Energy Saving Opportunities Through the Intelligent Application of Refrigeratioon Evaporators
One area I find which does not get the attention it deserves in the drive to economize refrigeration systems is evaporator operation. Here are four things YOU can do (for your customer) to save energy and YOUR (their) money, and possibly, win a few points for yourself.
The four things you can do are:1.Budget Fan Horsepower, 2. Utilize VFD’s, 3. Defrost Smarter, and 4. Be Energy-Wise.
Budget Fan Horsepower
You or your customer should budget fan horsepower. Budgeting fan horsepower, like budgeting in other areas of life, forces discipline and ultimately wiser choices. When you exercise this discipline, however difficult, it demonstrates you care about the well being of your customer.
Consider your choices of fan motors. The scatter chart in Figure 1 shows the fan motors combinations which could be selected for a series of three fan evaporators. The evaporators illustrated are all ceiling hung evaporators, galvanized coils with a fin spacing of 4 fins per inch. The evaporators in this example range in size from 16.5 TR to 53.5 TR.
You have many fan HP to coil paring possibilities and combinations. The important thing to note is the number of choices available and their impact on the fan HP to delivered cooling ratio. Weight your choices carefully. The economic considerations are substantial.
As a further consideration, Figure 2 shows for a typical evaporator the impact of increasing the fan HP. Certainly increasing the fan HP and the air flow volume through the evaporator will allow the evaporator coil to perform at a higher level. But at what cost? For this evaporator the increase in fan HP, and the corresponding increase in air flow, increases the capacity of the evaporator about 37%. From fan motors of 1 HP to 1.5 and 2 HP the increase in evaporator capacity roughly tracks the increase in airflow through the evaporator unit.
The change in air flow volume and fan HP significantly diverge as the fan HP increases to 3 HP. The increase in power consumed from 2 HP to 3 HP is 50%. The increase in capacity is 17%. Who would wish to defend to a customer the idea of paying 50% more in power for a capacity increase of 17% as being a good investment?
Some may argue that the 1 HP increase in fan energy is insignificant. If the increase was limited to 1 HP, the argument might be valid. But adding 1 fan HP has an impact on the entire refrigeration system and the operating cost borne by the operator.
Figure 3 illustrates a cost analysis of the cost of 1 HP at an evaporator. And this does not include any “downstream” costs.
So what are the “downstream” costs? Where does this “1” horsepower go? How does it manifest itself within a refrigeration system? We know the answer though we may not wish to acknowledge it. The fan exerts work on the air within the refrigerated facility and heat is added to the air. Fan motor heat is removed by the refrigeration system.
The path of the fan motor heat is known: It is absorbed by the refrigerant, the refrigerant is compressed by the booster compressor (assuming a two-stage system), the heat travels thru an intercooling vessel, the refrigerant vapor is compressed by the high stage compressors, and the vapor finally loses the evaporator fan motor heat to the atmosphere via the evaporative condenser.
A conservative calculation will show that one (1) fan HP adds +/- 0.6 HP to the work required by the entire refrigeration system. And this calculation does not include any losses incurred by the refrigeration system resulting from pressure losses in the piping network, added refrigerant pumping or delivery costs.
Let’s look at an example. Consider the case of a 60,000 square feet freezer which is 30 feet in height. For this example we will assume the loading is 425 ft2/TR. (This is a reasonably conservative value for estimating a cooling load). If the cooling requirement assumption is accepted, the resulting refrigeration load could be projected to be 141 TR at a -20°F SST. The assumption will also assume four (4) 35 TR evaporators are required. We will further assume the evaporators will have a fin spacing of 3 FPI, designed to operate at a 10F° TD and these evaporators will have a static pressure requirement of 0.25”.
A blind selection by a third party resulted in the following six evaporator selection. The only requirement stated for the blind selection was one selection to be the most inexpensive evaporator unit offered in his product line which fit the criteria and one had to “appear” to be the most energy efficient offering. The selections offered are shown in Table 1. The selections are listed no particular order.
The units ranged in size from 35.0 TR to 37.1 TR. Airflows varied significantly, from 38,323 CFM to 74,514 CFM. (The discussion of how much air is needed will be reserved for another day)! For each evaporator the HP/TR ratio was calculated. The ratio ranged from a low of 0.111 to a high of 0.632.
Table 2 shows the total connected fan HP of the individual evaporators and their cost of operation. The values used in the calculation are those shown in Figure 3. For this, and subsequent calculations, Evaporator #1 was the base against which all evaporators were compared.
As expected, the higher HP fan motors cost more to operate. Table 3 shows the costs imposed on the refrigeration system when the fan motor heat is included in the calculation.
Table 4 shows the cumulative costs of operation for one year when the cost of operating the compressor room is included, as it should be.
Table 5 shows the results of the analysis. Shown in the table is: the fair market value of the six evaporators (what a user/owner might pay), their price compared to the “base” evaporator; the yearly power cost above the base unit; the first years operation above base evaporator; and finally, the cost of operation after five years above the base evaporator.
The results are staggering. Evaporators #2 and #5 were the least cost evaporators to operate. They had a HP/TR ratio of 0.25 and 0.111, respectively. The “least expensive evaporators”, #3 and #4, are fantastically expensive to operate. They had a HP/TR ratio of 0.413 and 0.632, respectively. Note the second most expensive evaporators would pay for themselves in just over 3 years.
Obviously, not all fan brake horsepower ends up being whole integers. In truth, the horsepower consumed by evaporator fan motors can vary from the nameplate rating, both up and down. A proper analysis for you and your customer would require you obtain this information. But the central truth remains: fan horsepower does add load to a refrigeration system and it is significant.
Truth #1: Budgeting fan horsepower will add to your first cost, but it will save you (significant) money over time.
Truth #2: If you do not budget your fan horsepower someone else will! And they will not be as concerned about your cost of operation as you are.
Consider your fan horsepower carefully. Certainly, as a start, consider targeting 0.4 HP/TR or less for your evaporators. (This suggested value is based on evaporators expected to operate at a 10F° TD).
Utilize VFD’s
Utilize VFD’s, and/or consider re-powering existing evaporators. Properly controlled, the VFD’s allow the fan motors to respond to the refrigeration requirements of the conditioned space. Refrigerated facilities do not operate at full load 100% of the day. Why purchase energy to remove heat that is not there?
One concern in using VFD is the diminished rate of air change within the cooled area when the VFD is operating the fan motor at reduced speeds. One study of which I have knowledge looked at cooler evaporators fitted with VFD’s. The study found the forklift traffic in the coolers had a significant beneficial impact on the “stirring” of the cooler air. The forklift induced stirring working in conjunction with the cooler fans operating at very low speeds was quite sufficient. The coolers maintained the proper air temperatures.
On the west coast, one operator looking to save operating cost explored the option of rewinding his existing fan motors. The basic idea was a fan motors rotating at a slower speed had a reduced power input. In their “test” facility, freezer evaporator fan motors were rewound. The operating speeds were changed from 1200 RPM to 600 RPM. Their test indicated an air temperature increase of only about 0.5 F° to 0.75 F° over a distance of 300 feet. The only thing lost as a result of the rewinding was a drain on the company treasury! Temperatures were satisfactory and the refrigeration load was diminished. The operator estimated they saved $750,000.00 per year. And this was several years ago when power rates were lower.
Chart 1 Chart 1 illustrates the reason for their success. Operating a fan at 50% speed requires only 12.5% of the power the fan motor would consume at 100%.
Another control scheme would be to operate evaporators (and their fan motors) for only half a day. The idea is to operate outside of the peak power rate times. Is this a viable strategy? This is, one owner stated, the proper way to save energy and operating costs. I suggested that operating with VFD’s was a far more energy efficient approach.
His system in a particular room held four (4) evaporators each fitted with (2) 5 HP fan motors. He manually limited the run time of his evaporators to 12 hours. Consider the math:
Time of day cycling: 4 units x 2 fans x 5 HP/fan x 12 hours/day = 480 HP-Hours
To “equalize” the comparison, I “locked” VFD’s at 50% fan speed: 4 units x 2 fans x 5 HP/fan x 24 hours/day x 12.5% = 120 HP-Hours
In fact, the VFD “equipped” system could operate at 79.3% fan speed and not exceed the energy consumed by the time of day cycling.
VFD’s applied to evaporator fan motors have the potential to conserve large amounts of energy. And this saving is multiplied when you consider unnecessary heat is never allowed to enter the refrigeration system.
Defrost Smarter
Do you defrost the way your Grandfather, or someone else’s grandfather, did? Are you defrosting the same way you did 20 or 30 (or more) years ago? Welcome to the 21st Century! We have better and smarter ways to defrost evaporators. One of those ways is to utilize Liquid Drainers.
Table 6 lists many of the advantages of utilizing Liquid Drainers for defrosting. Three of the biggest benefits are:
- The ability to allow condensing pressures to fall as low as the 120 PSIG range while maintaining quality defrosting, and
- The fact the liquid defrost drainer passes virtually no high pressure vapor (a.k.a. hot gas) into the “lowside” of the refrigeration system thereby preventing the adding of nonuseful work to the refrigeration system, and
- When defrosting is complete the liquid drainer is self terminating and stops draining, i.e. no “artificial” gas load is imposed on the refrigeration system due to a timer not being satisfied.
When a liquid drainer style of defrost is chosen, the defrost piping network pressure can be set to operate at 75 to 100 PSIG with most evaporators. This “frees” the system from changes in vapor flow rates due to condensing pressure fluctuations.
Liquid drainer style defrosting relies on the latent energy required to condense vapor, not the sensible heat of large volumes of vapor passing (and condensing) through an evaporator. And, since liquid drainers are designed to drain and transfer liquid refrigerant, a liquid drainer style of defrost can be forgiving if the evaporator is not completely evacuated prior to the beginning of the defrost period.
Liquid drainer defrosting has been advocated since the early 1980’s as a proven technology. I know from personal experience liquid drainer defrosting works and works quite well.
In Photograph 1, the drainer is shown beneath an evaporator. A liquid drainer’s location is not limited to a gravity drain position beneath the evaporator. There are many liquid drainers located above evaporators at the valve stations on the roof above. There is a small penalty to be paid as there is the requirement to “push” the condensed refrigerant uphill. However, the advantages of the low vapor supply pressure and the self-terminating defrost considerably outweigh the pressure penalty.
Be Energy-Wise
Consider equipping your evaporators with defrost hoods. A defrost hood fitted to a penthouse style evaporator is shown in Photograph 2. A defrost hood benefits your evaporator in several ways.
First it contains the heat resulting from the defrost cycle. The defrost heat is “captured” as it cannot rise and disperse within the refrigerated space. Defrosting time is improved as the defrost heat is held in close proximity of the evaporator’s coil. A larger amount of the defrost heat does useful work. A reduced amount of high pressure vapor flow is required. The “non-useful” high pressure vapor load imposed on the cooling system is reduced.
Secondly, a defrost hood minimizes the potential for moisture, resulting from the defrosting cycle, transferring from one evaporator to another. And trapping the moisture within the defrost hood also prevents this moisture from freezing to the structure, walls and roof of the refrigerated enclosure.
Lastly: Clean your evaporators! This is probably the easiest thing to do and the least done. No one would install an evaporator with dirt and grime attached to the tubes and fins. And we can all agree a clean evaporator is more efficient in heat transfer and moves more (or the proper amount of) air. Why then do we tolerate dirty evaporators?
A private test at a grocery distribution facility indicated the following after a “good” cleaning of dock evaporators: Air leaving the evaporator decreased by 6.5F°; Air velocity increased by 64%; The evaporator’s capacity increased by +/–30%.
I cannot substantiate the values above. I do not know firsthand the condition of the evaporators prior to the cleaning. I can tell you simple mechanical common sense would indicate a clean coil does more cooling than a dirty coil. I do know the owner was so pleased with the results the other evaporators were cleaned.
Cleaning an evaporator may not be exciting work, but the benefits cannot be denied. Most facilities have a maintenance staff or a scheduled third party maintenance service. Put “clean evaporators” on the list of maintenance procedures. Real benefits can be derived from a small investment in time.
Conclusion
Evaporators do not operate in a vacuum (no pun, or a rewriting of the thermophysical laws, is intended). Like every other mechanical device in the system, their application and their maintenance can either add to cost of operation or reduce the cost of operation. You can make the decision as whether to add or reduce the operating cost of your, or your customers’ refrigeration system.
Reducing the cost of evaporator operation really is not difficult. But, like all the other components within a refrigeration system, an evaporator’s operation simply requires the attention it is rightfully due.
