Patent Description:
Air-cooled (non-evaporative), ammonia refrigeration systems struggle to start during low-ambient conditions. As the compressor discharges superheated vapor into the condenser, the cold condenser coils immediately condense any vapor, preventing the discharge pressure to increase. Screw compressors require a minimum pressure delta across the housing to maintain proper oil flow to the compressor's components. The air-cooled condenser surface area is too large, due the very low ambient conditions (very high temperature differences) to allow the delta pressure to build at start-up. Chlorofluorocarbon refrigerant (CFC, HFC, HCFC) systems have utilized isolating valves on the outlet of condenser coils, which force liquid to back up in the condenser, reducing the surface area of the coil that is capable of condensing vapor. However, this requires significant charge that must be stored elsewhere in the system during normal operation. This is not acceptable to achieving low-charge and critically charged ammonia refrigeration systems. A known low charged packaged refrigeration system is disclosed in <CIT>. A method for start-up of a refrigeration system including a sequential start of several condenser coils is known from <CIT>.

The present invention overcomes the problems of the prior art by allowing the condenser coils to isolate individually during the startup period, allowing individual sequencing of the coils until the condenser is warm enough to maintain discharge and oil pressure. This invention also eliminates the need for a stand-alone oil pump to maintain oil pressure during start-up. A refrigeration system according to the invention is defined in independent claim <NUM>; a method for start-up of an air-cooled low charged packaged ammonia refrigeration system according to the invention is defined in independent claim <NUM>; and a method for modifying an air-cooled low charged packaged ammonia refrigeration system according to the invention is defined in independent claim <NUM>. Optional features of the invention are defined in the dependent claims.

Several components provide the control required to stably and reliably operate the system during start-up: Motorized valves can be installed on all or one of the condenser coil inlets, a main compressor discharge motorized valve is installed, a bypass pressure regulator valve in the main compressor piping is installed, check valves on the condenser outlets are installed and speed control of the condenser fans. The condenser inlet motorized valves provide precise control of gas feed or act as an on/off valve for the condensers allowing pressure to build without collapsing the oil pressure. The motorized valves provide precise control of the gas flow at a very low pressure drop or provide on/off control as needed. The air-cooled condensers may be any style: tube and fin or microchannel, etc. in horizontal or vertical tube arrangements. The condenser coil outlet contains vertically-oriented inline check valves to prevent liquid backflow when a coil is isolated. This allows each condenser coil to be isolated without trapping significant liquid refrigerant charge in a low-charge ammonia, refrigeration system. Trapping an appreciable amount of liquid in the condenser coils upsets startup of a packaged ammonia refrigeration system. The compressor discharge line contains a single motorized valve for regulating discharge pressure. The motorized valve is used for coarse gas control at start-up. The motorized valve in the compressor discharge piping also includes a bypass with a mechanical pressure regulator to allow precise regulation at the minimum discharge pressure. Once discharge pressure rises above the minimum setpoint, the condenser inlet solenoid coils will open one at a time. The discharge pressure regulating motorized valve will simultaneously regulate the discharge pressure until the condenser coil has warmed up enough to maintain discharge pressure. Fan speed control is also utilized to maintain stable operation at start-up.

<FIG> is a process and instrumentation diagram for a single compressor, air-cooled (non-evaporative) condenser, low charge packaged penthouse refrigeration system according to an embodiment of the invention. A blow-up of the upper right quadrant of <FIG> is presented in <FIG>. <FIG> is a process and instrumentation diagram for a dual compressor, air-cooled condenser, low charge packaged penthouse refrigeration system according to an embodiment of the invention. A blow-up of the upper right quadrant of <FIG> is presented in <FIG>.

The system includes evaporators 2a and 2b, including evaporator coils 4a and 4b, respectively, condenser <NUM>, compressor(s) <NUM>, expansion devices 11a and 11b (which may be provided in the form of valves, metering orifices or other expansion devices), pump <NUM>, liquid-vapor separation device <NUM>, and economizer <NUM>. According to one embodiment, liquid-vapor separation device <NUM> may be a recirculator vessel. According to other embodiments, liquid-vapor separation device <NUM> and economizer <NUM> may one or both provided in the form of single or dual phase cyclonic separators. The foregoing elements may be connected using standard refrigerant tubing in the manner shown in <FIG>. As used herein, the term "connected to" or "connected via" means connected directly or indirectly, unless otherwise stated.

According to the embodiment shown in <FIG>, low pressure liquid refrigerant ("LPL") is supplied to the evaporator by pump <NUM> via expansion devices <NUM>. The refrigerant accepts heat from the refrigerated space, leaves the evaporator as low pressure vapor ("LPV") and liquid and is delivered to the liquid-vapor separation device <NUM> (which may optionally be a cyclonic separator) which separates the liquid from the vapor. Liquid refrigerant ("LPL") is returned to the pump <NUM>, and the vapor ("LPV") is delivered to the compressor <NUM> which condenses the vapor and sends high pressure vapor ("HPV") to the condenser <NUM> which compresses it to high pressure liquid ("HPL"). The HPL is delivered to the economizer <NUM> which improves system efficiency by reducing the high pressure liquid ("HPL") to intermediate pressure liquid ("IPL") then delivers it to the liquid-vapor separation device <NUM>, which supplies the pump <NUM> with low pressure liquid refrigerant ("LPL"), completing the refrigerant cycle.

<FIG> also include numerous control, isolation, and safety valves, as well as temperature and pressure sensors (a. indicators or gages) for monitoring and control of the system.

Referring to the single compressor embodiment (<FIG> and <FIG>, and particularly <FIG>), motorized condenser inlet <NUM>, <NUM> and <NUM> valves are installed on the inlet of the condenser coil bundles. The motorized valves can function as variable control valves or on/off valves.

A single condenser bundle is open to ensure proper surface is available during start-up. As the system begins increasing load, valves <NUM>, <NUM> and <NUM> will begin to open. Once all valves are open, variable fan control takes over pressure control. The sequencing of the use of valves and fan operation can vary, based on system operation and design.

Motorized valve <NUM> and ammonia pressure regulator valve <NUM> provide precise ammonia gas control during start-up of the system in low ambient conditions. During start-up, all motorized valves are closed and the pressure regulator provides compressor differential pressure control to ensure proper oil flow. The ammonia pressure regulator <NUM> provides low volume flow control. As the compressor begins to load, more ammonia gas flow is generated. Motorized valve <NUM> begins to open and control the discharge pressure, compressor differential pressure and oil flow.

The next step during system start-up is to begin opening the condenser motorized valves <NUM>, <NUM> and <NUM> and concomitant staging the startup of the condenser fans.

Check valves <NUM>, <NUM>, <NUM> and <NUM> installed at the outlet to the condenser bundles are utilized to ensure liquid ammonia does not backflow into the condenser or other coil bundles during periods of downtime or normal operating periods.

Each of valves <NUM>, <NUM>, <NUM> and <NUM> are activated by attached microcontrollers or PLC (programmable logic control). A central microcontroller or PLC monitors the status of each valve, as well as discharge pressure, and directs the action of the valves accordingly for sequential startup of the condenser coils while maintaining gas and oil pressure.

Not all valves are required for a every ambient condition. In fact, above a certain ambient temperature, low ambient control may not be required. Therefore, valves can be installed and arranged to optimize operation at startup based on the ambient temperature.

<FIG> and <FIG> show a process and instrumentation diagram for a dual compressor, air-cooled condenser, low charge packaged penthouse refrigeration system. The dual compressor design utilizes and isolated compressor concept. The compressors use different oil separators, oil coolers, and condenser bundles.

Motorized valves <NUM>, <NUM>, <NUM> and <NUM> are installed on the inlet of the condenser coil bundles. The motorized valves can function as variable control valves or on/off valves.

During startup, motorized valves 111and <NUM> will be opened to a minimum position to allow ammonia gas flow to the condenser coil. As the system begins increasing load, valves <NUM> and <NUM> will open to <NUM>% and valves <NUM> and <NUM> will begin opening. Once all valves are open, variable fan control takes over pressure control. The sequencing of the use of valves and fan operation can vary, based on system operation and design.

Fine ammonia gas control during start-up of the system is provided by:.

The next stage is to begin opening the condenser motorized valves (<NUM>, <NUM>, <NUM> and <NUM>) and staging the condenser fans accordingly.

Check valves (<NUM>, <NUM>, <NUM> and <NUM>) are utilized to ensure liquid ammonia does not backflow into the condenser or other coil bundles during periods of downtime or normal operating periods.

As with the single compressor embodiment, each of valves <NUM>-<NUM> is activated by attached microcontrollers or PLC. A central microcontroller or PLC monitors the status of each valve, as well as discharge pressure, and directs the action of the valves accordingly for sequential startup of the condenser coils while maintaining gas and oil pressure. Not all valves are required for every ambient condition. In fact, above a certain ambient temperature, low ambient control may not be required. Therefore, valves can be installed and arranged to optimize operation at startup based on the ambient temperature.

According to various embodiments, the evaporator is housed in the evaporator (penthouse) module, and the remaining components of the system shown in <FIG> (except for the condenser coils and fans and associated structures) are housed in an enclosure such as a machine room module. The condenser coils and fans may be mounted on top of the enclosure or machine room module for a complete self-contained rooftop system. The air-cooled condenser may optionally be fitted with an adiabatic air pre-cooling system. The entire system may be completely self-contained in two roof-top modules making it very easy for over-the-road transport to the install site, using e.g., flat bed permit load non-escort vehicles. The penthouse and machine room modules can be separated for shipping and/or for final placement, but according to most preferred embodiments, the penthouse and machine room modules are mounted adjacent to one-another to maximize the reduction in refrigerant charge. According to a most preferred embodiment, the penthouse module and the machine room module are integrated into a single module, although the evaporator space is separated and insulated from the machine room space to comply with industry codes. According to an alternative embodiment, the evaporator coil may be mounted in a refrigerated space adjacent to, below, or remote from, the machine room module.

Claim 1:
A refrigeration system comprising:
a refrigerant evaporator coil (4a; 4b),
vapor/liquid separation structure (<NUM>) connected to an outlet of said evaporator coil via a refrigerant line configured to separate low pressure refrigerant vapor from low pressure refrigerant liquid;
a refrigerant compressor (<NUM>) connected to an outlet of said liquid-vapor separation device via the refrigerant line and configured to compress refrigerant vapor from said vapor liquid separation structure;
a compressor discharge motorized valve (<NUM>) connected to an outlet of said refrigerant compressor via the refrigerant line and configured for coarse regulation of discharge pressure during system start-up;
a bypass pressure regulator valve (<NUM>) connected to an outlet of said refrigerant compressor via the refrigerant line and configured for precise regulation of discharge pressure during start-up; an air-cooled refrigerant condenser (<NUM>) comprising a plurality of condenser coils connected to said compressor discharge motorized valve and said bypass pressure regulator valve via the refrigerant line and configured to condense refrigerant vapor produced in said compressor to refrigerant liquid,
a motorized valve (<NUM>; <NUM>; <NUM>) connected to an inlet of at least one of said condenser coils configured to provide control of gas feed to the condenser coil to allow pressure to build without collapsing oil pressure;
a vertically oriented inline check valve (<NUM>; <NUM>; <NUM>; <NUM>) connected to an outlet of at least one of said condenser coils configured to prevent liquid backflow;
a collection vessel (<NUM>) connected to an outlet of said condenser via refrigerant line for receiving refrigerant liquid from said condenser;
the refrigerant line connecting an outlet of said collection vessel to an inlet of said vapor/liquid separation structure and configured to deliver refrigerant liquid to said separation structure;
said vapor/liquid separation structure having a liquid outlet that is connected via the refrigerant line to an inlet of said evaporator coil;
a pre-packaged modular enclosure;
and wherein said vapor/liquid separation structure, said compressor, and said collection vessel, are situated inside the pre-packaged modular enclosure.