Description

Professor Weisensee
DHW 3 – Dehumidification

The objective for this design homework is to analyze the cost and amount of energy three different technologies would need to keep the air from condensing in the refrigeration display. For the design homework I chose to analyze the volume of only the refrigeration display cases. My three technologies that I will be looking at are the refrigeration cycle, a solid desiccant system and a compression system.

Assumptions:
– Volume of the display cases is 61.1645 m3. This is assuming that for every 3 display cases that the volume is 2.03381 m3,[1] with 15 display cases in one isle and that there are 5 isles of refrigerated display cases in the grocery store [2]. I thought this was reasonable because I think that is about how many display cases there could be in a grocery store. I also did not choose to do the whole grocery store because there is plenty of space that does not need to be kept at 5 ˚C and 55% relative humidity.
– I will also assume that the average relative humidity in the display cases is around 55% [3]. I think this is a reasonable level, and from the psychrometric chart you can see that the dew point is lower than the desired temperature so you know the moisture would not condense. Also, that is what is said online.
– The cost for electricity is 7.94 cents per kWh in the state of Missouri [4]. I think this is reasonable because we are in Missouri and to get a cost analysis, I needed to have a cost for kWh.
– I will also assume that the technology is running 24 hours a day throughout the whole year. This is reasonable because you need to keep the product cold to stay fresh.
– Starting temperature and relative humidity are 20 ˚C and 55% humidity respectively and the temperature inside the refrigerated display case is 5˚C [5].
– The Desiccant system is based off of the Ebac DD1200 240 V Industrial Desiccant Dehumidifier, where it has a maximum power output of 7,200 Wh [13].
– The compressor system is based off of the Sullair ShopTek 10-HP Base Mount rotary Screw Air Compressor, where its maximum power output is 121 kWh [9].
– Assume that all the air is circulating through the system ever second with no obstruction. This means that the mass flow rate is constant and that the all the air is being cycled out every second. This probably isn’t the best but makes the math easy for when calculating cost.

Diagram/ Schematic:

Calculation for amount of water removed:

Fig. 1 In this figure the dot on the right represents state 1(outside the refrigerated display case) and the dot on the left represents state 2(inside the refrigerated display case). The black lines are to help with seeing the temperature, humidity and the dew point for each state. The blue lines help show the closest volume each state is at. It is also important to note that the dew point for state 2 is below 5 ˚C so there would be no condensation on the display case.

Table 1 In this table shows the necessary values for each state for the calculations. These values were from the assumptions and from psychrometric chart in fig. 1. The volume for state 2 was near 0.79 (m3/kg) so I chose a number just above the value of the line.
State Temperature (˚C) Relative Humidity (%) Enthalpy (kJ/kg) Volume (m3/kg)
1 20 55 40 0.84
2 5 55 13 0.791

In order to find the amount of energy used in the system, the difference in mass first needs to be calculated. First solving for the amount of water that needs to be removed, take the difference in the reciprocals of the volumes and get:
1 1 1 1 𝑘𝑔 𝑔
𝐴𝑚𝑜𝑢𝑛𝑡 𝑜𝑓 𝑤𝑎𝑡𝑒𝑟 𝑟𝑒𝑚𝑜𝑣𝑒𝑑 = 𝑣2 − 𝑣1 = 0.791 − 0.84 = 0.0737 𝑚 3 = 73.7 𝑚 3

Answer:
Amount of water need to be removed (g/m3) 73.7

Discussion of working principles:

In the compression system, such as the one assumed for the calculations, it takes air and compresses it. This will not only increase the pressure but the temperature of the air and decrease the amount of moisture the air can hold. So, when the air condenses it can hold less moisture cause some of the moisture that was in the air to form droplets in the chamber [9]. From my source [10], the air is pressurized up to 85 bars from a high-pressure pump. Filters and separators are then used to collect the droplets that have formed. But this does not extract any of the vapor that is still in the air. With the air now saturated with the rest of the moisture that it was holding further heating needs to be done in order to remove more. The downside to this process is that it needs to be followed up by another one, such as desiccant or coils. But the most common way to further reduce the moisture in the air after it goes through compression is to use a dryer [12]. If the moisture is not removed it can cause corrosion on a part of the machine and then the machine can become inefficient or fail.

Calculations of electricity input:
Refrigeration cycle
So, since a volume of 61.1645 m3 is assume we can find the mass in each state from dimensional analysis:
𝑉 𝑚3
𝑚1 = 𝑣 = 𝑚 ⁄𝑘𝑔) = 72.814 𝑘𝑔
0. (
𝑉 𝑚3
𝑚2 = 𝑣 = 𝑚 ⁄𝑘𝑔) = 77.33 𝑘𝑔
0. (
And the difference between the masses is:
∆𝑚 = 𝑚2 − 𝑚1 = 77.33 − 72.814 = 4.516 𝑘𝑔 Now doing an energy balance around the whole system we have:
𝑚 𝑤
So then we can say:
𝑘𝐽
𝑤 𝑘𝐽
𝑘𝑔
Now I am assuming that all the air passes through the system ever second ever hour, so I get 19.783 kWh of power. From this I can calculate the cost for my system to remove the moisture from the air.
𝐶𝑜𝑠𝑡 $99,121/year
𝑑𝑎𝑦 𝑦𝑒𝑎𝑟 𝑘𝑤
Desiccant system

𝐶𝑜𝑠𝑡 /year
𝑑𝑎𝑦 𝑦𝑒𝑎𝑟 𝑘𝑤
Compression system

𝐶𝑜𝑠𝑡 /year
𝑑𝑎𝑦 𝑦𝑒𝑎𝑟 𝑘𝑤
Table 2 Answer of cost for each system
System Cost ($/year)
Refrigeration 99,121
Desiccant 5,853
Compression 139,426

Cost Comparison:

References:
[1] https://www.centralrestaurant.com/Kratos-Refrigeration-69K-825-Swing-Glass-Door-
Merchandiser—3-Doors-81-inW-72-Cu-Ft-White-Exterior-c182p619347.html

[2] https://i.pinimg.com/originals/00/03/6c/00036c33dc4c4681f92741216849d06d.jpg

[3]https://www.researchgate.net/publication/279572201_Humidity_effects_on_supermarket_refr igerated_case_energy_performance_A_database_review

[4] https://www.eia.gov/electricity/monthly/epm_table_grapher.php?t=epmt_5_6_a

[6] https://www.sylvane.com/ebac-dd1200-industrial-desiccant-
dehumidifier.html?product_id=94388&s_cid=cse_gpl&gclid=CjwKCAjwguzzBRBiEiwAgU0F
T7C7IEZELUJWhM0Ao5tMKTA1tLAAtXZXlqxVGd2KjIhXRzdg2VBzDRoCwQ0QAvD_Bw
E

[7] https://www.nrel.gov/docs/legosti/old/7010.pdf

[8] https://www.swtc.edu/Ag_Power/air_conditioning/lecture/basic_cycle.htm

Air compressor
[9] https://www.compressorworld.com/shoptektm-10-hp-base-mount-rotary-screw-aircompressor-230v-single-phase-
125psi.html?matchtype=&network=g&device=c&adposition=&keyword=&gclid=CjwKCAjwgu zzBRBiEiwAgU0FTzjcm480vHV-
l5X634otOz0SFEAYCyAd9uOKVNZwJ_FuC1BdCsHYXBoCxv8QAvD_BwE

[10] https://www.condair-systems.eu/knowledge/system-comparison/technologies

[11] https://smarterhouse.org/cooling-systems/types-cooling-systems

[12] https://www.thecompressedairblog.com/how-to-remove-moisture-from-compressed-air

[13]https://www.sylvane.com/ebac-dd700-industrial-desiccant-
dehumidifier.html?product_id=94394&s_cid=cse_gpl&gclid=CjwKCAjwguzzBRBiEiwAgU0F T8_zlrsxcHuLbOJVcurXleIYLj4Vw0lavcNwJ31rwyVoZQqhUfT6FxoCnsEQAvD_BwE

[14] https://3fficient.com/managing-energy-costs-in-grocery-stores/

[15] https://thesoothingair.com/how-to-improve-dehumidifier-efficiency/