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Types of NH3 Systems & How They Work

 

Controlled Pressure Receiver-Fed Systems

Over the coming months I hope to find time to post these system descriptions to my website.  In view of the many visitors my site sees and what they’re looking for, these system descriptions I trust will fill a need.  This paper is first in a coming series, this of a controlled pressure receiver-fed system (CPR).  It also happens to be of an actual system located here in the upper Midwest region of the U.S.  It is atypical and I believe one of the first (if not the first) CPR liquid overfeed systems built, dating back to the early 1960’s.  Today, CPR systems have become commonplace in the U.S.  These have numerous advantages but along with those advantages come the disadvantages.  Once properly set up (adjust liquid feed rates to evaporators, set pressure regulators), these give excellent service.  However, if not properly set up, a CPR-fed system can become a pain in the butt.  But, the same thing can also be said of a mechanically-pumped liquid overfeed system as well which many believe is the “Cadillac” of systems.  So, let’s delve into the figure which follows and I’ll walk you through this pup.

Figure 1 – Process Flow Diagram, CPR-Fed System

 

A CPR-Fed System

Ok – let’s start at the evaporative condenser – at its outlet – the high pressure liquid drain line, identified in Figure 1 as warm liquid (165 psig).  This pressure is equivalent to 90 ºF condensing[1] at 78 ºF ewbt.  The system shown is single stage compression.  Liquid flows by gravity from the condenser outlet into a small pilot receiver.  Liquid level in this pilot is maintained by a high-side float.  If the liquid level rises above the float, the downstream expansion valve modulates open.  If the liquid level falls below the float level, the expansion valve modulates towards the closed position.  Upon passing through the expansion valve, the exiting refrigerant is now under a pressure of 65 psig.  During this throttling process, a portion of the upstream high pressure liquid flashes to vapor.  It is this rate of change in refrigerant volume that provides the force required to move liquid through the liquid piping distribution system.  Considering both upstream and downstream pressures (180 psia and 80 psia), the mass ratio of refrigerant liquid given up to flash vapor during adiabatic irreversible throttle at these conditions may be found from:

                                                       (1)

where:

  • hx = enthalpy of liquid refrigerant at condensing conditions
  • hf = enthalpy of refrigerant liquid at downstream saturation conditions
  • hg = enthalpy of refrigerant vapor at downstream saturation conditions

Eq (1) solves to 10% flash at the conditions stated.  However, the volume of the refrigerant changes vastly more than a measly 10%.  From 180 psia to 80 psia, the volume increases by roughly 130 times.  Since this expansion process occurs inside of a system having a fixed volume, the resulting force generated from the expansion process becomes available to lift liquid uphill against gravity while overcoming friction losses in piping and valves.  Actually, the foregoing explanation becomes a really nasty calculus problem probably best described with a third-order differential equation, so we’ll let Eq (1) do for now.  Upon exiting the expansion valve, the liquid and vapor enter a controlled pressure receiver, CPR in Figure 1.  Upon entering, liquid separates from vapor by gravity.  Liquid then becomes available for circulation to the various evaporators located throughout the facility.

The flash vapor formed across the expansion valve exits the CPR through a back pressure regulator and then enters the piping conveying liquid and vapor returning from the evaporators, upstream of the accumulator.  This pressure regulator is the governor of vapor pressure within the CPR.  Cold liquid leaves the CPR at 65 psig (45 ºF at saturation but subcooled slightly to ~ 40 ºF – more on this liquid subcooling later) and journeys out to the various evaporators through a network of piping. 

 

CPR-Fed Evaporators

Whenever an evaporator manufacturer selects a particular evaporator having x many rows of tubing (assuming an air-cooling evaporator) with y many fins per inch, it is commonplace for that manufacturer to assume that when liquid arrives in their evaporator, that it will boil immediately – no having to wait around for it to warm up to its saturation (boiling) temperature.  This assumption marks a critical decision because it is the strong force in overall evaporator effectiveness – the operating h[2] for the evaporator.  If liquid enters an evaporator at a temperature below its boiling temperature, the ability of this liquid to take heat away from the surface of the tubing it is in contact with becomes seriously compromised. 

Just about every manufacturer has learned to ask the question “Is this evaporator going to be fed via CPR or from a mechanical drive pump?”  If fed from a CPR, we then must establish an “overfeed rate” – the ratio of mass liquid to mass vapor exiting the evaporator.  For a CPR-fed system, this ratio should be low, on the order of 1.5:1[3].  If fed via a mechanical drive pump, the overfeed rate is normally higher, ~3:1.

 

Back-Regulating an Evaporator

As I’ve mentioned in our classes at U/W, these evaporators should not be back regulated:

  • Top-fed, mechanically-pumped liquid overfeed evaporators.  Don’t do this at all – don’t even think about it!  However, if you get your fanny in a bind, there is a way out of the subsequent brining dilemma – download a copy of the Evaporator Brining paper from the downloads section of this website – this will give you a clue.
  • Bottom-fed mechanically-pumped liquid overfeed evaporators (up to 10 ºF entering liquid subcooling permissible in certain cases)

These evaporators can be back-regulated:

  • Direct expansion, top and bottom-feed, and
  • Gravity flooded (regulator in dry suction line from surge drum), and
  • CPR-fed, top or bottom feed

So – here’s one of the advantages of a CPR-fed system – the evaporators can be back regulated to higher temperatures without fear of encountering evaporator brining (within limitations).

 

The Refrigerant Path – Evaporators to Compressor

Cold liquid (65 psig, 40 ºF) passes through a solenoid valve and a hand expansion valve at each evaporator.  After passing through the hand valve, the pressure has fallen from 65 psig to 30 psig (17 ºF at saturation).  During this second throttle process, flash vapor again forms across the hand expansion valve which becomes mixed with the remaining cold liquid, now at 17 ºF.  This mixture is then distributed to each circuit for boiling inside the finned tubing.  The admission of some flash gas along with saturated liquid raises the entering circuit fluid velocity.  This results in a “scrubbing” action that increases overall effectiveness and facilitates heat transfer.

A thermostat cycles the liquid solenoid in response to space temperature; the fans run continuously (except during defrost – valves and piping not shown for simplicity).  Vapor plus a little liquid (very little by volume, <0.2%) exits each evaporator then combines in a common “wet” suction line (also known as a High Temperature Recirculated Suction HTRS here in the U.S.).  Upon entering the accumulator, liquid then separates from vapor by gravitational forces (versus the drag force from upward moving vapor).  Vapor exits the top of the accumulator and returns to the compressor(s) for processing.  Liquid falls by gravity out of the accumulator into a small vessel known as a “pumper drum” or “liquid transfer”.  As liquid enters this vessel, it displaces an equal volume of vapor back through a three-way valve (de-energized position, common port to low pressure port) and into the accumulator.  When liquid reaches the upper site glass, a float switch (not shown in Figure 1) repositions the ports within the three-way valve, closng the LP port and opening the HP port to common.  Hot gas, regulated to the setpoint of the outlet pressure regulator, flows into the top of the pumper drum.  This action instantly subcools all liquid contained within this vessel as the gas pressure in the vapor head space increases.  This pressure change also causes the upstream check valve to close while simultaneously opening the downstream check valve.  Now liquid begins to flow out of the pumper drum, through a liquid transfer line and into the CPR.  Whenever ammonia liquid becomes pressurized (by using high pressure vapor), its temperature changes but slowly.  This now colder 17 ºF liquid pressurized to 65 psig mixes with the saturated liquid inside the CPR which subcools the whole mass.  This subcooling becomes available to overcome liquid static lift + friction pressure losses within the liquid supply piping network without forming vapor bubbles.

From the accumulator, dry suction vapor at 30 psig flows back into the compressor where its pressure is raised to 165 psig, becoming very hot in the process (~245 ºF reciprocating compressors; 130 ºF liquid-injected screws; 185 ºF thermosiphon oil cooled screws).  This vapor then travels into the condenser for desuperheating and reliquification.

 

Alternate Control of CPR Vapor Head Space Pressure

In most CPR systems, it is common to see the hot gas piping shown in Figure 1 with a red dashed line along with its pressure regulator.  The purpose of this line and regulator is to maintain some preset minimum pressure within the CPR.  But one will notice that in essence, we now have dual regulators installed in separate lines controlling the same receiver pressure.  Can these “fight” each other?  If not properly set up, you bet your bippy!  In each and every case, this extra line and regulator has proven unnecessary, provided the system designer initially realized that a CPR-fed system has a limitation – that limitation being its overfeed rate (or recirculation rate[4] if you will).  When a CPR-fed system has been properly set up and adjusted, this second hot gas regulator and piping is not needed.  If the hot gas line and outlet regulator is installed, be absolutely sure that this optional regulator (Phot) is set for a lower pressure than the back pressure regulator (Pcold) in the cold vapor line.  A minimum of 5 – 8 psi difference is recommended.  If these settings are reversed (Phot > Pcold), your electric bill will go through the roof.  Why?  The regulator with the highest pressure setting (Phot in this case) overrides the lower pressure regulator, forcing it wide open.  The quantity of gas that will pass through these two regulators (now in flow series) becomes a function of the regulator port diameters and their respective pressure setpoints.  A few years ago, a class attendee mentioned that he had encountered this phenomenon at his plant.  After reversing the setpoints of the two regulators (Phot and Pcold), they shut down a formerly loaded 300 hp screw. 

 

Pumper Drum Liquid Return to CPR – Top or Bottom?

What makes the system in Figure 1 atypical are two features: the absence of a hot gas connection to the top of the CPR (just discussed) and the point of connection for the returning cold liquid from the pumper drum.  In the system shown, it connects to the top of the CPR (a horizontal vessel in this case).  However it is far more common to see this connection made to the bottom of the CPR, well below the vessel liquid freeboard[5].

Some years ago, I visited the refrigeration system shown in the next figure – a packaged ice plant in the Deep South.  Note where the liquid return from the pumper drum is made to the CPR in Figure 2.

Figure 2 – Pumper Drum Connected to Bottom of CPR

The vessel on the left in this photo is the accumulator; the little middle puppy is the pumper drum.  The vessel on the right is a CPR.  The float switch shown to the right of the pumper drum initiates a liquid transfer.  In the system shown (and that in Figure 1), cessation of transfer (de-energizing the three-way valve) is accomplished with a time-delay relay.  What distinguishes these two systems is the elevation of the liquid return into the CPR – above or below the CPR freeboard.

There is some disagreement regarding the placement of the liquid return connection to the CPR – should it be above or below its liquid freeboard?  Below is far more common.  This location has the advantage of maintaining more of the liquid subcooling effect of returning liquid from the pumper drum.  However it is my opinion that this connection should be made above the CPR liquid freeboard for technical reasons I won’t go into here.  True, some subcooling is given up as returning cold liquid falls through a warmer vapor head space.  However it is my opinion that the advantages outweigh the disadvantages and I’ll let it go at that.

 

Setting the Transfer Time Delay Relay

Nearly all CPR systems in the U.S. use a time-delay relay as the means of ceasing a liquid transfer from the pumper drum to the CPR as discussed.  A few (but only a few) systems use alternate means of ceasing transfer, those employing a second lower float switch (usually insufficient clearance available for this) or a float column with a resistance probe connected to a control panel.  However as far as shear numbers of them, the time delay relay wins out.  To set this relay, one must be in clear view of the site glasses on the pumper drum.  When liquid reaches the upper site glass, transfer automatically begins.  When liquid reaches the lower site glass, transfer should cease.  Don’t attempt to force all liquid out of a pumper drum – stay within the two site glasses.  That’s what they’re there for in the first place.  When insulating the pumper drum, remember to include an extended length frost shield for the lower site glass, otherwise you won’t be able to see anything because that lower glass will completely frost over.

If you’re considering a CPR-fed system, my recommendation is to return liquid to the top of the CPR as shown in Figure 1, or to employ the optional arrangement shown in the next figure. 

 

Hartford Return Loop

Up until the late 1890’s, the Hartford Steam Boiler Insurance Company (now Hartford Insurance Company) was the major policy issuer for insuring residential steam boilers.  Hartford was also paying out considerable bucks to replace boilers and rebuild houses because their insured boilers were blowing up left and right.  Then one day, an ingenious engineer with Hartford’s Boiler Division came up with the piping arrangement shown in Figure 3.  Up until then, boiler return connections were made below the boiler freeboard.  This method of connection commonly forced all the water out of the boiler when the steam pressure rose.  Raising the connection elevation solved the problem and the name stuck – Hartford Return Loop.  These are still used today.  If you wish to return liquid ammonia below the liquid freeboard, then my recommendation is to use this method.  Yes, you’ll pay for another vessel opening, an angle valve plus a tee, but this is only a one-time payment and one very well worth its initial investment.

Figure 3

 

Summary – Advantages & Disadvantages of CPR – Fed Liquid Overfeed Systems

Advantages:

  • No liquid recirculation pumps to install (cost savings)
  • Excellent evaporator heat transfer
  • Evaporators can be back-regulated
  • Slightly higher h than mechanically-pumped evaporators
  • Lower “active” evaporator refrigerant charge than mechanically-pumped evaporators

Disadvantages:

  • More complex to set up pressure regulators
  • Evaporator liquid overfeed is generally limited to ~1.5:1 (this could also be an advantage as well)
  • Frequent readjustment of liquid transfer time-out relay necessary (systems without downstream pressure regulator in hot gas branch to 3-way valve)

Do not use CPR-fed systems with high-stage screw compressors operating at different suction pressures, especially when flash vapor from CPR regulators (one per side port required) are fed into side ports of each screw.  This issue quickly becomes hugely complex; difficult to size port diameters of each regulator; side port gas/motor amperage overload is common when products of defrost are returned to the CPR.

Higher energy consumption than mechanically-pumped liquid overfeed systems when high pressure hot gas is used for pressurizing both the CPR and the dump trap for liquid transfer to the CPR.  An option to substantially reduce compressor energy involves installation of a transfer pump; CPR vapor is then used instead of higher pressure hot gas as the means of pressurizing the dump trap (seldom done but a heck of a good idea; lower risk than using high pressure gas for transfer).


[1] This facility is fortunate to have a large condenser.

[2] The thermal conductivity of the surface, Btu/ft2-hr-ºF

[3] Denkmann’s opinion, plus opinion of other practitioners also.

[4]  Recirculation rate = overfeed rate + 1

[5] Freeboard, as the term is used in this paper, denotes the relative elevation of the liquid level.  The term originates from the boiler industry and is equal to the square feet of steam disengaging area.  Because a rapid decrease in vapor head space pressure will result in flash gas, and this flash gas must escape from the liquid surface without carrying liquid with it, the term “freeboard” also becomes applicable to vessels containing saturated liquid ammonia.

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