Multizone Air-conditioning System With Reheat

Abstract

An improved air-conditioning reheat system having a control valve responsive to the airstream temperature leaving the reheat coil for restricting refrigerant flow through one of two parallel refrigerant condensers upstream of the reheat coil when increased reheat is called for thereby flooding one condenser with liquid refrigerant and reducing the total condenser capacity to allow a combination of saturated liquid and vapor refrigerant from the other condenser to be delivered to the reheat coil serially connected between the condensers and the evaporator. The reheat coil, which functions as a liquid-to-air heat exchanger during normal operation with low reheat, then becomes a condenser to provide additional transfer capability. A bypass capillary allows a trickle of liquid refrigerant from the flooded condenser into the refrigerant flow path to prevent the flooded condenser from becoming logged with oil and to control the amount of uncondensed vapor refrigerant being fed to the reheat coil.

Description

BACKGROUND OF THE INVENTION

This invention relates to air-conditioning systems and particularly to multizone systems having a reheat coil in the air flow path downstream of the evaporator to reheat air which has been cooled and dehumidified in the evaporator.

Copending application, Ser. No. 749,775 filed Aug. 2, 1968 by Hoaglund et al. now U.S. Pat. No. 3,540,526 discloses a rooftop multizone air-conditioning system with reheat. In this system a housing unit has an air intake at one end and hot and cold decks at the opposite output end. To cool and dehumidify all incoming air, an evaporator coil is placed in the housing assembly near its inlet end. A reheat coil is located downstream in the hot deck section of the housing and arranged to be in series with and normally act as a part of a condenser coil. The reheat coil is thus supplied with a flow of refrigerant emanating from the condenser coil. The exchange of heat, in the reheat coil, heats the air passing through the hot deck and subcools the refrigerant before it enters the evaporator coil. When higher levels of heating are required during periods when the furnace operation is not desired, a bypass valve places the condenser and reheat coils at least partially in parallel, and the compressor supplies some superheated refrigerant gas which is injected directly into the liquid refrigerant flowing into the reheat coil. A thermostat within the hot deck modifies zone thermostat command signals as a function of prevailing temperatures within the hot deck.

SUMMARY OF THE INVENTION

The present invention is an improvement to the reheat system of the prior application referred to hereinabove, the improvement being designed to increase the reheat capacity of the system.

This increase in reheat capability is achieved by blocking the refrigerant flow through one of the parallelly connected condensers when increased heat is called for, thereby flooding the condenser with liquid refrigerant and reducing the condenser capability to allow a combination of saturated liquid and vapor refrigerant from the condenser to be delivered to the reheat coil serially connected between the condensers and the evaporator. The reheat coil, which functions as a liquid-to-air heat exchanger during normal operation with low reheat, then becomes a condenser to provide additional transfer capability.

A bypass capillary allows a trickle of liquid refrigerant from the flooded condenser into the refrigerant flow path to prevent the flooded condenser from becoming logged with oil and to control the amount of uncondensed vapor refrigerant being fed to the reheat coil.

A BRIEF DESCRIPTION OF THE DRAWING

The drawing is a schematic view of an air-conditioning system embodying the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT

Except as described below in connection with the reheat arrangement and its mode of operation, the system described is the same as that disclosed in the prior application discussed above and therefore this prior application is incorporated herein by reference.

Turning now to the drawing, the refrigeration system generally indicated by numeral 2 comprises a compressor 4, parallelly connected fin and tube air cooled refrigerant condensers 6 and 8, a reheat coil 10, and an evaporator 12.

Three direct driven fans 44, only one of which is shown in the drawing, are provided for drawing cooling air over the condensers 6 and 8.

Conduit 14 and parallel conduits 15 and 17 connect the compressor discharge to the inlets of the condensers while parallel conduits 16 and 18 connect the outlets of the condensers to the conduit 20 leading to the reheat coil 10. Manually operated shutoff valves 22 and 24 in the conduits 16 and 18, respectively, are closed only during servicing and are opened during operation of the system. A solenoid operated liquid flood back valve 26 is located in one of the lines. When closed the valve 26 blocks the flow from the condenser 6 via conduit 16. A bypass capillary 28 having one end connected to the conduit 16 between the outlet of the condenser 6 and the valve 26 and the other end connected to the conduit 18 provides a controlled leakage around the valve 26 when the valve is closed.

Conduit 30 connects the outlet of the reheat coil to a refrigerant distributor 37 at the inlet of the evaporator 12. This conduit has a solenoid operated valve 32, referred to as the liquid line valve for identification, a filter dryer 34 and a thermostatic expansion valve 36 therein. Conduit 38 connects the evaporator outlet to the inlet of the compressor.

The solenoid valve 26 is connected to a thermostatic sensor 40 in the airstream leaving the reheat coil via a control 42, the thermostatic sensor causing valve 26, which is normally held open, to close when the temperature of the airstream leaving the reheat coil drops below a predetermined value.

As described in the copending application referred to above, the evaporator 12 is arranged near the inlet of the air-conditioning unit to cool and dehumidify all incoming air while the reheat coil is located downstream in the warm air duct or hot deck which receives a portion of the cooled and dehumidified air from the evaporator for heating before supplying those zones of the building requiring heating.

OPERATION

The system has two basic modes of reheat operation. In the first mode there is a relatively high cooling demand and low demand for reheat. Under these conditions high pressure, high temperature refrigerant vapor from the compressor 4 is supplied to the condensers 6 and 8 via conduits 14, 15 and 17. Here the refrigerant is condensed to a slightly subcooled liquid which flows to the reheat coil via parallel conduits 16 and 18 which lead to the conduit 20. In the reheat coil 10 the refrigerant is further subcooled in heat exchange with the airflow passing through the reheat coil in the hot deck. From the reheat coil the refrigerant flows through conduit 30, open solenoid valve 32, filter-dryer 34 to the thermostatic expansion valve 36 where the refrigerant is converted to a low pressure low temperature liquid and vapor phase which is fed to the evaporator pg,7 coil 12 via the distributor 37. In the evaporator coil the refrigerant cools the main airstream passing through the air conditioning unit, the refrigerant being heated to a low pressure superheated vapor which then is drawn into the compressor inlet via conduit 38.

The second mode of operation occurs in the demand for heating which causes more air to flow through the reheat coil. As the flow increases to the point where there is insufficient heat being supplied by subcooling of the liquid refrigerant to meet the demand the temperature of the air leaving the reheat coil begins to drop. At a predetermined temperature setting the thermostatic sensor 40 actuates the control system 42 to close the solenoid operated valve 26 at the line 16. This blocks the main flow of liquid refrigerant from the condenser 6. This condenser begins to fill up with liquid refrigerant while the other condenser 8 is subject to an increasing flow of refrigerant. Due to the reduction in the effective condenser surface area the second condenser 8 is not capable of extracting all of the latent heat of vaporization of the refrigerant flowing through it and therefore a combination of saturated liquid and vapor refrigerant flows to the reheat coil 10 via conduits 18 and 20. The reheat coil 10 which in the first mode of operation had been operating as a liquid-to-air heat exchanger now in effect becomes a condenser since it receives a mixture of vapor and liquid. The condensation of the vapor in the reheat coil increases the heat transfer capability of the coil in this mode of operation. The liquid refrigerant which is displaced from the reheat coil by the vapor collects in the condenser 6. This flooding of the condenser 6 with liquid refrigerant reduces to a minimum the heat transfer between the flooded condenser and the condenser airstream.

In this mode of operation the bypass capillary 28 provides a carefully controlled leak which bypasses the solenoid valve 26 to prevent a complete blocking of the flow through the condenser. This limits the amount of condenser vapor refrigerant being fed to the reheat coil due to the addition of liquid refrigerant to the stream and also prevents the condenser 6 from becoming logged with oil during prolonged periods of operation in this mode.

The three condenser fans 44 are used to maintain a relatively constant compressor discharge pressure. The first fan is on whenever the refrigeration system is in operation with no additional call for reheat. If reheat is called for this fan is shutoff. The second fan is in operation whenever the refrigeration system is operating in the second mode in which the condenser 6 is flooded. The third fan is on whenever the compressor discharge pressure exceeds some predetermined value and is shut off again when the compressor discharge pressure drops below a second value somewhat lower than the first. In this way two of the three condenser fans are cycled to maintain maximum condensing pressure which also maintains a relatively constant pressure across the expansion valve to allow the expansion valve to modulate the flow of the refrigerant to the evaporator coil more evenly under part load condenser.

Although not described herein the control 42 includes thermostatic temperature control means for regulating the refrigeration system as is well known in the art. For a complete description of the remainder of the system and its control reference should be made to the above mentioned prior application.

Although the system shown uses two condensers, one of which is blocked during period of high reheat demand, it will be apparent to those skilled in the art that a single condenser with multiple sections or more than two separate condensers can be connected in parallel during normal operation with any number of the sections or condensers being flooded during increased reheat demand to provide the mode of operation disclosed herein. In this case bypass capillaries would be provided from each of the flooded sections to the main refrigerant flow stream.

It will also be apparent that a modulating valve can be used to restrict the flow from one of the condensers rather than a two way valve. In this way abrupt cycling of the system between the different reheat modes can be avoided.

While I have described above the principles of my invention in connection with specific apparatus it is to be clearly understood that this description is made only by way of example and not as a limitation to the scope of my invention as set forth in the accompanying claims.

Claims

I claim:

1. An air-conditioning system of the reheat type having a compressor, a condenser comprising plurality of condenser sections parallelly connected, a reheat coil, an expansion device and an evaporator coil serially connected to form a closed refrigerant loop, and means for bringing an airstream to be conditioned in heat exchange relation with the evaporator coil and the reheat coil, the improvement being control means responsive to the airstream temperature leaving the reheat coil to flood at least one of the condenser sections with liquid refrigerant when the airstream temperature goes below a predetermined value.

2. The air-conditioning system of claim 1 including restricted passage means between the outlet of the flooded sections of the condenser and the reheat coil.

3. An air-conditioning system comprising a compressor, a condenser, a reheat coil, an expansion device and an evaporator serially connected to form a closed refrigeration loop, the condenser having separate parallel first and second condenser sections connected by parallel inlet and outlet conduits, means for bringing an airstream to be conditioned in heat exchange relation with the evaporator and the reheat coil, valve means in the first condenser section outlet conduit responsive to the airstream temperature leaving the reheat coil for restricting said outlet conduit when the airstream temperature drops below a predetermined value, and a capillary tube connected between the first condenser section outlet conduit upstream of said valve means and the second condenser section outlet conduit.

4. The air-conditioning system of claim 3 wherein the valve means comprises a solenoid operated two-way valve responsive to a thermostatic control which senses the airstream temperature leaving the reheat coil.