiver incorporates liquid level sensing, an automated pressure equalization valve and automated oil return valves.

14. An air conditioning system according to claim 13, including a three stage heat of rejection process that incorporates an ambient condenser, a sub-cool heat exchanger and a super-cool heat exchanger.

15. An air conditioning system according to claim 2, including a three stage heat of rejection process that incorporates an ambient condenser, a sub-cool heat exchanger and a super-cool heat exchanger.

16. An air conditioning system according to claim 4, including a three stage heat of rejection process that incorporates an ambient condenser, a sub-cool heat exchanger and a super-cool heat exchanger.

Description

BRIEF DESCRIPTION OF DRAWINGS

[0033] Having briefly described the general concepts involved with the present invention, a preferred embodiment of a direct expansion air conditioning system will now be described that is in accordance with the present invention. However, it is to be understood that the following description is not to limit the generality of the above description.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENT

[0034] In the drawings, FIG. 1 is a flow diagram of a preferred embodiment of a direct expansion air conditioning system in accordance with the present invention. For ease of understanding, the following description will first provide a general overview of the flow diagram of FIG. 1, followed by a more detailed explanation (in a table) of the different elements of the flow diagram.

[0035] In general terms, shown in the flow diagram of FIG. 1 is a direct expansion air conditioning system where return air Y from the conditioned space O is psychrometrically treated separately (and thus in parallel) from the outdoor air X (for ventilation) to achieve independent control of latent and sensible cooling. Following this separate treatment of the return air Y and the outdoor air X, the two air streams are then mixed to provide a single conditioned space supply air stream Z to be delivered to the conditioned space O.

[0036] The cooling process for the return air stream Y is principally sensible cooling, which is conducted in the return air sensible cooling treatment stage represented in this embodiment by a sensible evaporator H placed in the return air stream Y. The cooling process for the outdoor air stream X is principally latent cooling, which is conducted in the outdoor air latent cooling treatment stage represented in this embodiment by a dehumidification evaporator G placed in the outdoor air stream X.

[0037] As will be outlined in the following tabular description of the various components in the preferred embodiment, the thermal capacity of the dehumidification evaporator G is regulated in response to a desired conditioned space humidity set point, together with a load driven variance from that set point. Also, the thermal capacity of the sensible evaporator H is regulated in response to a desired conditioned space dry bulb temperature set point, together with a load driven variance from that set point.

[0038] The following table (Table 1) provides a more detailed explanation of the various elements illustrated in the flow diagram of FIG. 1:

Table 1—Identification of Components in FIG. 1

[0039] It will be appreciated that Table 1 above describes the different air conditioning componentry that makes up the physical form of the preferred embodiment. The following table (Table 2) describes the logic algorithms that form the basis of the control of the preferred embodiment. [0040] [Table 2 commences on page 14]

TABLE-US-00001 TABLE 2 Logic Algorithms for Control of Components in Figure 1 Strategy Functionality Sensible cooling If the space dry bulb temperature is 0.1 degrees or greater control strategy than set point, initiate the outdoor air economy cycle increasing deviation strategy and incrementally increase the outdoor airflow above set point economy cycle set point. If the space dry bulb temperature is 0.1 degrees or greater than set point, energize the return air evaporator liquid refrigerant stop valve to open. If the space dry bulb temperature 0.5 degrees greater than set point, commence incremental opening of the return air evaporator liquid refrigerant metering valve. If the return air evaporator liquid refrigerant metering valve opens to a position greater than 50%, commence incrementally increasing the supply air flow set point. If the supply air flow set point increases to greater than 70% output, energize the outdoor air evaporator liquid refrigerant stop valve to open and commence incremental drive of the outdoor air evaporator liquid refrigerant metering valve. If the outdoor air evaporator liquid refrigerant metering valve position actuator valve opens to a position greater than 50%, commence incrementally decreasing the compressor suction pressure set point. Sensible cooling If the space dry bulb temperature is 0.1 degrees or greater control strategy below set point, commence incrementally increasing the increasing deviation compressor suction set point. below set point If the space dry bulb temperature is 0.5 degrees or greater less than set point, commence incremental drive of the outdoor air evaporator liquid refrigerant metering valve position actuator valve to close. If the space dry bulb temperature is 0.5 degrees or greater less than set point, and the outdoor air evaporator liquid refrigerant metering valve has been driven closed, de-energize the outdoor air evaporator liquid refrigerant stop valve to close and commence incremental drive of the return air evaporator liquid refrigerant metering valve to close. If the return air evaporator liquid refrigerant metering valve drives to a position less than 70%, commence incrementally decreasing the supply air flow set point. If the return air evaporator liquid refrigerant metering valve has been driven closed to 0%, de-energize the return air evaporator liquid refrigerant stop valve to close. If the supply airflow set point has reduced to less than 50%, incrementally decrease the outdoor airflow economy cycle set point. Latent cooling control If the space dew point temperature is 0.1 degrees or strategy increasing greater than set point, energize the outdoor air evaporator deviation above set liquid refrigerant stop valve to open. point If the space dew point temperature is 0.5 degrees greater than set point, commence incremental drive opening of the outdoor air evaporator liquid refrigerant metering valve. If the outdoor air evaporator liquid refrigerant valve opens to a position greater than 50%, commence incrementally increasing the outdoor air flow set point. If the outdoor air flow set point increases to greater than 70%, commence incrementally decreasing the compressor suction set point. Latent cooling control If the space dew point temperature is 0.1 degrees or strategy decreasing greater below set point, commence incrementally deviation below set increasing the compressor suction set point. point If the space dew point temperature is 0.5 degrees or greater less than set point, commence incremental drive of the outdoor air evaporator liquid refrigerant metering valve to close. If the space temperature is 0.5 degrees or greater less than set point, and the outdoor air evaporator liquid refrigerant metering valve position actuator valve has been driven to less than 50%, commence reducing the outdoor air flow set point. If the space temperature is 0.5 degrees or greater less than set point, and the outdoor air flow set point has reduced to minimum, and the outdoor air evaporator liquid refrigerant metering valve has been driven closed to 0%, de-energize the outdoor air evaporator liquid refrigerant stop valve to close. Supply air flow set The supply airflow set point will be maintained between a point minimum set point and a maximum set point. The supply airflow set point will be adjusted by the sensible cooling control strategy. The supply airflow will be calculated by the addition of the outdoor airflow and the return airflow. Supply air flow greater If the supply airflow is greater than the supply airflow set than set point point, incrementally decrease the supply air fan speed. Supply air flow less If the supply airflow is less than supply airflow set point, than set point incrementally increase the supply air fan speed. Outdoor airflow set The outdoor airflow set point will be maintained between a point minimum ventilation set point, a maximum ventilation set point and a maximum outdoor airflow set point for economy cycle operation. The outdoor airflow set point will be adjusted the greater of any of the latent cooling strategy, the outdoor air economy strategy or the CO.sub.2 concentration strategy. Outdoor air flow If the outdoor airflow is greater than the outdoor airflow set greater than set point point, incrementally close the outdoor air throttling damper. Outdoor air flow less If the outdoor airflow is below the outdoor airflow set point than set point incrementally open the outdoor air throttling damper. CO.sub.2 concentration If the concentration of CO.sub.2 within the return air rises above strategy set point, increase the outdoor ventilation airflow set point. If the concentration of CO.sub.2 within the return air falls below set point, decrease the outdoor ventilation airflow set point. Outdoor air economy If the ambient dewpoint is less than the space dew point, strategy and the ambient dry bulb is less than the space dry bulb, enable operation of the outdoor air throttling damper economy cycle. If the ambient dew point is greater than the space dew point, and the ambient dry bulb is greater than the space dry bulb, disable operation of the outdoor air throttling damper economy cycle. Return airflow The return airflow calculation will be determined utilizing calculation the return air evaporator coil differential sensor, the entering return air evaporator coil dew point sensor and the leaving return air evaporator coil dew point sensor. The return air coil airflow will be determined in accordance with a wetted coil dew point calculation. Outdoor airflow The outdoor airflow calculation will be determined utilizing calculation the outdoor air evaporator coil differential sensor, the entering outdoor air evaporator coil dew point sensor and the leaving outdoor air evaporator coil dew point sensor. The outdoor air coil airflow will be determined in accordance with a wetted coil dew point calculation. Compressor start The compressor variable performance drive will be energized in response to the low liquid refrigerant level compressor start sensor, providing there is a minimum of opening of either the outdoor air evaporator liquid refrigerant thermal expansion flow metering valve or the return air evaporator liquid refrigerant thermal expansion flow metering valve. If the low liquid refrigerant sensor detects an absence of liquid refrigerant, energize the liquid receiver gas relief stop valve to open for a time period and energize the compressor variable performance drive to operate at low speed. Liquid receiver low The liquid receiver refrigerant low level will be regulated by level charging the low liquid refrigerant level sensor. If the low liquid refrigerant level sensor detects an absence of liquid refrigerant, increase the frequency of the compressor variable performance drive. Liquid receiver high The liquid receiver refrigerant high level will be regulated level charging by the high liquid refrigerant level sensor. If the high liquid refrigerant level sensor detects the presence of liquid refrigerant, decrease the frequency of the compressor variable performance drive. Compressor stop The compressor variable performance drive will be de- energized in response to the high liquid refrigerant level compressor stop sensor. If the high liquid refrigerant level compressor stop sensor detects the presence of liquid refrigerant, de-energize the compressor variable performance drive. Compressor oil return Oil will be returned to the compressor in response to the transition liquid refrigerant level sensor. When the transition liquid refrigerant level sensor detects a change of state from an absence of liquid refrigerant to a presence of liquid refrigerant/oil, energize the oil return stop valve to open and drive the oil return metering valve open to a fixed position. After a time duration, de-energize the oil return stop valve to close and drive closed the oil return metering valve. Compressor suction If the liquid refrigerant mass flow rate is not adequate to pressure set point satisfy sensible and latent cooling, the compressor variable frequency drive will be advanced by either the sensible or latent cooling strategies through detecting the refrigerant suction pressure. The set point at commencement of a signal from either the sensible or latent cooling strategies will be the pressure at commencement. On receipt of a lowering or raising suction pressure signal from either the sensible or latent cooling strategies, lower or raise the refrigerant suction pressure set point. The lowering of set point adjustment will take precedence over the raising of set point signal. Compressor suction The compressor variable performance drive will increase or pressure decrease frequency in response to the compressor suction pressure set point. The frequency will be increased incrementally when the suction pressure is higher than the suction pressure set point. The frequency will be decreased incrementally when the suction pressure is lower than the suction pressure set point. Condenser fan start The condensing fans will be energized to maintain condensing liquid temperature near ambient conditions. If the differential temperature between the sub-cooled high pressure liquid refrigerant temperature and the entering outdoor air dry bulb temperature is greater than the start differential temperature set point, energize the condenser fan variable frequency drive and commence operation at minimum frequency. Condenser fan stop The condensing fans will be de-energized to conserve energy when condensing temperature approaches ambient conditions. If the differential temperature between sub-cooled high pressure liquid refrigerant temperature and the entering outdoor air dry bulb temperature is less than the stop differential temperature set point, de-energize the condenser fan variable performance drive. Condensing The condensing head pressure will be regulated by the temperature control high pressure liquid refrigerant temperature and ambient dry bulb temperature. If the differential temperature between the sub-cooled high pressure liquid refrigerant temperature and the entering outdoor air dry bulb temperature is greater than the control differential set point (and rising), increase the condenser fan variable performance drive frequency. If the differential temperature between the sub-cooled high pressure liquid refrigerant temperature and the entering outdoor air dry bulb temperature is less than the control differential set point (and falling), decrease the condenser fan variable performance drive frequency. Sub-cooler fan start The sub-cooler fan will be energized to maintain liquid temperature near ambient wet bulb conditions. If the differential temperature between the sub-cooled high pressure liquid refrigerant temperature and the outdoor air wet bulb temperature is greater than the start differential temperature set point, energize the sub cooler fan variable performance drive and commence operation at minimum frequency, and energize the sub-cooler water circulating pump.. Sub-cooler fan stop The sub-cooler fan will be de-energized to conserve energy when condensing temperature approaches ambient conditions. If the differential temperature between sub-cooled high pressure liquid refrigerant temperature and the entering outdoor air wet bulb temperature is less than the stop differential temperature set point, de-energize the condenser fan variable performance drive and the sub- cooler water circulating pump. Sub-cooler reservoir The sub-cooler reservoir will be water filled at the fill commencement of compressor operation daily. At the commencement of the first compressor operation of the day, de-energize closed the water discharge stop valve and energize open the water feed stop valve. When the reservoir high level water sensor detects water, de-energize to close the water feed valve and energize the adiabatic cooler circulating pump in conjunction with the sub-cooler fan operation. When the reservoir mid-level water sensor detects an absence of water, energize open the water feed stop valve. De-energize the cooler pump when an absence of water is detected at the reservoir low level sensor. At the cessation operation of the supply air fan, the water discharge valve will be energized open and the feed water valve will be de-energized closed. Sub-cooler The sub-cooler temperature will be regulated by the high temperature control pressure liquid refrigerant temperature and ambient wet bulb temperature. If the differential temperature between the sub-cooled high pressure liquid refrigerant temperature and the entering outdoor air wet bulb temperature is greater than the control differential set point (and rising), increase the sub-cooler fan variable performance drive frequency. If the differential temperature between the sub-cooled high pressure liquid refrigerant temperature and the entering outdoor air evaporator wet bulb temperature is less than the control differential set point (and falling), decrease the sub-cooler fan variable performance drive frequency. Head pressure safety If the head pressure safety switch rises above the safe to operate set point, trip the manual reset switch to open the start circuit to the compressor contactor. The head pressure safety switch monitoring relay will change state from normally open to closed to identify a head pressure safety switch intervention. Head pressure safety If head pressure sensor rises above the head pressure incident warning set point, reduce the compressor variable frequency drive incrementally and increase condenser fan variable frequency drive incrementally to reduce the head pressure to a safe operating pressure differential below the head pressure warning set point and raise a head pressure safety incident notice. If head pressure sensor falls below the safe to operate pressure differential following a head pressure safety incident, resume compressor, liquid refrigerant and condenser head pressure control strategies and raise resumption of automatic head pressure control notice. Low pressure safety If suction pressure safety switch falls below a saturated suction temperature (SST) pressure of 0° C., trip the manual reset switch to open the start circuit to the compressor contactor. The low pressure safety switch monitoring relay will change state from normally open to closed to identify a low pressure safety switch intervention. Low pressure safety If suction pressure sensor falls below a SST pressure of incident 4° C., reduce the compressor variable frequency drive incrementally and raise a suction pressure safety incident notice. If the suction pressure sensor rises to above a SST pressure of 8° C. following a suction pressure safety incident, resume compressor control and raise resumption of automatic suction pressure control notice. Low suction If the suction temperature sensor falls below 6° C., reduce temperature safety compressor variable performance drive frequency incident incrementally and raise a suction temperature safety incident notice. If the suction temperature sensor rises to above 10° C., following a suction temperature safety incident resume compressor control and raise resumption of automatic suction temperature control notice. Super-cooler operation Operation of liquid refrigerant super-cool heat exchanger will be managed utilizing the condensate tray water level sensor, the condensate circulating pump, the condensate return automated shut off valve and the condensate transfer automated shut off valve. Operation of the valves and pump will be sequential as follows: When the mid level condensate tray water level sensor indicates the presence of condensate, the condensate return stop valve will be energized to open and the condensate circulating pump will be energized. When the high level condensate tray water level sensor indicates the presence of condensate, the condensate transfer stop valve will be de-energized to open and the condensate return stop valve will be de-energized closed. When the high level condensate level sensor indicates the absence of condensate, the condensate return stop valve will be energized to open and the condensate transfer stop valve will be de-energized to close. When the low level condensate level sensor indicates an absence of condensate, the condensate pump will be de- energized. At the cessation operation of the supply air fan, the condensate transfer valve will be de-energized open, the condensate return valve will be de-energized closed and the condensate pump be energized for a time period to evacuate condensate from the tray. Refrigeration sensors Compressor head pressure safety switch Compressor discharge temperature Compressor discharge pressure Condenser exiting liquid temperature Sub-cooler exiting liquid temperature Liquid receiver temperature Liquid receiver pressure Super-cooler exiting liquid temperature Compressor suction pressure safety switch Compressor suction temperature Compressor suction pressure Ambient air sensors Entering evaporator dry bulb Entering evaporator dew point Carbon dioxide Evaporator pressure differential Exiting evaporator dry bulb Exiting evaporator dew point Return air sensors Dry bulb remote space located Dew point remote space located Carbon dioxide remote space located Entering evaporator dry bulb Entering evaporator dew point Evaporator pressure differential Exiting evaporator dry bulb Exiting evaporator dew point Supply air sensors Dry bulb Dew point Water sensor Condensate tray high level Condensate tray mid level Condensate tray low level Sub-cooler reservoir high level Sub-cooler reservoir mid level Sub-cooler reservoir low level Comfort reporting The comfort of occupants will be evaluated consistent with ASHRAE PMV calculator.The conditioned space dry bulb, dew point and a quotient of the supply air quantity will be used as input. A trend of the notional PMV will be logged. Energy Efficiency Compressor COP calculating The compressor COP will be calculated at intervals utilizing the following input devices: Compressor kW - extract energy consumption from compressor variable frequency drive high level interface Ambient dry bulb Ambient dew point Leaving outdoor air evaporator dewpoint Leaving outdoor air evaporator dry bulb temperature Return air dry bulb Return air dew point Leaving return air evaporator dewpoint Leaving return air evaporator dry bulb temperature Outdoor airflow Return airflow The compressor refrigeration effect will be calculated. Compressor COP will calculated. The compressor COP will be trend logged. System COP The system COP will be calculated at intervals utilizing the following high level interfaces from the compressor refrigeration effect calculation and the following variable performance drive high level interfaces: Compressor Supply air fan Condenser air fans Sub cooler fans The system COP will be trend logged. Trend and event All input sensors and output events will be trend log logging capable. Alarm events All input sensors and variable frequency drives will be alarm event capable. Operator interface All controlled devices, input sensors, operator set points, trend logs and alarm identification and cancelling will be accessible to for via a tiered access review through a graphical operator interface. External interface The operator interface will be accessible through an internet web based external connection. Alarming events will generate internet email messaging to identified plant operators.

[0041] As will be appreciated from Table 1 and Table 2, the use of evaporators to separately manage space dry bulb and space humidity enhances occupant comfort, while the use of control algorithms to regulate liquid refrigerant to the evaporators in response to space load conditions enhances the utilization of evaporator heat transfer capabilities to extend the refrigeration effect of the circulating refrigerant mass flow. Also, the preferred use of sub-coolers and super-coolers in the heat of rejection condensing process extends the refrigeration effect of the circulating refrigerant mass flow and reduces compressor head pressure, and the preferred use of liquid refrigerant sensing decouples the volumetric dependence of compressor head pressure from compressor suction pressure.

[0042] In conjunction with the preferred use of liquid refrigerant metering valves and suction gas set point management, the adoption of these components configured in the manner described enables evaporators to operate at higher saturation temperature, thereby reducing compressor pressure lift requirements for generation of an enhanced refrigeration effect.

[0043] Additionally, the preferred use of volumetric calculation of outdoor air flow and supply airflow in response to feedback signals to provide ventilation amenity and space comfort, reduces both heat load and air circulation energy consumption.

[0044] A person skilled in the art will understand that there may be variations and modifications other than those specifically described. It is to be understood that the invention includes all such variations and modifications. The invention also includes all components, steps, functions and features referred to, or indicated in this specification, individually or collectively, and any and all combinations of any two or more of those components, steps, functions and features.