DXInstallation

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DX Installation May 2011

Expansion Valves and Ammonia in Direct Expansion SystemsTerry Chapp, PE Danfoss Industrial Refrigeration

Background

In todays industrial refrigeration facility, there are several options available forsupplying cold liquid refrigerant to the evaporators. One of the oldest methods of liquid feed control is direct expansion (also known as dry expansion) using thetraditional thermostatic expansion valve.

The concept is fairly simple. Direct expansion expands high-pressure, high-temperature liquid to a much lower pressure and temperature, resulting in a two-phasemixture. This mixture is then distributed evenly to one or more coil circuits of anevaporator. The flow of the liquid is metered in such a way that the amount vaporizedmatches the heat load on the evaporator. Finally, after all of the refrigerant liquid hasevaporated, the gas is further heated to a level high enough to ensure that no liquidreturns to the compressor. The number of degrees above the evaporating (or

saturation) temperature of the liquid is referred to as superheat. There is very littlecooling effect resulting from this additional heat but the evaporator must make roomfor the extra vapor. It is desirable from a size and cost perspective to minimize theamount of superheat required. How low the superheat can go is highly dependentupon the type of valve and control system being used for this process.

The potential advantages of using direct expansion include the minimization of therefrigerant charge and liquid line sizes, as well as a reduction of equipment first costs.

There are limitations, however, to consider when designing a direct expansion system,including the selection of expansion valves and the use of ammonia as a heat transferfluid.

Equipment Requirements

While equipment requirements will vary depending on the level of sophistication of the design, the basic elements required to measure and control the liquid flow to theevaporator includes:

superheat monitoring and control devices refrigerant distributors flow metering/expansion valves.

There are a number of different means of expanding high-pressure liquid refrigerantto a lower pressure and lower temperature mixture. The following three expansionvalves are the most common types used in ammonia applications:

Thermostatic Expansion Valve (TXV)

With a successful history as one of the oldest means for the automatic expansioncontrol, the TXV consists of

An orifice across which the liquid is expanded and metered; An internal diaphragm that moves according to the balance of forces around it

and meters the flow of refrigerant through the orifice;


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A power head, which is used to sense the temperature leaving the evaporator.Through the rise and fall of temperature, the pressure of the gas in the bulbrises and falls, and ultimately creates one of the forces on the aforementioneddiaphragm.

The TXV, however, is a mechanical device which does not actually measuresuperheat but, rather, is controlled by an implied superheat. Its function relies on aninternal force balance which leads to some limitations.

For one, the pressure differential across the valve must be maintained at a fairly highlevel in order for this internal force balance to function properly. And, because thepower head senses only an implied superheat, its range of temperatures (and pressures)is limited to the conditions for which the power head was designed. This forces theuser to operate within a specific range of pressures which can, in many cases, forcethe end user to operate at higher discharge pressures than necessary.

Under widely varying loads, the valve might hunt for the proper position, which can

create a very unstable condition within the evaporator. Response speed can also belimited, as the valve operates only in present time. If conditions change faster than thevalves ability to respond, instability of operation can occur.

Cross-sectional View of a Thermostatic Expansion Valve and the Force Balance

Pressure from gascharge in power head

External EqualizationPressure

Spring ForceHighPressureLi uid


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Schematic Diagram of Equipment Requirements Using TXV Expansion Valve

Pulse Width Modulating Expansion Valve

An earlier form of expansion device that utilized electronic control, the pulse widthmodulating expansion valve is simply a special solenoid valve modified to meterliquid and handle very high cycles.

The actual operation is simple. The valve receives an ON signal from an electroniccontroller which tells it to open for a certain period of time within each cycle,(nominally 6 seconds in length); and typically remains open from one to six seconds.The brain is an electronic controller that receives temperature and pressure inputsfrom sensors on the evaporator suction line. Internal to the controller are refrigeranttables which determine the actualrather than impliedsuperheat. The controller

dynamically processes the past, present and future characteristics of the system usingan internal algorithm and then sends a signal to the valve.

Because of its pulse modulation, this valve is particularly well-suited for air coilapplications which operate frequently under partial load conditions. When thecontroller calls for liquid, the valve is completely open. At startup and when comingout of defrost, superheat is generally quite high. While pulsing, the valve is sendingthe full flow of liquid. Under part load conditions, this translates to high velocitieswhich can lead to annular flow and optimum heat transfer coefficients.

For the same reasons that the pulse width valve is well suited for part load conditionsin air coils, it can also have a detrimental effect on heat exchangers with low internalvolumes such as plate and frame heat exchangers. Because the valve opens and closescompletely during operation, the amount of refrigerant passing through the valve canbe quite high in relation to the internal volume of the heat exchanger, especially atstartup when the evaporators internal temperature is relatively high. This can lead toa flooded condition in the heat exchanger during this transition period resulting incarryover of liquid downstream of the heat exchanger.

Because this is a pulsing device, the valve is not well suited for 1:1 systems as thepulsing condition can create significant pressure fluctuations in the suction line and,

To Suction

Liquid From High Pressure LiquidSupply

Thermostatic Expansion Valve

Eva orator

Power Head


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ultimately, to the compressor. Unless certain precautions are taken, the pulsing actionof the valve can lead to premature wear in some types of solenoids and pressureregulators.The pulse valve also requires a minimum pressure differential across it in order tooperate properly. It is not always well suited for applications utilizing controlledpressure receivers (CPRs) because of their lower operating pressure.

Cross-sectional View of a Pulse Width Modulating Expansion Valve

Motorized Expansion Valve

This valve is also controlled electronically, but unlike the pulse width valve whichregulates flow through a pulsing action, the motorized valve regulates flow bychanging the flow area (and C v) through the valve.

A well-designed motorized expansion valve should have the following characteristics: A large turndown ratio that allows the valve to meter fluid through a wide

range of conditions; A fast-acting motor and valve which will respond as rapidly as the controller

requires; Cavitation-resistant materials that can withstand the harsh environment created

by flashing refrigerant through the valve port and orifice.

Motorized valves typically operate with no pressure differential requirements. This

allows the valve to function independently of discharge pressure and is well suited forthose applications where pressure differential is very low. Common applicationsinclude:

Air coils Chillers Liquid Injection Valves Liquid Make-up Valves Intercoolers Desuperheaters


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Cross-sectional view of two similar motorized expansion valves

Schematic Diagram of Equipment Requirements Using Electronic Control

Ammonia Use

As described in Table 1, as a heat transfer fluid, ammonia is one of the most effectiverefrigerants available in terms of heat capacity. A small amount of ammonia can go along way. This is beneficial when it comes to minimizing refrigerant charge vesselsize, and pipe size in the liquid state.

On the other hand, the high heat capacity of ammonia adds a unique challenge to theaccurate measure and control of superheat. If liquid refrigerant is detected in the outlet

PressureTransmitter

Superheat Controller

Liquid From High Pressure LiquidSupply

Power Wiring

Motorized or Pulse Width Expansion Valve

TemperatureSensor

To Suction

Eva orator


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piping of the evaporator, the expansion valve will close. In most superheat controlsystems, the valve closes upon detection of low superheat levels and will not re-openuntil any liquid is cleared from the sensor location, either through evaporation orsome other means of movement away from the sensor.

With ammonia, the amount of heat required to evaporate the same amount of liquidby mass can be seven or more times that of a halocarbon. To solve this problem, it isrecommended that the temperature sensing element be placed in the vertical outletpiping.

The excellent heat capacity of ammonia serves direct expansion applications well inminimizing superheat. However, the high specific volume of the ammonia vapor mayerase the savings and potentially increase the evaporator size in order to minimizepressure drop if superheat is not minimized. These limitations are important to factorinto the design of a direct expansion system.

Comparison of Refrigerant PropertiesTemperature F-20 0 20

NH3 583 568 553

R22 97 94 90Heat of Vaporization

(BTU/LBM)R507 80 76 72

NH3 14.719 9.14 5.92

R22 2.08 1.37 0.935Specific Volume(FT3/LBM)

R507 1.36 0.902 0.617

Table 1 Comparison of ammonia properties with typical halocarbons

Conclusion

The use of direct expansion in industrial refrigeration has been on the rise over thepast decade thanks to the availability of electronic control systems and valves whichrespond at the speeds necessary to mate with the control system. It is not the intent of this article to widely promote direct expansion but rather to give confidence to thosewho feel that it is an appropriate technology for certain situations.

It is important to understand the basic physics of direct expansion, the type of equipment that is appropriate, and the equipments limitations. It is especiallyimportant to understand the characteristics of ammonia as a heat transfer fluid.

This paper is not intended as a comprehensive document in the design of DX systemsbut rather is meant to give the designer and operator an overview of the benefits,issues, and technology associated with its use in todays facilities.

As with all design approaches, direct expansion has its place in the industry but itmust also be remembered that there are limitations. When applied appropriately andcorrectly, direct expansion systems can offer cost effective approaches to many designchallenges in industrial refrigeration.

Terry Chapp is a Key Account Manager in the Industrial Refrigeration group and can be reached at [email protected]

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