Refrigeration & Air Conditioning - Science topic

Dear Cenker Aktemur,

According to information provided by Saeed Sayadi, there is a complication when total exergies of the flows are used to define fuel and product in a heat exchanger. Besides the dependence on the reference ambient temperature, there is the inconvenience of applying the disaggregation of exergy into thermal and mechanical components. Such disaggregation has already been criticized in the literature due to potential arbitrariness.

On the other hand, there are proposals in the literature to define fuel and product in heat exchangers without the aforementioned problems. Please look for the UFS and A&F Models, which were designed for exergo-economic and exergo-environmental analysis of refrigeration and heat pump systems.

Best regards,

Atilio.

Remember that selecting the right compressor is just one aspect of designing an efficient heat pump system. The entire system, including components like the heat exchanger, air handler, and controls, must work together seamlessly to achieve optimal performance and energy efficiency. Therefore, it's essential to take a holistic approach to heat pump system design and consult with professionals when necessary.

Aside from the obvious required phase change temperatures, for any refrigerant one needs to consider the contaminants, physical, metallurgical, sensor ranges, and chemical interactions - especially lubricant oils ( like Polyester POE Oil ) from the compressor and the seals. It isn't simply the refrigerant itself, there is a lot of complexity, especially over the total lifetime of the system. There is also a huge thermodynamics design challenge involved, heat capacity and transfer (like surface to volume ratio and fluid flow (gases and liquids ) determines the dimensions and material choices of everything.

Ali D. Salman thank you very much. In this case, will the temperature of my room be -28oC or -34oC?

Dear Subash S. ,

I suggest you to see links and attached file on subject.

Use parametric or conditional study, atleast one parameter should be unknown. For your case it might be two. So you need to fix that. Also check adding other stream too.

Pressure-temperature-concentration can be available in Heat and mass transfer text books. Every chemical temperature and concentration is discussed.

Use the frequency invertor .

First, the heat removed you mentioned above is for steady state condition analysis. Second, your calculation is probably ideal condition. Let's assume your refrigeration system is already ideal condition.

Then, is your 100 kJ load is your experiment? if yes, is it perfectly thermal insulated?

In real world, there always small heat loss when you use insulation material. So you need to consider this heat loss. How? The simple way is to analyze the transient temperature increasing when your object is cold and without cooling system (your refrigeration system is off).

Sampath Suranjan Salins I am talking about aspen plus not ansys fluent.

A very useful link to find the thermodynamic parameters ( △G°, △H° and △S° ) form experimental data.

Thank you for your answer Mr. Rocha,

Answering the question, yes, it is expected to have a constant pressure.

Kind regards

Dear Cenker Aktemur I found a report on the use of potassium formate/water (see attached) in which some severe corrosion problems were observed. They are listed on p. 21 of this report:

Dear Cenker Aktemur I'm not an expert in this field and therefore cannot provide you with a final answer. However, I found a report on the use of potassium formate/water (see attached) in which some severe corrosion problems were observed. They are listed on p. 21 of this report:

Dear Sir, thank you for your reply. We have already defined the properties of the sCO2 by using Fluent Console (NIST tables for CO2 is current in the Fluent database). Although the entire graph is very reasonable, the pressure change along the flow direction is interesting as seen in the attached file. When calculated theoretically (considered friction depending on both wall shear and momentum change) by using Ansys data (wall shear stress, density, axial velocity, etc.), the pressure drop is found as expected (full decreasing along flow direction).

Equation of Nanofluids

Density .........................(1)

ρ_nf = (1-φ)ρ_f + φρ_s

Heat Capacitance ..............................(1)

(ρC_p )_nf= (1-φ) (ρC_p )_f+𝛗(ρC_p )_s

Thermal Expansion Coefficient .............................(1)

β_nf = (1-φ)β_f + φβ_s

Thermal Diffusivity ..............................(1)

α_nf =Κ_nf/(ρC_p )_nf

Effective Thermal Conductivity ...............................(1)

Κ_nf/Κ_f = (Κ_s+2Κ_f-2φ(Κ_f-Κ_s))/(Κ_s+2Κ_f+2φ(Κ_f-Κ_s))

Viscosity ..................................(1)

Brinkman model

µ_nf = µ_f/〖(1-φ)〗^(2.5)

Pak and Cho correlation

µ_nf = µ_f ( 1+39.11φ+533.9φ^2 )

Reynolds number

Re= 4Ϻ/(π.D.µ)

Ϻ= (Re.π.D.µ)/4;

where,

[M= Mass flow rate]

[Re= Reynolds number]

[D= pipe diameter]

Reference

1. Combined convection flow in triangular wavy chamber filled with water–CuO nanofluid: Effect of viscosity models

Dear Kenkar,

In diffusion absorption coolers regarding to the heat performance of the system, ammonia/water couple with alumina (Al2O3) particles in nano-size can be a better option. It can provide better absorption of heat from the generator and faster evaporation of the cooler from the cooling/absorption fluid. Considering the effect of fluids containing nanoparticles the connection units of the heat transfer in the system operation time of the system might be reduced due to shorter heat transfer periods.

Ashish

Hi Adity,

I think you can keep your cells in -80 for several years. However I strongly suggest that after the Covid-19 situation, cultivate your cells again and prepare your nitrogen source from fresh culture. Do not put them directly into Nitrogen Tank.At least make an experiment first. For example, put some cryovials into liquid nitrogen and after couple of days, cultivate them. If they are alive you can transfer your cells into liq. nitrogen tank too.

The first concern for me is contamination.

Second, depen on my experiences, I realized that cells dye if they stay first too long in -80 and consequenlty transfered to nitrogen. With too long, I mean more than 2 months.

Cells are normaly alive -80 for years. I agree with that. However, if you put these cells from -80 after 3 months into liquid nitrogen, unfortunately cells may dye. That occured to us several times. That's why we never keep our cells more than 3 weeks in -80 after crypreservation protocol. If we forget them in -80, we use them but never put them into nitrogen tank again.

I hope your cells will be Ok.

Good luck!

Cigdem

Dear all

Any body implement Model predictive control based EMS ?

i need help

The log P-h diagram of R134a refrigerant sented in attachment. you can use this diagram to design. Note that compression refrigeration cycle designing depend on to several parameters. main parameters is ambient temperature and cooling load. Also you can use coolpack software to design refrigeration cycles.

This is a heat transfer rate maximization problem in the evaporator , in which you have to find an evaporator with the maximum LMTD which means the highest efficiency. Theoritically you can cool down the water as fast as you want but you have design restriction such as refrigrerant inlet temp to the evaporator. In the best scenario the refrigerant inlet temp to evaporator cannot be higher than 2c for your design case and it leads to local water solidification which acts as an insulator. To solve this problem you can recirculate water in tank or add antifrost to tank if you are not using it for sanitary purposes. Higher evaporator efficiency inflicts alot more costs.

Cooling power is the amount of energy/sec require to derive the cooling system like vapor compression or vapor absorption and mass flow rate of refrigerant is directly proportional to the cooling capacity of system.so the cooling power and mass ratio can be calculated by using refrigeration table

Ejector refrigeration systems and absorption chillers are the most common systems which use heat sources instead of electricity. please check the following articles:

Hi, Rahul

The diagrams show the effect of coil tube diameter on unit refrigerant total charge and coil circuiting number with identical compressor size ( compressor displacement, rotation, oil type and oil charge quantity.)

Hopefully this helps.

Thank you, Prabakaran! I will look at some of them that I did not before.

There are different coolers/freezers. Some products just operate at relatively stable ambient and product conditions and no pull-down operations are needed,, some require quick chilling/freezing, and some require minor pull-down capacity. Therefore, it depends on what your product designs require.

RefrigLab cloud eTools (www.refriglab.com) provide components designs and unit overall performance tuning including pull-down, ambient adaptation, charging optimising etc.

The solubility of refrigerant like R290 in POE32 for example can be calculated using the compressor suction temperature and pressure, i.e., the function equation y=f(x1,x2). The solubility equations are available from lubricant companies or compressor companies. Hopefully this helps.

Dear Tom,

thank you very much for all the references, comments and reflexions :)

Here I found a document that relates the cocentration gradient in the Fick law with the chemical potential gradient, but, as you said, I think it is obtained using analogy...

Regards,

Cristina

Thanks a lot, Dear Gholampour, Eidan, and Kareem. We decided to use a pulse free pump with an accumulator compensating any pressure wave.

I am creating some eTools based on cloud platform and cloud computing for product designs and services. If you are design or service engineer, what eTools do you want to have on your smartphone or laptop?

this link is very useful for your question

http://www.freepatentsonline.com/4653288.html

kindly, see the attachment

Thank you Mr. Abdalla for your information !

This is because winter air-conditioning demands heating and humidification.

Dr DB JANI

Resistive-type relative humidity sensors: Resistive-type sensors are composed of interlocked metal electrodes that are deposited on a substrate. The resistance, which is measured by the sensor, decreases as the humidity increases. The advantages of resistive type sensors include being generally accurate in high relative humidities (>95% RH). it is not accurate for HR<15% (opposite to capacitive). Both capacitive and resistive types are affected by the variations of RH and temperature and the deviations between the measured and the actual values depend on the accuracy, sensitivity and the range of the model type. It was proved that the resistive type is more accurate within the specificed range of measurements and both types show a sensitivity for both RH and T.

Hello, it's a interesting question and I'd like to share my opinion on it.

I don't think "the smaller, the better" holds for the generic heat exchanger.

The smaller size will increase the surface-to-volume ration, which helps increase the heat transfer surface area A. However, the heat transferred is determined by Q=hAΔT. Once you decrease the tube size too much, the flow inside the tube will turn from turbulent to laminar flow, and the heat transfer coefficient h will suffer from a huge loss. As a result, the final performance is the determined by the trade-off between h and A. So it's hard to conclude "the smaller, the better".

Yes, we can find out the optimum charge mass of refrigerant in a domestic refrigerator/freezer and air conditioner to achieve optimal energy efficiency and/or capacity at a certain ambient condition range for a specific unit. The charge amount is decided by the refrigerant vapor and liquid ratio in components and the internal volumes of each components including the evaporator and condenser coils, filter internal void, liquid pipe volume, and compressor lubricant oil type. Proper charging is key to a product running safety and performance.

We have e-Tools to help estimate the optimal charge quantity of a CAP tube, TEX, or EEV refrigeration/AC unit available at the website www.refriglab.com. Both smart phones and laptop/desktop computers can work friendly with the website.

For unit services, it is always right and necessary o check the product brand plate/manual to get the proper charge amount. Remember that overcharge may damage your compressor, and undercharge will decrease the output of the unit.

With CapS/e-Tool at www.refriglab.com, CAP tube contacting factor=0 for detached/separate, =1 for common winding on, =2 for tube-in-tube arrangement with suction line pipe. Contacting Factor varies from 0 to 1.5 for winding on suction line to stand for different contacting degree.

Dear / Alaa Al-Badri

Please try to use the hygrometer. The simplest hygrometer - a sling psychrometer - consists of two thermometers mounted together with a handle attached on a chain. One thermometer is ordinary. The other has a cloth wick over its bulb and is called a wet-bulb thermometer. This instrument in not expensive as well as it is more effective.

Dear researchers,

In the following publication from our research group all of the irreversibilities of the cycle were modeled:

Regenerate 4A molsieve at ~250 deg-C. How long you need to regenerate it for depend on the amount of moisture/water adsorbed on it.

In a lab setup you can weight your sieve every 1 to 2 hours. When the reach a constant weight stop and cool it down gradually.

Keep in mind the sieve will lose some of its activity after regeneration.

How can I define R600a refrigerant in Fluent. is it need udf?

I want simulate heat transfer with assumption that R600a be ideal gas?

There is an increasing interest towards R1234yf and R1234ze to replace R134a, in spite of their low flammability.

Spray the drain water on condensing coils , it will improve the condenser performance.  

I'm having the same problem, the external performance data for modeling the absorption chiller using Type 107 requires:

- Part load fraction to chiller

- inlet hot water temperature

- Inlet water temperature to the evaporator 

- Inlet cooling water temperature 

I think the absorption chiller you mentioned above is the Yazaki WFC SC5, for this and other size, the manufacturer defines the performance of the chiller using the last three parameters, nothing about the part load fraction chiller.

To be more specific in the manufacturer datasheet is shown the effect of water inlet temperature and hot water generator inler temperature on the design heat input and chilling power factor, giving the fraction of design heat input and the values of fraction of related chilling capacity.

What I'm trying to create is a simple model that allows to predict Yazaki's performance using the equations for the absorption chiller heat balance.

www.solarbuildings.ca/.../Seasonal_Stratified_Thermal_Energy_Storage_Exergy_Ana.

yes there is always a chance of leakage.....

.

.

and the solid i am talking about can be anything but most probably solidified working fluid....

The following publications helpful to you.

http://www.healthyfacilitiesinstitute.com/a_319-Liquid_Desiccant_Dehumidification_For_Challenging_Environments 

You can use various commercial software's available ...you can also do in  MATLAB ( in order to study the effect of diff refrigerant.). but my advice would be that you should do programming in C rather than that pen and paper stuff...make your program more of user defined like ...properties of referigerants which he will enter....this would be best and most easier stuff as per your current state...

Shell and tube exchanger has relatevely the best heat exchanging rate form contact surface area,thermal mass flow rate and configration point of view. However the effect of design style of chiller has certain effects and this may be taken into consideration.

In a throttling process if there is a change in phase say from liquid to vapor as in refrigeration cycles definitely the temperature will change from condenser temperature to evaporator temperature otherwise refrigeration process will not take place.

Take a look to a T-s diagram of a refrigerant: you'll find that isoenthalpic transformation from saturated liquid have a negative slope, leading to a lowering in temperature for non-reversible adiabatic transformations. 

Another way to understand it is that during throttling liquid pressure lowers but as it is at boiling point these leads to some evaporation, as the process is adiabatic, transition heat is obtained thorugh a lowering in fluid temperature down to boiling temperature at the lower pressure.

You should do energy balance based on the required temperature for your process.

You will probably find good options from Micropump gear pumps (http://www.micropump.com/product_list.aspx?ProductFamilyID=5) and hydracell (diaphragm pumps) http://www.hydra-cell.com/product/F20-hydracell-pump.html

I see this question for a while and wait for a answer from others, but still not. One possible reason is that people don't understand some words. I search 'espin method' but no source. If you can show a configuration of the system, we might be easy to follow. So far, I can only guess that your injector doesn't work well.   

If a heat exchanger is overdesigned, heat duty will increase, resulting in higher coolant exit temperature and lower hot fluid exit temperature. For a condenser, provided that the vapor mass flow rate  is limited/constant, the condensing vapor pressure will decrease and the heat duty remain almost unaffected.

You can use EES program, It is really a very good solution.  

The maximum power output is attained by differentiating the power output W

 in Equation (23) in the attached file with respect to the ratio of the load resistance to the internal resistance and setting it to zero

Please read the attached file

o

Dear Khaleel

Using hydrocarbonic gasses as refrigerants is still in R&D and its cost approach to the ordinary refrigerants today but in the next five years I think it will be more commercial than these refrigerants.

Regards

Hi, Cody:

I think submerged evaporator should not be a problem in terms of technical feasibility, after all it is already used in most air source heat pump domestic water heater on the market. Indeed, for most air source chillers, they do use evaporator to  generate chilled water. The only difference here is they do not store the chilled water in tank, they circulated the chilled water in a loop.

Best,

Chuan

Designing a CaCa cooler is not done step by step but is a complete calculation. The maximum dimensions of fans for the external air is very often the start of the calculation. Is sound silencing nessecary? This gives extra pressure loss resulting in less air for the cooler. We design and build these CaCa coolers a lot in all kind of different designs. If you have more information what you are looking for please send.

Hi Kevin

Thank you very much Kevin for your assistance. I know that you left Edinburgh. I hope that everything going are excellent. my best

The evacuated tube collectors have an outer and an

inner casing of borosilicate glass 3.3. These tubes are

sealed at each end and between them a vacuum is maintained

to allow minimal heat loss due to radiation from the

absorber plate. As solar radiation is incident on the outer

layer of the borosilicate glass tube, a part of it is

absorbed, part reflected back into the atmosphere and

most of the radiation is transmitted to the next layer of

glass tube. The properties of the outer borosilicate tube

are given in Table 2. This arrangement is shown in Figure

2. Since,

τ = 0.92

α = 1- 0.92 - 0.04 = 0.04

From the empirical relations, the insolation at Dubai’s

latitude and longitude incident on a horizontal collectors is

measured to be approximately = 8.35 KWh/m

2

day. Figure

3 depicts a typical solar radiation curve over a clear day.

The readings on the graph are been plotted every 15 min

to obtain accuracy and correctness of the readings. The

graph clearly explains the gradual rise and fall of the

intensity levels of radiation. The area under the curve of

this graph gives the value in KWh/m

2

day as radiation is

only available from 7 AM to nearly 7 PM depending upon

the season. The data calculated indicates a value of 8.35

KWh/m

2

day incident in Dubai, it represents the entire

area under the curve of the Figure 3. To obtain the peak

value of insolation, it is required that this value is divided

with an approximate number of bright sunshine hours.

Assuming that May 15 is a clear day with no clouds and

8 h of bright sunshine, the radiation received/m

2

at every

incident would be = 8.35/8 ~ 1050 KW /m

2

. The further

calculations are based on this value of incidence solar

radiation flux.

G α sun = αlow temp σ (T

4

– T

4

surr)

Assuming the value of α sun = 0.12 as given in Rai (2009),

and the Tsurr= 40°.

The temperature of the outer glass plate is calculated to be 232C

I want to model an adsorption air conditioning process with zeolite and water. I want to study how the temperature varies throughout one cycle of the system. Using Thermolib, I can model the evaporator and condenser, but how can I model the adsorber and desorber?

Dear Vipin Nair,

First of all, domestic refrigerators use capillary expansion devices to create a flow restriction, not expansion valves. The capillary does not adjust the restriction according to the demand as pressure or thermostat valves.

Second, most of the domestic refrigerators have a ordinary On/Off compressor (NOTE: there are many available in the market with variable speed), which means when is powered on the compressor run in one speed only.

Considering those two conditions, yes, the compressor ratio varies according to the ambient temperature. This logically implies in variation of discharge pressure with ambient temperature.

There isn't much that the compressor can do when the ambient goes from cold to warm, besides keep pumping gas. See, the compressor will build a gas flow and a positive pressure difference. But the flow and pressure restriction is controlled by the circuit, mainly by the expansion device (in this case, capillary tube).

So, in the overall, when the refrigerator is running on summer, the pressure ratio is bigger (bigger condensing pressure, small change in evaporating pressure) and the compressor have to run longer time (because the thermal load in the cabinet is bigger, and the temperature difference between evaporator and cold compartment is smaller). In winter, the pressure ratio is smaller (smaller condensing and evaporating pressure) and the compressor run short time (small thermal load in the cabinet and big temperature difference between the evap. and cold compart.)

Keep in mind here that the cooling compartment temperature has the same set point in both cold or warm ambient.

Any doubt feel free to contact me.

Regards

Dear Nilesh:

You can plot or tabulate from Fluent the local heat transfer coefficient h(x) for example in the direction of flow, then numerically integrate these values to get the average heat transfer coefficient. For Tref in your equation, this is the bulk mean temperature of the fluid that is could be the arithmetic mean between inlet and exit.

Look at this doc it may be helpful for you topic. Good luck

To find out whether air temperature lowers in an isenthalpic expansion you may use a state diagram of air. As enthalpy is a state property the final temperature will depend on final pressure and density regardless of the process. Be careful to check whether the process is really isenthalpic, evaluating especially kinetic energy: if inlet and outer ducts of a throttling valve have the same area kinetic energy at the outlet will be higher than at the inlet thus enthalpy will be lower

Look at this doc please it may be helpful for your topic. Good luck.

And don't forget to see if the cappilary can hold the pressure difference. According to the mechanical of material, the stress due the pressure is calculated by: s = p.r/t (normal stress equals to pressure times radius divided by thickness). Use the maximum pressure difference between internal and external environment.

Thank you D.B. Jani

compressors basically divided into two categories

1-positive displacement

2-rotodynamic