Refrigeration – Automatic control – Of external fluid or means
Reexamination Certificate
2000-01-21
2001-06-12
Buiz, Michael (Department: 3744)
Refrigeration
Automatic control
Of external fluid or means
C062S435000
Reexamination Certificate
active
06244058
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The subject invention generally pertains to tube and shell heat exchangers and more specifically to an evaporator that provides a chiller water temperature marginally above freezing.
2. Description of Related Art
Many chiller systems include a closed loop refrigerant circuit comprising a compressor, a condenser, a flow restriction, and an evaporator. Expanded, cold refrigerant in the evaporator cools a secondary closed loop chilled fluid circuit. The chilled fluid, such as water or a water-based solution, is distributed to and circulated through various smaller heat exchangers. The smaller heat exchangers cool various comfort zones, such as rooms or other areas within a building.
In many cases, one or more chillers are dedicated to a single building. However, in some cases one large central cooling system, comprising one or more chillers, serve several distinct buildings. The chilled water is typically piped a great distance to reach the various buildings. Such a chiller system is often referred to as a “district cooling system.”
As chilled water is conveyed through a relatively long network of pipes, the water takes on heat before reaching its various designated heat exchangers. To ensure that the chilled water is sufficiently cold upon reaching the heat exchangers, it is usually desirable to have the evaporator reduce the temperature of the water as much as possible. However, if the water gets too cold, it may freeze inside the evaporator. Freezing, of course, can destroy the evaporator and/or its associated piping.
To avoid freeze up, the chilled water solution may be a glycol and water solution or some other solution having a lower freezing point than pure water. However, with district cooling systems, an appreciable amount of glycol or other solution that may lower the freezing point can be rather costly due to the large volume contained within the chilled water piping that interconnects the evaporator and the remote heat exchangers. Consequently, current district cooling systems use water solutions that consist of primarily water with perhaps small amounts of water treatment chemicals. Since such solutions have a freezing point near 32 degrees Fahrenheit, evaporators are typically operated at a temperature safely above that.
To this end, many chillers control the leaving chiller water temperature (LCWT) in response to a temperature sensor installed immediately downstream of the evaporator or situated within an outlet water box of the evaporator (see U.S. Pat. Nos. 5,083,438 and 5,355,691). The outlet water box serves as somewhat of a manifold or collection point into which the numerous heat exchange tubes within the evaporator shell discharge. The temperature sensor, whether in the water box or immediately downstream of the evaporator, usually provides a generally good indication of the LCWT.
However, the sensed temperature is only an average of the actual water temperature discharging from each individual tube of the evaporator. In a tube and shell heat exchanger the discharge temperature at each tube often varies from one tube to the next, depending on its location within the shell and the conditions under which the system is operating. Thus, to avoid freeze up at any individual tube, chillers are usually controlled to provide an average LCWT that is well above freezing, typically 37 degrees Fahrenheit or higher.
Unfortunately, when leaving the evaporator at 37 degrees, the chiller water temperature may rise to an unacceptable high temperature by the time it reaches the remote heat exchangers of a district cooling system.
In some chiller systems, such as the one disclosed in U.S. Pat. No. 5,782,131, a temperature sensor senses the temperature of the refrigerant inside an evaporator, as opposed to directly sensing the temperature of the chilled water. However, with such a system it may be difficult to determine what minimum allowable refrigerant temperature still avoids freezing the water. For example, in some cases, a refrigerant temperature of 30 degrees might only be able to chill the water to 38 degrees Fahrenheit.
SUMMARY OF THE INVENTION
To minimize the LCWT of a tube and shell evaporator, it is an object of the invention to monitor the temperature of the chiller water discharging from generally one or just a few of the very coldest tubes, as opposed to just sensing the average LCWT.
Another object is to control the operation of a chiller system in response to feedback from a temperature sensor that senses the temperature of the chiller water discharging from one or just a few of the very coldest tubes, as opposed to just sensing the average LCWT.
Another object is to maintain the temperature of the chiller water discharging from one or just a few of the very coldest tubes to a temperature of no more than 36 degrees Fahrenheit.
For chiller systems operating from part load to full load, another object is monitor the chiller water temperature at a location between the coldest tube at part load and the coldest tube at full load.
For chiller systems subject to refrigerant loss, another object is to monitor the chiller water temperature near the coldest tube during a normal operating condition as well as during a condition of low refrigerant charge.
In some embodiments, another object of the invention is to monitor the chiller water temperature at an elevation within the upper third of the tube bundle, where the refrigerant tends to boil most dramatically.
In some embodiments, further object of the invention is to monitor the chiller water temperature just below the top row of tubes to avoid sensing at an elevation where the refrigerant is in a primarily gaseous state.
In some embodiments, a still further object is to monitor the chiller water temperature at about the third row of tubes from the top where the refrigerant is a mixture of both liquid and gaseous refrigerant.
Another object is to monitor both the average LCWT and the temperature of the chiller water discharging from one or just a few of the very coldest tubes, whereby the average LCWT provides an indicator of the chiller system's overall operating performance, while the monitoring the coldest water temperature provides feedback that helps in optimizing that performance.
Another object is to monitor the refrigerant temperature within the evaporator in addition to monitoring the temperature of the chiller water discharging from one or just a few of the very coldest tubes, whereby the refrigerant temperature can be lowered well below 32 degrees Fahrenheit without significant risk of freezing.
These and other objects of the invention are provided by a tube and shell evaporator that includes a temperature sensor that senses the temperature of chiller water discharging from one or just a few of the very coldest tubes, whereby the sensed temperature is less than the average leaving chiller water temperature.
The present invention provides an evaporator that uses a refrigerant to chill a water solution. The evaporator comprises a housing defining an inlet water chamber, an outlet water chamber, and a refrigerant chamber; and a plurality of tubes each of which have an exterior surface exposed to the refrigerant chamber and an interior surface adapted to convey said water solution from the inlet water chamber to the outlet water chamber. The plurality of tubes are adapted to transfer heat from the water solution to the refrigerant to provide an average leaving chiller water temperature within said outlet water chamber. A first tube of the plurality of tubes is disposed at a higher elevation than a second tube of the plurality of tubes. The first tube and the second tube are adapted to convey the water solution at a first temperature and a second temperature respectively. The first temperature is less than the second temperature and is less than the average leaving chiller water temperature. A temperature sensor is situated closer to the first tube than the second tube and is adapted to sense a water solution temperature that is less than the second tempera
Duga Joel S.
Pitts Steven J.
Roberts John H.
Ali Mohammad M
American Standard International Inc.
Beres William J.
Buiz Michael
Ferguson Peter D.
LandOfFree
Tube and shell evaporator operable at near freezing does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Tube and shell evaporator operable at near freezing, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Tube and shell evaporator operable at near freezing will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-2470630