The LED lamp article was about the consequences of utilizing LED lamps in streetlights.  Apparently during a heavy snow, the snow collected on the streetlights rendering the specific light partially hidden or completely covered.   Unfortunately, this created some traffic issues such as accidents and an increase in traffic citations.  From what I have been able to learn the LED lamps burn much cooler than an incandescent light bulb.  Without the warmth of the lamp to melt the snow, the specific color of the lamp was not visible to drivers.

Ammonia refrigeration systems can be affected like this also.  In an attempt to lower the energy use, what are the underlying barriers?

Let’s say you decrease the discharge pressure.  That saves energy, right?  Yes, but what happens if the discharge pressure is too low and you cannot defrost the evaporators?  Do you install head pressure control valves to elevate the discharge pressure for hot gas defrosts?  Or, do you raise the discharge pressure so the defrost cycle can work as it currently does?

You want the discharge pressure to be low to save energy, however in some systems control valves are added to provide an artificially higher pressure.  The need for the higher hot gas pressure could be for defrosting the  evaporators or sufficient pressure to push liquid ammonia to the expansion valves.  Another consequence could be additional oil carryover due the gas velocity exceeding the separator design limits.

One of the questions observed in the LED streetlight issue concerned the problem of the snow accumulation.  If the lamp is too cool to melt the snow, how do you solve the problem?  One statement was to add resistance heaters!  It’s logic like this that is hard to argue with.  Saving energy in one form and creating a need for energy to support the fundamental reason for the change in the first place?

So, when you are working with energy saving projects on ammonia refrigeration systems who pays for the resistance heaters when the plan does not work?  If the projected energy savings are based on the lower discharge pressure, who pays for the difference when the dischare pressure has to controlled at a higher pressure  to make the system work properly?

Something to think about…

The four basic components of a mechanical refrigeration system consist of the following:

1. Compressor (a means of increasing the refrigerant vapor to a high pressure)
2. Evaporator (heat absorption device)
3. Throttling device (expands liquid from a higher pressure to a two-phase mixture at lower pressure)
4. Condenser (heat rejection device)

In any mechanical refrigeration system you will find these four basic components.

A fifth component is also required to allow the four items listed above to work together.  This is the piping, which connects the four components into a system.  Otherwise you would just have parts and pieces that do nothing.  If the four basic components are properly selected with either; the piping not being installed correctly or an improper pipe diameter chosen, the performance of the system will be less than expected.

Anyone with a basic understanding of a residential refrigerator or air conditioning system already knows how an ammonia refrigeration operates.

The compressor increases the pressure of the vapor to a pressure sufficient for condensing the vapor into a liquid based on the available heat sink temperature.  The heat sink temperature is either water, air, or a combination of both.  Cooler heat sink temperature’s will allow lower condensing pressures.  Conversely, high heat sink temperature’s require the condensing pressure to be even greater.

Once the vapor is condensed into a liquid the liquid flows to the throttling device.  The liquid is expanded from the condensing pressure to the lower pressure desired in the evaporator.  This pressure reduction of liquid creates a two-phase mixture of liquid and vapor.  The vapor is called flash gas, which is the energy expended to cool down the high pressure liquid to a low pressure liquid.

In some system arrangements the flash gas and cold liquid are fed directly into the evaporator.  This is what occurs in direct expansion liquid feed systems.  This liquid flow control could be accomplished by a capillary tube or thermostatic expansion valve.

In other configurations the flash gas is separated and the cold liquid is fed solely into the evaporators.  This occurs in liquid overfeed systems or gravity flooded evaporators.  The flash gas and the vapor formed inside of the evaporator are returned to the compressor to increase the pressure for condensing to occur once again.

In the condenser the high pressure, high temperature vapor is desuperheated and condensed.  All of this heat is removed by circulating water, air or both over the condenser heat transfer surface.

The process is continuous for as long as cooling is required to maintain the temperature of the space.

In short, the biggest difference between ammonia refrigeration systems and a residential refrigerator is based on this: various types of heat exchangers and liquid feed systems are used with a different refrigerant.

There are several more slight difference though.  In an ammonia refrigeration system the components are a lot larger and ammonia stinks (to some).

The tariffs are often segregated into ON-Peak and OFF-Peak durations.  In these two categories the utility informs you how much they will charge for electricity.  The tariffs will also stipulate the time periods when these occur.

This can be further complicated by two additional terms: energy (kWh) and demand (kW).  Therefore during an ON-Peak period you can expect to pay for energy and demand at an increased rate.  This can be an expensive time to operate a refrigeration system.  During the OFF-Peak period the energy and demand charges are usually considerably less expensive.

Becoming aware of this it might behoove you to investigate your refrigeration system operation.  For example, can you operate your system more during the evening in the OFF-Peak period?  Doing so can dramatically reduce your monthly utility charges.

On the other hand, if your operation occurs strictly during the ON-Peak period you can expect to pay the higher rates.

By investigating your electrical rate tariffs you can uncover the opportunities within it to see if these can be exploited to your benefit.

If you would like some assistance with this, please contact us.

Compounding this is the application of those components to different  requirements for cooling or freezing.  In addition to the above, the system designer has to make the entire system work through varying weather conditions while continuing to operate efficiently through a range of loads (full load to part load).

In some instances you might see recommendations where flooded evaporators are used without regard to operating charge because that’s the way it is done.  In others you might see very large high-pressure receivers for a once in a lifetime pump down.  You may see head pressure controls provided to ensure high-pressure for hot gas defrost systems or liquid transportation.

Secondly, the application of these components to a specific refrigeration duty may stipulate the need for larger refrigerant charges or certain ancillary devices, such as surge drums, pressure regulators, etc.

Up to this point, the system is still on paper.  You still need to get it installed properly.  Then, there are control systems that will need to be installed.  Therefore as you can see there are a lot of opportunities where things can go wrong.

How could this process be improved?

Training is an opportunity to describe the basic operational requirements of each component, its operating limits, and how that operation/performance can be impacted by the interaction with other components.  Another aspect is; has the system designer accounted for all operating conditions or only full load conditions in the summer?

Coordination is another issue.  Will the controls company have the same viewpoint for controlling the system as the system designer envisioned?  Will all required information be passed along to the parties that come after the system is designed on paper?

Installation begins after the job foreman is tasked with making all of this work.  Then at start-up let’s ask another question.  Who does the commissioning of the system to ensure the rated performance capabilities are achieved?

Each of these topics are an opportunity for training to be passed along from one area to another in hopefully a coordinated fashion.  The overall impact of the decisions made in the above areas also has a bearing on the success of energy conservation aspects of the refrigeration system operation.

When this occurs many report an issue of not being able to keep the compressor(s) running.  Another common complaint is the discharge pressure is too low. Both are related.  When the ambient temperature decreases even an evaporative condenser operating dry can be troublesome.  With the low temperatures and a lot of exposed pipes in the condenser coil the refrigerant condenses very quickly.  This is further aggrevated by the start-up condition where you are only starting one compressor, because the suction pressure will rapidly decrease if you start too many.

This entire situation is a result of low liquid feed pressures to the expansion devices.  If  the liquid feed pressure is too low, the expansion device has very little flow capacity.  Consequently, with little to no liquid flow through the expansion device the evaporator does not have sufficient liquid to boil.  Hence, lower than normal suction pressures to keep the compressor operating.

This situation is a bit like the old addage: Which came first? The chicken or the egg?

If you cannot deliver a sufficient quantity of liquid to the evaporator(s) at an adequate pressure it is difficult to build suction pressure to keep the compressors operating.

This past week one of our clients experienced this in two different facilities.  One system experienced start-up issues and one had no problems with the cold weather.  The system that did not have any issues with the cold weather was recently modified with methods developed by us.

If you are experiencing similar conditions and want to implement a reliable soltuion, please contact us.

In older systems it is quite common to see flooded evaporators.  They are simple to design and install and only require periodic maintenance, such as oil draining.  In the attempt to limit the ammonia inventory it is necessary to find other heat exchanger types that fulfill the same duty.  For fluid cooling applications a brazed plate or plate and frame heat exchanger will work quite well.  These heat exchangers can be selected for almost any common cooling function with greatly reduced refrigerant charges.

Other applications where gravity-flooded air-cooling evaporators are used may dictate the use of another liquid feed methodology.  These may be:

• Heat exchange systems such as direct expansion (DX) are already considered as a low refrigerant charge device.  The use of excess liquid is not required and in fact limited by the very nature of the control valve; a thermostatic expansion valve.

• Liquid overfeed systems typically consist of a mechanical pump that circulates an excess volume of refrigerant to achieve the rated performance of the evaporators.  Numerous articles have been published as to what is required for the minimum accepted overfeed rate.  Higher overfeed rates tend to dictate the use of larger piping, which in turn holds more liquid refrigerant.

In practical terms, we have to ask ourselves: how can you provide the desired cooling effect with the lowest volume of refrigerant in the system?  Large high pressure receivers are used to provide pump down capacity, but these inherently have some volume of liquid within them to provide submergence of the dip tube.  The liquid from this vessel is traditionally used as the supply for all of the evaporators or other vessels within the refrigeration system.  This brings up a secondary safety issue: what is inherently safer?  High pressure liquid ammonia or low pressure liquid ammonia?  In the event of a release the high pressure liquid will generate a substantially larger cloud of ammonia when it flashes from a high pressure to atmospheric pressure.

Condenser piping practices commonly in use balance pressure gradients in the condenser coil with specific piping practices.  If these are improperly installed or designed this can allow liquid refrigerant to back-up into the condensers by affecting the ability to drain by gravity.  The extent of the deficiencies in this piping can increase the total refrigerant charge also.  Not only can this increase the volume of refrigerant required, it can also increase the condensing pressure and the size of the high pressure receiver.

The use of modified liquid overfeed systems and other methods to provide lower refrigerant charges are technically feasible.  We have been involved with several that have been installed and in successful operation for several years now.  Our approach is based on the general concepts suggested above and applied to the specific requirements of the performance parameters dictated by the facility.  These systems also greatly lend themselves to operation at reduced discharge pressures.  In effect, the utilization of these methods offers multiple benefits of significantly lower refrigerant charges, increased energy reduction, while greatly reducing issues associated with cold weather operation.

It is helpful to think of superheat as temperature (what I call quality of heat).  Having a high temperature available such as the discharge gas stream seems to imply a lot of heating potential.  While a high temperature may exist when ammonia is utilized as the refrigerant the fraction of high temperature heat is low when compared to the total quantity of heat energy available if condensing is considered.

Desuperheating is a sensible heat transfer process.  You are only transferring sensible (heat you can feel) energy to the fluid being circulated.  Once the superheat is reduced to a limit dictated by the heat exchanger used no addition heat is transferred.  Therefore, having a small percentage of available heat at high temperature will limit how much heat you can recover and re-use.

Alternative methods may use the latent heat of condensation, which is a phase change.  Being able to recover the larger fraction of low-grade heat may provide the ability to heat a larger volume of fluid in circulation, however this would be performed at a much lower temperature.  In many cases this lower temperature will approximate the condensing temperature of the refrigerant at the expected operating conditions.  It would certainly seem reasonable at this point to take fluid warmed up and then use the superheat to actually increase the fluids temperature above the condensing temperature of the refrigeration system.

In some instances, the use of industrial heat pumps can amplify the temperature to provide a more useful heating source.  In other circumstances it might be prudent to utilize refrigerant as the heat transfer fluid.

Remember, using a sensible heat transfer process requires a larger heat exchanger than a heat exchanger sized for a phase change heat transfer process.  The differences in film coefficients between sensible and phase change heat transfer can be substantial.

When you approach a heat transfer process for heat recovery, be sure to investigate the actual process requirements, not just the equipment that seems to be popular.

The steam was actually from two distinct sources: a smokestack and a large bank of cooling towers.  Here was a perfect example of wasted energy.  Energy flowing up the smokestack was the result of energy consumed for generating heat or power, while the cooling towers were rejecting heat from some process directly to the atmosphere.  This is all too often seen.  Putting this into context with an industrial refrigeration system we have heat energy being moved from one source to another location where the rejected heat is not objectionable.

If we were to reverse these roles, we would be transferring heat energy from a source where excess heat is available to another location where the recovered/transferred heat energy would be beneficial to the overall process.  That perfectly describes a heat pump and illustrates how operating issues may be utilized for reducing energy!

Heat pumps are essentially refrigeration systems that recover heat by moving energy in a useful and beneficial direction.

One of the most intriguing applications of heat recovery is to utilize common industrial refrigeration components and equipment to achieve this useful transfer of energy.  Low grade energy recovery has two common issues; what I call quantity and quality.  Herein after referred to as temperature and BTU’s (British Thermal Units).  These two constraints need to be balanced within the heat recovery process.  Bluntly put, you need high quality heat to achieve the final process temperature and a sufficient quantity of heat energy (BTU’s) to provide the desired heating effect for the process.

It remains to be seen though whether this technology ever catches on here in the US.  Since these systems are not catalog ready for purchase and the perception that this is new and unproven technology, many shy away from utilizing this branch of industrial refrigeration technology.  Again from a slightly ironic view point, this is a simple application problem not unlike designing any other custom refrigeration system.  For further reading this may be helpful.

The use of hand expansion valves in a mechanically pumped liquid overfeed system are similar to control-pressure receiver (CPR) systems.  The HEV is controlling the flow of liquid into the evaporator.  As with the CPR system the valve is selected for the operating pressure differential (liquid feed pressure – evaporating pressure) and the required mass flow to achieve the rated performance of the evaporator.

The one major difference with this application from the others is that a refrigerant pump is used to circulate the liquid ammonia to the evaporators.  This adds another layer of complexity to the overall system, since the pump performance can be defined by improperly adjusted (or improperly selected) hand expansion valves.  It is important to remember that while the refrigerant pump may be selected for a specific flow and head requirement, any operating condition that changes may also impact an associated device.

In liquid overfeed systems the hand expansion valve also provides a secondary function of balancing liquid refrigerant flow to each evaporator.  If the HEV is adjusted too far open (whether due to selection or adjustment) the pump head can be drastically lower.  This can force a re-balance of the pump such that the pump flow actually increases.  This is sometimes called riding the pump operating curve.  When this occurs, the Net Positive Suction Head Required (NPSHR) increases.  This can create periodic issues with cavitation and be difficult to find without being aware of the interaction between pump curves and hand expansion valves.

This commonly occurs when the refrigerant pumps are selected with very conservative operating conditions, lack of effective commissioning, and not being aware of the dependencies of the interactions of the associated components and equipment.

In closing this series, the discussion has been on the application and selection of hand expansion valves.  Like most application scenarios it is common to see the same component applied in many different ways.  The fundamental tasks of the system designer is to understand the requirements of the application to ensure adequate performance is achieved.   It is also important to determine the full range of operating conditions that may impact the performance of the selected component or associated equipment.   If the full range of operating conditions has not been carefully evaluated, various seasonal weather conditions may dictate periodic adjustments of the component.

If you have questions, please add your comments and I’ll try to address them.  Thanks for visiting the blog.

A float column will have two connections on the associated vessel (top and bottom).  The lower float column connection allows liquid from the vessel to reach an equilibrium level with the liquid level in the vessel.  The upper connection is for equalizing pressures and gas formation in the float column with the pressure in the associated vessel.

As you may have noticed, the liquid level in a float column will fluctuate during operation.  This may be the result of pressure changes in the system, which result in boiling of liquid within the float column.  This can cause the apparent liquid level to change much like the level of water appears to increase in a pan of boiling water.

One method of minimizing the variation in the liquid level in the float column is to use a hand expansion valve in the top equalizing connection from the float column to the vessel.  By using the HEV to partially restrict the gas flow through this top equalizing line you limit the pressure changes within the float column.  Thus, by mitigating the pressure fluctuations within the float column the boiling action is minimized and results in a stable liquid level.  This may help to reduce nuisance actuation of float switches.