Minimizing Ammonia Charge in Industrial Refrigeration Systems, Part 2

Minimizing Ammonia Charge in Industrial Refrigeration Systems, Part 2

Ammonia has been and continues to be the predominant refrigerant used in the cold storage and food processing industry. However, as a result of industrial accidents, the potential as a terrorist target, drug trafficking, and growing local concerns, its use has come under increased scrutiny over the past decade.

The first part of this series examined how an advanced direct expansion system and a CO2/NH3 hybrid system would operate with reduced ammonia charges.

This article examines packaged systems and CO2/NH3 with pumped volatile brine.

Azanechiller
Azanefreezer

CO2/NH3 with Pumped Volatile Brine (PVB)
The CO2/NH3 system with pumped volatile brine has many similarities to the CO2/NH3 Cascade System. In this system, however, there is no primary CO2 compressor. It is possible that there may be a small CO2 compressor for the hot gas defrost system but there are also other options which allow for defrost without adding a compressor for that purpose. To many, this system would be described as a chiller system; however, what distinguishes this system most from what is typically thought of in terms of a chiller is the word “volatile”. Most chiller systems use water or some formulation of glycol/water and the cooling coil is no longer an evaporator but rather just a sensible heat exchanger. This has an enormous impact, not only, on the amount of heat transfer fluid that is required to meet the cooling load but also on pump size, pumping power, piping size, and the heat exchange size. Compared to a traditional chiller system, the CO2/NH3 pumped volatile brine system has higher energy efficiency and, usually a lower installed cost.

Packaged Systems
The Azanechiller and Azanefreezer

The Azane product line is produced by a subsidiary of Star Refrigeration out of the UK. As a packaged system, there are a number of basic advantages when it comes to reducing ammonia refrigerant charge including:

• Elimination of the machine room
• Elimination of the large scale vessels required for holding ammonia charge
• Minimization of distribution piping throughout the facility.
• This product also introduces the concept of critically charged systems. A critically charged system is one in which the refrigerant is always undergoing change: expansion, compression, heat transfer. That is, there is no excess refrigerant residing in the system waiting for the moment when more refrigerant is needed in some part of the process. This is not a new concept for smaller residential and commercial air conditioning and refrigeration systems; however, it represents a significant deviation from the typical pumped recirculated liquid industrial refrigeration system.

• The Azanechiller uses ammonia as the primary refrigerant in “chilling” a secondary heat transfer fluid for the medium temperature rooms of a cold storage facility. This secondary heat transfer fluid is usually glycol. The glycol is cooled by use of a plate exchanger located in the package and then distributed to the docks and coolers as required. The condensers may be air cooled or water cooled. Azane has developed seven different models of air cooled units capable of providing between 60 and 300 tons of refrigeration, depending on model. A number of advanced features incorporated into the design include variable speed fans for the air cooled condensers, high effectiveness condenser coils, fully automatic oil return, and a fully integrated PLC control system.

• The Azanefreezer is an industrial condensing unit which utilizes ammonia as the only refrigerant for the entire system. As with the Azanechiller, refrigerant charge is minimized as a result of the packaged approach along with a number of advanced design features including high effectiveness evaporator and condenser coils, an ultra- low charge low pressure receiver, and highly efficient defrost methodology. The defrost methodology for the Azanefreezer is worth noting in that the system really acts like a heat pump. During the defrost part of the cycle, the evaporators act as condensers in giving up heat to melt the ice on the evaporators and the condenser acts as an evaporator in re-vaporizing the defrost condensate. Star believes that the net result is a defrost cycle which is faster and more energy efficient than a traditional hot gas defrost cycle. The key component in the reverse cycle defrost is a four-port valve located within the condensing unit. This valve was developed in the 1980’s specifically for this application and has proved to be very reliable in operation. The 4 port valve is the only valve required for the reverse cycle system which results in a greatly simplified system, with far fewer valves than a traditional hot gas defrost system.

• The Azanefreezer uses a unique low pressure receiver (LPR) system design, which allows all of the control valves to be located within the condensing unit. This eliminates the need for valve stations at the evaporators, greatly reducing the number of potential leak paths in the system and has the added benefits of greater efficiency and simplified maintenance. Star’s low pressure receivers have been installed in numerous European facilities for over 20 years. One additional feature of the system which helps to greatly minimize charge is the low overfeed rate of the evaporators. The combination of the low pressure receiver (LPR) and advanced aluminum evaporators has led to a highly efficient system with minimum refrigerant charge, no pump, and no valves or mechanical joints in the refrigerated room.

Ammonia Refrigerant Charge
There are a number of factors which will enter into actual refrigerant charge of these units: type of condenser, location of packaged system in relation to the evaporator location and, of course, the size of the package in relation to the actual load requirements. The total number of units required for the target facility is:

• Blast Freezers – Qty. 9 GP5
• Freezers – Qty. 4 AF45
• Docks – Qty. 1 AA120

The total ammonia refrigerant charge equals 3,245 pounds or approximately 4.3 lbs/TR.

The NXTCOLD™ System
The NXTCOLD system represents a dramatic advance in the use of refrigerant by utilizing a number of proprietary technologies in both liquid feed to the evaporator and a unique liquid feed control algorithm. This system represents the lowest ammonia charge of all systems under review in this document. The NXTCOLD system was first conceived six years ago. The first test cell went into operation in 2012 and since then a number of beta sites have been installed. In 2013, the first NXTCOLD complete refrigeration system penthouse unit went into operation in place of a flooded ammonia system which held approximately 500 pounds of ammonia charge. The NXTCOLD penthouse unit runs with approximately five pounds of ammonia charge. That charge can be increased by 40% as a safety factor if desired which brings the total charge up to seven pounds (approximately .5 pounds per ton).

NXTCOLD Ammonia Refrigerant Charge
As noted earlier, the ammonia charge for the NXTCOLD system is the lowest per ton of refrigeration of all systems under study in this report. The total charge for a facility with the requirements of the baseline system would be 385 pounds or 0.51 lbs/TR. 

Terry L. Chapp, PE, is national business-development manager for Danfoss Industrial Refrigeration. He has been involved in all aspects of HVACR, with particular emphasis on heat exchangers, valves, and controls, over the last 35 years.

Note: The Azanefreezer™ and Azanechiller™  are trademarked systems by Star Refrigeration Group. The NXTCOLD™ system is trademarked by Distribution Property Solutions, Inc.

References
    Welch, J. (2013). DX evaporator installation – Final project report. Retrieved from http://bit.ly/Welch_DX
    Nelson, B.I. (2013). DX ammonia piping handbook. Colmac Coil Manufacturing. Retrieved from http://bit.ly/Nelson_DX
    PG&E. (2009). Cascade C02/NH3 refrigeration system efficiency study. PGE 0707. Emerging Technologies Coordinating Council.

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