Exploring Two-Stage Direct-Fired Absorption Chiller-Heaters: Efficient and Eco-Friendly Heating and Cooling

Two-Stage Direct-Fired Absorption Chiller-Heaters

Discover the advanced technology behind two-stage direct-fired absorption chiller-heaters, which offer an environmentally friendly and cost-effective alternative to traditional electric energy systems. By using natural gas instead of electricity, these chillers not only provide efficient cooling but also generate hot water, often eliminating the need for a separate boiler. Learn how the chilling cycle operates with water as the refrigerant and lithium bromide as the absorbent, facilitated by a nearly complete vacuum within hermetic vessels. This blog details the six-step chilling process, from solution pumps and heat exchangers to the absorber, explaining how the system efficiently maintains low temperatures. Additionally, explore the heating cycles with and without a hot water heat exchanger, highlighting the dual functionality of these innovative systems.

 

 Direct Fired Absorption Chiller offers an environmentally friendly alternative to the skyrocketing costs of electric energy.

Since Direct-Fired chillers uses inexpensive natural gas rather than electricity, These Chiller also provides hot water and typical applications will not require a Boiler saving even more.

How It Works : Chilling Cycle

Efficient two-stage absorption refrigeration cycle uses water as the refrigerant and lithium bromide as the absorbent. It is the strong affinity these two substances have for each other that makes the cycle work. The entire process occurs in hermetic vessels in an almost complete vacuum.

 

The large diagram above indicates the complete chilling cycle. The six steps are detailed below, with corresponding numbers in the diagram to show where each step takes place. Two-stage absorption chilling cycle is continuous; however, for the sake of clarity and simplicity, it is divided into six steps

 

1. Solution Pump/Heat Exchangers

 A dilute solution of lithium bromide and water descends from the Absorber to the Solution Pump. This flow of dilute solution is split into two streams and pumped through heat exchangers to the First-Stage Generator and to the Second-Stage Generator

 Exclusive two-way split of solution flow virtually eliminates the possibility of crystallization (solidification) by allowing the unit to operate at much lower solution concentration and temperatures than series flow systems

 Note: There are two heat exchangers, but only one is shown for illustrative purposes

 

2. First-Stage Generator

 A heat source heats dilute lithium bromide coming from the Solution Pump/Heat Exchangers. This produces hot refrigerant vapor which is sent to the Second-Stage Generator, leaving a concentrated solution that is returned to the Heat Exchangers

 

3. Second-Stage Generator

The energy source for the production of refrigerant vapor in the Second-Stage Generator is the hot refrigerant vapor produced by the First-Stage Generator.

This is the heart of remarkably efficient two-stage absorption effect. The refrigerant vapor produced in the First-Stage Generator is increased by 40%~at no additional expense of fuel. The result is much higher efficiency than in conventional systems.

This additional refrigerant vapor is produced when dilute Solution from the Heat Exchanger is heated by refrigerant vapor from the First-Stage Generator. The additional concentrated solution that results is returned to the Heat Exchanger. The refrigerant vapor from the First-Stage Generator condenses into liquid giving up its heat, and continues to the Condenser.

 

4. Condenser

Refrigerant from two sources~(1) liquid resulting from the condensing of vapor produced in the First-Stage Generator and (2) vapor produced by the Second-Stage Generator enters the Condenser. As the liquid refrigerant enters the low pressure of the condenser it flashes to vapor The two sources of refrigerant vapor combine and condense to liquid as they are cooled by the condenser water. The liquid then flows down to the Evaporator.

 

5, Evaporator

 Refrigerant liquid from the Condenser passes through a metering valve and flows down to the Refrigerant Pump, where it is pumped up to the top of the Evaporator. Here the liquid is sprayed out as a fine mist over the Evaporator tubes. Due to the extreme vacuum (6mm Hg) in the Evaporator, some of the refrigerant liquid vaporizes, creating the refrigerant effect. (This vacuum is Created by hygroscopic action-the strong affinity lithium bromide has for water-in the Absorber directly below.)

The refrigerant effect cools the returning system chilled water in the Evaporator tubes. The refrigerant liquid/vapor picks up the heat of the returning chilled water, cooling it  from 54°F to 44°F. The chilled water is then supplied back to the system.

 

6. Absorber

As the refrigerant liquid/vapor descends to the Absorber from the Evaporator, a concentrated solution coming from the Heat Exchanger is sprayed out into the flow of descending refrigerant. The hygroscopic action between lithium bromide and water—and the related changes in concentration and temperature—result in the creation of an extreme vacuum in the Evaporator directly above. The dissolving of the lithium bromide in water gives off heat, which is removed by condenser water entering from the cooling tower at 85°F and leaving for the Condenser at 92°F. The resultant dilute lithium bromide solution collects in the bottom of the Absorber, where it flows down to the Solution Pump.

The chilling cycle is now completed and begins again at step 1.

 

 

 

Heating Cycle Without Hot Water

Heat Exchanger

1. First-Stage Generator

An energy source heats dilute lithium bromide solution in the First-Stage generator. This produces hot refrigerant vapor which travels through the open valve VD and also moves through the Second-Stage generator and Condenser to the evaporator

 

2. Evaporator

The hot refrigerant vapor travels through the evaporator where it gives up heat to the system hot water, and changes state from vapor to liquid as its heat is removed. The refrigerant liquid collects in the bottom of the evaporator

 

3. Refrigerant Pump

The refrigerant pump removes the refrigerant liquid from the evaporator and pumps it into the second stage generator where it mixes with intermediate lithium bromide solution to form dilute lithium bromide solution.

 

4. Heat Exchangers/Absorber

The dilute lithium bromide flows to the solution heat exchanger where it combines with the concentrated solution from the First Stage generator to form intermediate solution. The intermediate solution then flows through the absorber.

 

5. Solution Pump/Heat Exchangers

The solution pump moves the intermediate solution from the absorber through the solution heat exchangers and into the first and second stage generators and the process begins again.

Heating Cycle With Optional Hot Water Heat Exchanger

1. First-Stage Generator

Dilute lithium bromide solution is heated by energy source in the first stage generator. This drives off refrigerant vapor and leaves concentrated solution.

 

2. Hot water Heat exchanger

The hot refrigerant vapor from the first stage generator gives up its heat to the system hot water heat exchanger and condenses liquid as the heat is removed. The condensed refrigerant liquid returns to the first stage generator and cycle begins again.

 

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