1Performance Comparison Analysis of the Heat Pump Water Heater with Internal and External Coil

Abstract: The storage buffer tank of air source heat pump water heater consist of inner tank,outer tank, heating exchange coil, insulation material etc. The heating exchange coil is thecondenser in the heat pump system, through the heating exchange coil, the refrigerant with hightemperature high pressure transfer the heat energy to the water, and increase the watertemperature. According to the structure of coils, it can divided into the heat pump water heaterwith internal coil, the heat pump water heater with external coil. This article introduces thetechnology applied to the buffer tank with internal coil.Key words: Internal coil heat pump water heater, heating exchange coil, stainless steel buffer tank,anti-corrosion performance, corrosion protection wall

.IntroductionAt present, a kind of heat pump buffer tank with internal stainless steel coil which applied to heatpump water heater(Patent no.: 201420041158.9) was developed successfully by HuangshiDongbei Mech-Electrical Group Solar Energy Co., Ltd, and applied for a number of patentprotection.As the main equipment in heat pump water heater, the storage tank played an important role forthe performance of the water heater, such as the heating exchange performance(internalcondenser), insulation performance, hot water output rate, the corrosion performance of innertank. But at present market, the external coil enamel water tank, plastic water tank has manyquality effects, such as: the performance external heating exchange coil is very bad, the enamel isnot complete, the heat-expansion and cold-contraction lead to crack in the enamel, and theenamel water tank have to be protective by Mg rod, and it will affect the water quality after theMg sacrifice. The differences water quality in different areas made it very difficult to maintenancethe Mg rod. The stainless steel buffer tank produced by Donper has some features such aspressurized, clean, high heating efficiency, strong anti-corrosion ability(the anti-corrosion patentno.:201220394501.9), long service time etc. These advantages made our stainless steel watertank became a famous products for the heat pump water heater, boiler heating system and solarheating system.

.Comparison analysis of the performance of external and internal coil.1Heating exchange efficiencyThe heating exchange efficiency is a important indicator, it impact the coefficient of performanceand energy efficiency ratio (COP) of the heat pump water heater.In the recently heat pump water heater, the heating exchanger of water tank mainly is coilstructure. Divided by the material of coil, they are : cooper coil,aluminum coil and stainless steelcoil.Divided by structure of coil, they are: external coil and internal coil. For the external coil,because that inner tank is twined by the coil outside, the heat in the coil transferred to the waterthrough the inner tank plate, this process made the heating efficiency lower. Especially for theenamel water tank, the heat transferred by the inner tank plate and enamel layer, it has twiceheating transfer, and it has thermal resistance in every heating transfer, and the enamel layer is

2non-metallic material, so its heat transfer coefficient is much lower than metallic material, so theheat transfer efficiency of the heat pump buffer tank with external coil is much lower than theheat transfer efficiency of the eat pump buffer tank with internal coil.By the way, in the design of heat pump system, the size of condenser(coil) is decided by thecompressor, if the area of condenser(coil) is too large, it will lead to burnout of compressorbecause the lubricating oil of compressor cannot flow back. But if the area of condenser(coil) istoo small, it will made the temperature of condenser(coil) is to high, the compressor will be shutdown, and it will shortage the service life of compressor. So if the heat pump water tank withexternal coil want to reach the same heat exchange amount as the internal coil, though itincrease the heat exchange area of coil, it still cannot reach to the same heat transfer efficiencyand energy efficiency ratio (COP) of the internal coil.

For the structure of copper coil twined type, it can be divided into 3 types: 1. traditionalround tube, 2. D type external coil, 3. Fluted type coil. For the traditional round coil, the contactarea is the line of coil and the surface of inner tank, the effective heat transfer area is very less.For the D type external coil, the contact area is the surface of coil and the surface of inner tank, itwill be better than former one, but the effective heat transfer area is still very less, the heattransfer speed is very slow, and the energy efficiency ratio is very low, and the copper coil will beinfluenced by heat-expansion and cold-contraction phenomenon which will change the contactarea between the external coil and inner tank, the heat transfer efficiency will be decrease everyyear. And the two corners of D type coil are very easy to be broken because that their force areuneven and under heat-expansion and cold-contraction condition. For the fluted type coil, theinner tank have to be shaped in waves type, the copper coil installed into the fluted, the contactarea is higher than D type coil, but the max. Contact area only have 50%. And its heat transferringstill have to though the inner thank, so is energy efficiency ratio is also very low. And because theneed of fluted technology, so the inner tank becomes more thinner, the press process of flutedwill impact the quality of welding connector, and impact the service life of inner tank.The requirement of structure design of copper coil, it requires the coil and inner tank are clingingclosely, and you have to pay attention to the space between the coils, if the interval is too small,the utilization efficiency of condenser tube is very low. And if the interval is too big, the heatexchange area is not enough. To increase the heat exchange efficiency, the coil should be twinedtightness, which will lead to overlength of the condenser system, and the inside flow resistance isvery big, and made the flow velocity of refrigerating medium in the coil is low, the heat exchangeefficiency will be decrease.But for the heat pump water heater with internal coil, the coil(condenser tube) put into the innertank directly, it is contact with the water directly. The tube is fully participate in heat transfer, theheat transfer temperature difference is low, the pressure of condenser is lower, it will not impactthe service life of compressor. For the heat exchange tube with same length, if the effective heattransfer area is high, its heat exchange efficiency is good. And the surface of coil contact waterdirectly, so whichever the coil is, its heat transfer efficiency is high.Please check following the calculation of the heat resistance of external coil and internal coil.

For built-in coil heat pump water heater, do a heat transfer coefficient equation, whichsubject to the outboard area of coil:

3Heat resistance

In the equation, stands for total heat transfer coefficient, stand for external diameter

and internal diameter, stands for heat coefficient of coil material, stands for heat

coefficient of external surface and internal surface, stand for area of external surface

and internal surface, stands for length of coil.

In this equation, stands for radius of curvature of spiral coil, stands for slave cernet

number, stands for Reynolds number, stands for Prandtl number, stands for fluid

density, stands for fluxion of internal coil, stands for kinematic viscosity.

In this equation, stands for thermal conductivity, stands for Grashof number, stands

4for acceleration of gravity, stands for volume expansion ratio of fluid, stands for

temperature difference.For external coil heat pump water heater, heat transfer coefficient equation is as below:

In this equation, stand for external diameter and internal diameter of inner tank,

stands for heat conductivity coefficient of inner tank, stands for heat transfer coefficient of

inner tank, stands for external area of inner tank.

Compare with heat resistance of built-in coil heat pump water heater and external coil heatpump water heater:

In this quotation, stands for forced convection heat resistance, stands for heat

conduction of coil, stands for natural convection of external surface coil,

stands for heat conduction of inner tank, stands for natural convection heat resistance of

internal surface of inner tank.As for

In a similar way

Because the internal diameter of inner tank is longer than external diameter of coil. That is

5And

Clear from the quotation, the heat resistance of external coil type is bigger than the heatresistance of built-in coil type, that means heat exchange performance of external coil type isworse than built-in coil type. At the same time that, compare with heat resistance of externalcoil and heat resistance of built-in coil, the former has heat conduction resistance of inner tank

, but the latter has not. Change external coil convection heat resistance to

inner tank convection heat resistance and performance reduction. If use enamel water

tank, adding enamel layer in the inner tank. But enamel layer is nonmetal, which has bigger heatresistance and bring about the worse heat exchange performance of coil for heat pump watertank and longer heating time of whole system.2Effect for service life of compressorCompressor is like a heart of whole system, which effect the COP of whole system. As their ownfactor of external coil heat pump is bad heat exchange performance, coil covered with insulationlayer, refrigerant in the coil is in a state of high temperature for a long time, while the system isrunning. This will cause the compressor suction, exhaust temperature, high overload, whichmakes serious shorten the service life of a compressor at long time.The solution of solving the phenomenon above from most heat pump manufacturers is to reducethe amount of refrigerant charge. Through research and analysis ,if adopt to this solution to solveproblem, will result in not enough refrigerant in heat pump system and less heat absorption ofheat pump , also cause the time of heating will longer than before. For compressor, the lessrefrigerant charge, the less flow will be got. Overheat of compressor makes heat absorptiontemperature rise and inhalation more volume of steam. And this will cause exhaust temperature,compressor overheat and then cause compression increases, the power consumption increases,which is very unfavorable for the energy saving and service life of the whole system. All result inthe COP of heat pump.

3The analysis of anti-corrosionWe have discussed that heat exchange performance of built-in coil type is better than externalcoil type. Even though, most heat pump water tank are adopt to external coil in the market.Though survey and analysis, most buffer tank manufacturer can not solve the corrosion problemof built-in coil in the water and corrosion phenomenon is horrible especially at the bad waterquality district. For example, almost all broken buffer tank are corroded built-in coil at Yichuancity, He Nan province, China. In order to avoid the corrosion, most manufacturers are adopt to

6produce external coil heat pump water tank.Huangshi Dongbei Mech-Electrical Group Solar Energy Co.,Ltd has original patent anti-corrosiontechnology(patent number: 201220394501.9) ---Corrosion Protection Wall, which means thesurface of water tank components are on passivating treatment, such as inner tank, water pipeand coil which should contact with water. Passivation is the key for the anti-corrosion technology.Finally compact and even passivation coat with content of chromium will be formed, whichcompletely covered the surface of metal.

The comparative experiment between SUS304 stainless steel with/without anti-corrosiontreatment:

Experiment 1:Put two pieces of SUS304 stainless steel welded by full-automatically argon arc(one is withanti-corrosion treatment,the other is without) into the 5% salty water and pure waterrespectively.

Results: After two-month immersion,the observation shows that SUS304 stainless steel withanti-corrosive treatment hardly corrodes,while the one without anti-corrosive treatmentcorrodes terribly.Attention:Places such as Northwest and North China,which have the worst water quality(Cl inwater) is better than the experimental 5% salty water(NaCl liquid).

Experiment 2:Put two pieces of SUS304 stainless steel welded by full-automatically argon arc(one is withanti-corrosion treatment,the other is without) into 5% salt spray test box for 16h(form 1)

Form 1 Salt spray test

No. Test time Photos Results

Diagram 2Diagram 1

5% salty water

With passivatingtreatment

Withoutpassivatingtreatment

Pure water

7SUS304stainless steelwelded byfull-automatically argon arcwithoutanti-corrosivetreatment

Before test

After test

Welding partscorrode terribly

SUS304stainless steelwelded byfull-automatically argon arcwithanti-corrosivetreatment

Before test

After test

Welding partsdo not corrode

In experiment 1,the piece in diagram 2 is without Passivation treatment. SUS304 stainless steelitself has anti-corrosion ability and when it exposes to the air, a protection layer(Cr2O3)will formon the surface of it, which can prevent the steel from corroding, but after a series of procedures:cutting, cold working and heating during welding, the protection layer will be damaged to somedegree. Since water contain Cl-, it wil corrode the stainless steel whose protection layer isdamaged. However, in form 1, the stainless steel, treated with Donper Solar s innovativeanti-corrosive technology patent do not corrode.

.Perfect quality originates from 48-year management and manufacture genesDonper Solar,as a subsidiary company of Donper Group, inherits 48-year management and

manufacturing genes. Donper Solar has an excellent R&D team,who aims at peoples need fordifferent types of new energy(heat pump, solar, HVAC)and R&D various types of buffer tanks.Donper Solar has 10-year experience in R&D stainless pressure buffer tank and advancedmanufacturing equipments. The buffer tank with internal coils integrates many technical skills,including the anti-corrosion and welding skills of stainless steel and the corrosion protection walltechnical skills.

Donper Solar possesses leading stainless steel technology and enjoys praises from consumerswith perfect after-sale services and no compliant feedback for 10 years. In heat pump industry,solar industry and HVAC industry, buffer tank of Donper Solar has become the appointedproducts for most consumers.

8Bibliography1.The use of heat insulation and preservation types in technics system design. HG/T 20570. 11-952. Qian Jian ,Wu Jianghong. 2008. Performance Comparison Analysis of Heat Pump Water HeatersJ.26( 4) : 10 -14.3. Hao Jibo, Wang Zhihua,J iang Yuguang.2013. Performance Comparison Analysis of Air SourceHeat Pump Water HeatersJ.13( 1) : 59- 624.Chen Zhiguang, Qin Zhaokui, Dai Wanneng.2010.Experimental Research on Heat TransferCoefficient of Spiral Tube and Numerical Simulation of Temperate FieldJ.30( 1) : 16-185. Zhu Honghua, Yang Xingtuan, Ju Huaiming.2012. A Research on Partial Heat Transfer Featuresof Water and Steam in Spiral TubeJ.Journal of Thermal Science and Technology,11( 1) : 27 -336. Yang Shiming, Tao Wenquan. Heat Transfer.7.The standard of stainless steel passivation salt spray test ASTM/G 85-1998 PassivatingTreatment of Anti-corrosive steel QQ-P-35, The Textbook of stainless steel parts passivation ASTMA380-068. Zhang Liangjun, Wu Jingyi, Wang Ruzhu.2006.Experiment and analysis on effects of charge onthe Performance of Heat Pump Water Heaters J .Journal of Shanghai Jiaotong University,2006,40( 8) :1307- 1311.9. Kong Xiangqiang, Zhang Dong, Li Ying.2010.Effect of Refrigerant charge on the performance ofa solar assisted heat pump water heater J .Journal of Shanghai Jiaotong University, 201044(10):1372-137710. Li Yuwu, Wang Ruzhu, Wang Taihua.2006. Experimental Study on the OperationalCharacteristics of Direct Expansion Solar Assisted Heat Pump Water Heater J .Journal ofEngineering Thermophysics, 27( 6) : 923- 925.