Design and calculation method for refrigeration air conditioner evaporator with variable superheat degree

Abstract

The invention discloses a design and calculation method for a variable superheat degree refrigeration air conditioner evaporator. The method comprises the steps that the evaporation temperature t0 of the evaporator is calculated according to input calculation parameters and a preset evaporator structure; the method comprises the following steps: presetting an outlet temperature t2 according to outlet enthalpy, checking a row number Z or correcting the outlet air temperature t2 according to cooling efficiency, determining flow arrangement of an air outlet evaporator, selecting countercurrent flow or downstream flow, inputting a refrigerant inlet state (condensation temperature and supercooling degree), checking and calculating an actual evaporation temperature t0jh according to an assumed evaporation temperature t0, and in the process of checking the evaporation temperature t0jh, checking the actual evaporation temperature t0jh. And meanwhile, the pressure drop dp and the temperature drop dt1 of the refrigerant in the evaporator tube are calculated, whether the evaporation temperature t0jh and the pressure drop dp and the temperature drop dt1 of the refrigerant meet the requirements or not is checked after calculation is completed, and if not, preset parameters need to be adjusted for recalculation. According to the application, the adjusted evaporation temperature and superheat degree can be checked and calculated, and the needed refrigerant evaporation temperature and liquid volume flow can be calculated by adjusting the dryness of the outlet of the evaporator.

Description

Technical Field

[0001] The invention relates to a design calculation method for a refrigeration air-conditioning evaporator with variable superheat, in particular to a design calculation method for an evaporator structure with adjustable superheat at an evaporator outlet applicable to different refrigerants. Background Art

[0002] The evaporator is one of the four major components of the refrigeration and air conditioning system, which has a great influence on the cooling capacity and operating energy efficiency. In actual product design, the temperature, flow rate, and process layout of the cooled medium often change when the evaporator is working. In order to improve the working efficiency of the evaporator and ensure the safe operation of the refrigeration system, the evaporator outlet is usually designed and selected according to the 5℃ superheat. Since evaporation and superheating in the evaporator tube are latent heat and sensible heat exchange processes respectively, the heat transfer coefficient in the tube is very different, and the heat transfer temperature difference in the countercurrent and downstream heat transfer processes is also different, and the theoretical calculation is very complicated. At present, the evaporator design calculation is generally calculated when there is no superheat and the outlet dryness is "1". After the calculation is completed, a certain design margin is added according to experience, and the design calculation is not accurate enough. In particular, the heat transfer effect of the countercurrent and downstream arrangements of non-azeotropic refrigerants with large temperature glide is very different. The non-azeotropic refrigerant has a temperature glide phenomenon during the evaporation process. The evaporation temperature changes due to the change in the gas-liquid phase ratio during the evaporation process of the non-azeotropic refrigerant. The calculation error of the evaporation process according to isothermal treatment is large. In order to adapt to the calculation of fluorine pump type evaporator with "circulation volume / evaporation volume">1, that is, when the superheat is "0" and the evaporator refrigerant outlet dryness is <1, it is necessary to accurately calculate the heat exchange effect of the evaporator. Summary of the invention

[0003] Purpose of the invention: The present invention aims to provide a design and calculation method for a refrigeration and air-conditioning evaporator with variable superheat. In order to be able to perform refined design and calculation of the evaporator, the present invention divides the heat transfer process of the evaporator into a superheat zone and an evaporation zone, and calculates the heat transfer coefficients of the superheat zone and the evaporation zone respectively, and solves the evaporation temperature by iterative calculation and verification of the heat exchange area.

[0004] Technical solution: A design calculation method for a variable superheat refrigeration air-conditioning evaporator, comprising the following steps:

[0005] (1) Setting calculation parameters and evaporator structural parameters, wherein the calculation parameters include cooling capacity Q0, air intake volume V, air intake state dry bulb temperature t1, and wet bulb temperature ts1; the evaporator structural parameters include heat exchange tube outer diameter d0, heat exchange tube wall thickness δt, evaporator tube spacing S, and evaporator row number Z;

[0006] The wind enthalpy h2 is calculated using the following formula:

[0007] h2=h1-3600·Q0 / (V·ρ air / (1+dair / 1000)),

[0008] Where h1 is the inlet air enthalpy, ρ air is the air density, d air is the humidity content of the incoming air;

[0009] (2) Assuming the outlet air dry-bulb temperature t2, obtain the outlet air state point on the air enthalpy-humidity diagram based on the outlet air enthalpy h2 and the assumed outlet air dry-bulb temperature t2, and calculate the intersection temperature ts and the enthalpy hs of the intersection point between the line connecting the inlet air state point and the outlet air state point and the relative humidity 100% line on the air enthalpy-humidity diagram;

[0010] (3) Calculate the cooling efficiency η, η = (h1-h2) / (h1-hs), and substitute the cooling efficiency η into the following formula to check the number of evaporator rows:

[0011]

[0012] In the formula, α a is the convective heat transfer coefficient outside the evaporator tube, a is the rib pass coefficient, Cp air is the air constant pressure specific heat, va is the air headwind speed, and the number of evaporator rows calculated by the above formula is defined as Z1. When Abs(Z-Z1)﹤0.2, the air outlet temperature is adapted to the evaporator structure. Otherwise, the air outlet dry bulb temperature t2 or the evaporator structure parameters need to be adjusted and steps (1)-(3) should be repeated.

[0013] (4) After the number of evaporator rows is determined according to step (3), at the set evaporation temperature t0, the heat transfer coefficient of the evaporation zone and the heat transfer coefficient of the superheating zone are used to calculate the heat exchange area A1JH1 of the evaporation zone and the heat exchange area A1JH2 of the superheating zone. The total heat exchange area of ​​the evaporator A1JH = A1JH1 + A1JH2;

[0014] When (A-A1JH) / A<0.01%, the calibration calculation is completed; otherwise, the actual evaporation temperature t 0jh Instead of the assumed evaporation temperature t0, iterative calculation is performed. Each time the iteration is performed, the assumed t0 data is refreshed once; until (A-A1JH) / A<0.01% is satisfied; A is the total surface area per unit length of the heat exchange tube;

[0015] After the calculation is completed, check whether the calculated evaporation temperature and evaporation temperature drop meet the designer's expected requirements. If not, adjust the evaporator structure according to the calculation results and recalculate.

[0016] Furthermore, the evaporator is a finned evaporator, and the heat exchange tubes of the evaporator are fin tubes.

[0017] Furthermore, the heat transfer coefficient of the evaporation zone and the heat transfer coefficient of the superheating zone are calculated as follows:

[0018] Heat transfer coefficient in evaporation zone:

[0019]

[0020] Heat transfer coefficient in superheated zone:

[0021]

[0022] Where: fin and heat exchange tube contact thermal resistance rcb, heat exchanger outer surface thermal resistance Rf, heat transfer coefficient λ guan , horizontal area per unit length of fin tube Ap, vertical area per unit length of fin tube Af, rib coefficient lhxs, ratio of surface area to base tube surface area lhxso, heat transfer coefficient ab in superheat zone gas , heat transfer coefficient ab in the evaporation zone tube, convection heat transfer coefficient α outside the evaporator tube a , Inner diameter of heat exchange tube d i , air treatment moisture coefficient x, heat exchange tube outer diameter d0. Where ξ=(h1-h2) / (Cp air (t1-t2))

[0023] Furthermore, the heat exchange area A1JH1 of the evaporation zone and the heat exchange area A1JH2 of the superheating zone are calculated as follows:

[0024] Check the calculated heat exchange area of ​​the evaporation zone A1JH1 = 1000·Q 0ZF / (K·Δt);

[0025] Check the calculated heat transfer area of ​​the superheating zone A1JH2 = 1000·Q 0GR / (K gas ·Δt gr );

[0026] Where: The cooling capacity generated by the refrigerant in the superheated zone is Q 0GR , the cooling capacity Q generated by the refrigerant in the evaporation zone 0ZF , heat transfer temperature difference in superheated zone Δt gr , heat transfer temperature difference Δt in the evaporation zone.

[0027] Furthermore, the heat transfer temperature difference Δt in the superheating zone gr The calculation method of the heat transfer temperature difference Δt in the evaporation zone is:

[0028] In case of evaporator countercurrent:

[0029] TZair=t1-(t1-t2)·Q 0GR / Q0

[0030] Heat transfer temperature difference in superheated area

[0031]

[0032] Heat transfer temperature difference in evaporation zone

[0033]

[0034] In downstream flow of the evaporator:

[0035] TZair=(t1-t2)·Q 0GR / Q0+t2

[0036] Heat transfer temperature difference in superheated area

[0037]

[0038] Heat transfer temperature difference in evaporation zone

[0039]

[0040] Where: the air temperature Tzair at the interface between the superheat zone and the evaporation zone, the temperature drop dt1 of the refrigerant at the evaporation temperature t0, and the refrigerant outlet superheat t gr .

[0041] Furthermore, the cooling capacity Q generated by the superheated zone refrigerant is 0GR The cooling capacity Q generated by the refrigerant in the evaporation zone 0ZF The calculation method is:

[0042] Calculate the condenser refrigerant outlet enthalpy Hi, Hi = Hpi-t gl CpLi, where the condenser refrigerant outlet saturated liquid enthalpy Hpi, condenser constant pressure specific heat CpLi, subcooling t gl ;

[0043] Calculate the evaporator refrigerant outlet enthalpy He, He = Hpp·GD + (1-GD)·Hp + t gr ·Cp gas , where the gas enthalpy Hpp at the saturation temperature of the evaporator refrigerant and the refrigerant outlet superheat t gr , refrigerant outlet dryness GD, refrigerant saturated liquid enthalpy Hp, refrigerant constant pressure specific heat Cp gas ;

[0044] Calculate the evaporator refrigerant inlet dryness GD1 = (Hi-Hp) / (Hpp-Hp),

[0045] Refrigerant mass flow rate mr = Q0 / (He-Hi),

[0046] Calculate the liquid refrigerant volume flow rate GV, GV = mr·vvL; vvL is the saturated liquid specific volume;

[0047] Cooling capacity Q generated by the refrigerant in the superheated zone 0GR=mr·t gr ·Cp gas ,

[0048] The cooling capacity Q generated by the refrigerant in the evaporation zone 0ZF =Q0-Q 0GR .

[0049] Beneficial effects:

[0050] It provides guidance for the design of finned evaporators for air cooling (dehumidification), can handle the design and verification calculations of various complex working conditions, and reasonably control the design margin.

[0051] When air-conditioning products need to perform evaporator load adjustment calculations for the cooling season energy efficiency ratio SEER (air-cooled type) and the integrated part load performance coefficient IPLV (water-cooled type), it may be necessary to adjust the evaporator outlet superheat or air volume calculation. The calculation method of this application can verify the adjusted evaporation temperature and superheat.

[0052] In the design of the fluorine pump type evaporator, the outlet evaporator superheat is set to "0". By adjusting the evaporator outlet dryness, the required refrigerant evaporation temperature and liquid volume flow rate can be calculated using this application.

[0053] There is temperature glide during the evaporation of non-azeotropic refrigerant (R407C). ​​The heat exchange effects are quite different when the evaporator is arranged co-currently or counter-currently. The evaporator design parameters can be optimized through calculation and the process can be arranged reasonably. BRIEF DESCRIPTION OF THE DRAWINGS

[0054] Figure 1 It is a state change diagram of the refrigerant and the cooled medium in the azeotropic refrigerant evaporator;

[0055] Figure 2 It is a state change diagram of the refrigerant and the cooled medium in the non-azeotropic refrigerant evaporator;

[0056] Figure 3 The figure is a calculation flow chart of the present invention. DETAILED DESCRIPTION

[0057] The technical solution of the present invention is further described below in conjunction with the accompanying drawings.

[0058] Taking the calculation of finned air evaporator as an example, the heat transfer process of the evaporator is simplified as follows:

[0059] The superheat heat transfer is smaller than the evaporation heat transfer, and the air side flow rate is the same. The air side heat transfer coefficients of the superheat zone and the evaporation zone are consistent during calculation; the superheat part is on the entire air side, the superheat air side temperature drop / evaporation zone air temperature drop = superheat heat transfer / evaporation heat transfer; the refrigerant pressure drop is calculated in the evaporation zone, and the temperature drop corresponding to the pressure drop is considered at the same time. The pressure drop is calculated on the refrigerant side of the superheat part, and the temperature drop is not calculated; the gaseous physical properties parameters (specific heat Cp gas , dynamic viscosity μ gas , specific volume vv, thermal conductivity λ gas , Prandtl number Pr gas ) is calculated based on the evaporator outlet saturation temperature t0.

[0060] A design calculation method for refrigeration and air conditioning evaporator with variable superheat. The heat transfer process of the evaporator is analyzed by taking the calculation of finned air evaporator as an example. Figure 1 (azeotropic refrigerants) and Figure 2 (Non-azeotropic refrigerants).

[0061] Figure 1 and Figure 2 The air inlet temperature is t1, the air outlet temperature is t2, the evaporator refrigerant outlet dew point temperature is t0, and the evaporator outlet superheat is t gr , the (imaginary) air temperature TZair at the interface between the superheat zone and the evaporation zone.

[0062] Take the evaporator heat transfer calculation steps of azeotropic refrigerant as an example:

[0063] Step 1: Input the air inlet state dry bulb temperature t1, wet bulb temperature ts1, atmospheric pressure B; input the evaporator structural parameters, heat exchange tube outer diameter d0, wall thickness δt, tube spacing S, heat exchange tube material, fin spacing e, fin thickness δp, fin material, and heat exchanger row number Z.

[0064] Calculate the inlet air enthalpy h1 and density ρ according to the inlet air parameters air , moisture content d air. .

[0065] Outlet enthalpy h2 = h1-3600·Q0 / (V·ρ air / (1+d air / 1000))

[0066] Where ρ air Air density, d air Inlet air humidity

[0067] Calculation 2 process: Input the assumed outlet air dry bulb temperature t2, calculate and determine the outlet air state point based on the outlet air enthalpy h2 and the assumed outlet air dry bulb temperature t2, and calculate the temperature ts and the enthalpy hs of the intersection point between the inlet air state point and the outlet air state point and the connecting line with the relative humidity 100% line on the air enthalpy and humidity diagram;

[0068] Calculation process 3: Based on the assumed parameters, calculate the horizontal area per unit length of the evaporator fin tube Ap, the vertical area per unit length of the fin tube Af, the total surface area per unit length of the fin tube A, the rib coefficient lhxs, and the ratio of the surface area to the surface area of ​​the base tube lhxso. The calculation process can be found in relevant information.

[0069] Calculate the cooling efficiency η, η = (h1-h2) / (h1-hs)

[0070] Calculate the air side heat transfer coefficient αa, αa = C1C2 (λ / de) (LL / de) n Re m

[0071] Where: C1, C2, n, m are coefficients. λ is the air thermal conductivity, de is the equivalent diameter of the air flow cross section, LL is the length of the fin in the air flow direction, and Re is the Reynolds number of the air flowing through the equivalent diameter. αa is the value for sequential arrangement, and is 1.1 times for staggered arrangement.

[0072] Check whether the calculated parameters such as evaporator outlet air temperature, number of rows, headwind speed, etc. are correct.

[0073]

[0074] Check the number of heat exchanger rows, (where αa is the evaporator tube external convection heat transfer coefficient, a is the rib pass coefficient, Z is the number of evaporator rows, Cp air The air constant pressure specific heat, va air headwind speed), the row number Z calculated by the verification row number formula is defined as Z1. When Abs(Z-Z1)﹤0.2, the air outlet temperature is adapted to the evaporator structure, otherwise the air outlet dry bulb temperature t2 or the evaporator structure needs to be adjusted to re-calculate "3".

[0075] Calculate the moisture separation coefficient ξ and fin efficiency ηf.

[0076] ξ=(h1-h2) / (Cp air (t1-t2))

[0077] ηf=th(m·l) / (m·l) (where m is the fin shape parameter and l is the equivalent fin height. Please refer to relevant materials for calculation.)

[0078] The air side equivalent heat transfer coefficient αae = αa·ξ·(ηf·Af+Ap) / A.

[0079] Calculation 4 process: Input condensation temperature tk, subcooling degree t gl , evaporation temperature t0, refrigerant outlet superheat t gr , number of refrigerant branches N, evaporator height H, refrigerant outlet dryness GD.

[0080] The heat exchange of refrigerant in the superheated zone tube is a gas phase sensible heat exchange process, and the heat transfer coefficient in the tube is ab gas It is related to the inner diameter of the heat exchange tube di, the Reynolds number RE2gas, the Prandtl number Prgas and other parameters. The heat transfer coefficient ab in the superheated zone can be calculated based on relevant data. gas (Different formulas are selected according to different RE2gas sizes) calculation.

[0081] The refrigerant in the evaporation zone tube is a gas-liquid two-phase latent heat exchange process.

[0082] Heat transfer coefficient in the evaporation zone tube ab=0.00573λ·μ -0.8 ·Ψ 0.4 ·vm 0.4 / di 0.6 , other algorithms can also be used.

[0083] (Where: Ψ heat flux density of the evaporation zone on the inner surface, Ψ = Q 0ZF / A iZF . (A iZF is the surface area of ​​the evaporation zone), λ is the thermal conductivity of the refrigerant liquid phase, μ is the liquid phase dynamic viscosity, vm is the mass flow rate of the refrigerant in the tube, and di is the inner diameter of the tube.

[0084] Calculate the heat transfer coefficient ab in the superheated zone of the evaporator tube gas When calculating the heat transfer coefficient ab of the evaporation zone in the evaporator tube, it is necessary to calculate the flow rate, flow velocity and other parameters in the evaporator tube. The calculation is relatively complicated. The general process is as follows:

[0085] According to the input condenser outlet parameter condensation temperature tk, the condenser refrigerant outlet saturated liquid enthalpy Hpi and condenser constant pressure specific heat CpLi are calculated. Then according to the subcooling degree t gl , calculate the enthalpy Hi at the condenser outlet (i.e., evaporator inlet when outlet resistance is not considered);

[0086] Hi=Hpi-t gl ·CpLi

[0087] According to the input assumed evaporation temperature t0, calculate the gas enthalpy Hpp and the refrigerant constant pressure specific heat Cp at the evaporator refrigerant saturation temperature gas , saturated gas specific volume vv, thermal conductivity λ gas , dynamic viscosity μ gas , Prandtl number Pr gas; Evaporator refrigerant saturated liquid enthalpy Hp, saturated liquid specific volume vvL, thermal conductivity λ, dynamic viscosity μ.

[0088] Calculate the evaporator outlet enthalpy He;

[0089] He=Hpp·GD+(1-GD)·Hp+t gr ·Cp gas

[0090] Evaporator refrigerant inlet dryness GD1 = (Hi-Hp) / (Hpp-Hp)

[0091] Refrigerant mass flow rate mr = Q0 / (He-Hi)

[0092] Cooling capacity Q generated by the refrigerant in the superheated zone 0GR =mr·t gr ·Cp gas

[0093] The cooling capacity Q generated by the refrigerant in the evaporation zone 0ZF =Q0-Q 0GR

[0094] When the outlet superheat t gr =0, outlet dryness ≤1, refrigerant circulation ratio = (Hpp-Hi) / (He-Hi). Calculate the liquid refrigerant volume flow rate GV, GV = mr·vvL.

[0095] The mass flow rate of refrigerant in the tube vm = mr / (LTJMJ·N) (where: LTJMJ is the cross-sectional area of ​​a single heat exchange tube.)

[0096] Refrigerant outlet saturated vapor flow rate Wgas = vm·vv

[0097] Reynolds number of saturated steam at refrigerant outlet RE2gas=Wgas·di / (μgas·vv)

[0098] Heat transfer coefficient in evaporation zone:

[0099]

[0100] Heat transfer coefficient in superheated zone

[0101]

[0102] (Where: Rcb is the contact thermal resistance between the fin and the heat exchange tube, Rf is the thermal resistance of the outer surface of the heat exchanger, λ guan Thermal conductivity of the heat exchange tube. Generally, the thermal resistance on the refrigerant side is 0, which is not indicated in the formula.)

[0103] Calculation of refrigerant resistance in the evaporator tube:

[0104]

[0105] (Where: is the average specific volume in the evaporator, l is the total length of each straight tube in the evaporator, WT is the total number of elbows in each tube, and f is the two-phase flow resistance coefficient of the evaporator, which can be calculated by referring to relevant data.)

[0106] According to the refrigerant Δp in the tube, the corresponding temperature drop dt1 at the evaporation temperature t0 of the corresponding refrigerant can be calculated.

[0107] Heat exchange area verification calculation (taking azeotropic refrigerant as an example), Figure 1 This is the fluid temperature state diagram of the azeotropic refrigerant evaporator.

[0108] 1. Backflow

[0109] TZair=t1-(t1-t2)·Q 0GR / Q0

[0110] Heat transfer temperature difference in superheated area

[0111]

[0112] Heat transfer temperature difference in evaporation zone

[0113]

[0114] 2. Downstream

[0115] TZair=(t1-t2)·Q 0GR / Q0+t2

[0116] Heat transfer temperature difference in superheated area

[0117]

[0118] Heat transfer temperature difference in evaporation zone

[0119]

[0120] Check the calculated heat exchange area of ​​the evaporation zone A1JH1 = 1000·Q 0ZF / (K·Δt);

[0121] Check the calculated heat transfer area of ​​the superheating zone A1JH2 = 1000·Q 0GR / (K gas ·Δt gr );

[0122] Check and calculate the total evaporator heat exchange area A1JH=A1JH1+A1JH2.

[0123] Conditions for judging the completion of the calibration calculation: When (A-A1JH) / A<0.01%, the calibration calculation is completed, otherwise the new t 0jh Instead of the assumed t0, iterative calculation is performed, and the assumed t0 data is refreshed every time it is iterated until the conditions are met.

[0124] For the case of Δtgr=0, A1JH2=0, in order to improve the calculation efficiency, the iterative method can be used instead of the direct solution method to calculate the evaporation temperature t 0jh Faster calculation speed.

[0125] After the calculation is completed, check whether the calculated evaporation temperature and evaporation temperature drop meet the requirements. If not, adjust the evaporator structure according to the calculation results and recalculate.

[0126] For non-azeotropic refrigerants (for near-azeotropic refrigerants such as R410A and R404A, the calculation method can be used for azeotropic refrigerants), since the temperature of the refrigerant will rise during the isobaric evaporation process, there is a temperature glide phenomenon. The temperature glide must be considered when calculating the heat transfer temperature difference. The temperature glide value is calculated in linear proportion to the change in refrigerant dryness. Figure 2 This is the evaporator fluid temperature state diagram for non-azeotropic refrigerants. The evaporator calculation method is the same as that for azeotropic refrigerants and will not be repeated here.

[0127] The present invention discloses a design calculation method for a variable superheat refrigeration air conditioning evaporator. The method is based on the refrigeration capacity Q0, refrigerant model, flow rate V of the cooled medium, air inlet state (dry bulb temperature t1, wet bulb temperature ts1, atmospheric pressure B), refrigerant outlet superheat t gr , calculate the evaporator evaporation temperature t0 according to the preset evaporator structure (the windward surface size structure of the evaporator can also be calculated according to the evaporation temperature).

[0128] Taking the calculation of the finned air evaporator for air treatment as an example, the preset evaporator structural parameters include the outer diameter d0 of the tube, the tube wall thickness δ t , tube material, number of rows Z, height H, number of branches N, single-flow tube length (evaporator internal dimension L), fin spacing e, fin thickness δ p , fin material. During the evaporator calculation process, the outlet temperature t2 needs to be preset according to the outlet enthalpy, and then the number of rows Z or the outlet temperature t2 needs to be corrected according to the cooling efficiency. Determine the flow layout of the outlet evaporator and choose countercurrent or cocurrent. Enter the refrigerant inlet state (condensing temperature, subcooling), and calculate the actual evaporation temperature t based on the assumed evaporation temperature t0. 0jh , in checking the evaporation temperature t 0jh During the process, the refrigerant pressure drop dp and temperature drop dt1 in the evaporator tube are calculated at the same time. After the calculation is completed, check the evaporation temperature t 0jhAnd whether the refrigerant pressure drop dp and temperature drop dt1 meet the requirements, otherwise the preset parameters need to be adjusted and recalculated.

[0129] The method described in the patent of this invention can also be used to design dry evaporators, and can also be extended to the calculation of steam heaters for superheated steam. These methods have been applied in the company's dry evaporator and steam heater design software.

[0130] Appendix: Measurement units in this article

[0131] Cooling capacity Q0, Q 0GR , Q 0ZF kW

[0132] Air volume V m 3 / h

[0133] Temperature t1, ts1, t2, ts2, ts, tk, t gr ,t gl ,t0,t 0jh ,,TZair,dt1℃

[0134] Air humidity d air. g / kg 干空气

[0135] Air enthalpy h1, h2, hs kJ / kg 干空气

[0136] Refrigerant enthalpy Hp, Hpp, Hpi, Hi, He kJ / kg

[0137] Specific heat Cp air 、CpLi、Cp gas kJ / (kg·K)

[0138] The rest are in international units and derived units.

[0139] The above is only a brief description of the implementation method of the present invention. It should be pointed out that for ordinary technicians in this technical field, without departing from the basic calculation method of the present invention, several design functions can be modified; the iteration step size and calculation accuracy can also be adjusted to change the design accuracy and calculation speed; these improvements and modifications should also be regarded as the scope of protection of the present invention.

Claims

1. A design calculation method for a variable superheat refrigeration and air conditioning evaporator, characterized in that: The following steps are involved: (1) Setting calculation parameters and evaporator structural parameters, wherein the calculation parameters include cooling capacity Q0, air intake volume V, air intake state dry bulb temperature t1, and wet bulb temperature ts1; the evaporator structural parameters include heat exchange tube outer diameter d0, heat exchange tube wall thickness δt, evaporator tube spacing S, and evaporator row number Z; The wind enthalpy h2 is calculated using the following formula: h2=h1-3600·Q0 / (V·ρ air / (1+d air / 1000)), Where h1 is the inlet air enthalpy, ρ air is the air density, d air is the humidity content of the incoming air; (2) Assuming the outlet air dry-bulb temperature t2, obtain the outlet air state point on the air enthalpy-humidity diagram based on the outlet air enthalpy h2 and the assumed outlet air dry-bulb temperature t2, and calculate the intersection temperature ts and the enthalpy hs of the intersection point between the line connecting the inlet air state point and the outlet air state point and the relative humidity 100% line on the air enthalpy-humidity diagram; (3) Calculate the cooling efficiency η, η = (h1-h2) / (h1-hs), and substitute the cooling efficiency η into the following formula to check the number of evaporator rows: In the formula, α a is the convective heat transfer coefficient outside the evaporator tube, a is the rib pass coefficient, Cp air is the constant pressure specific heat of air, va is the air headwind velocity, and the number of evaporator rows calculated by the above formula is defined as Z1. When Abs(Z-Z1)﹤0.2, the outlet air temperature is adapted to the evaporator structure. Otherwise, the outlet air dry bulb temperature t2 or the evaporator structure parameters need to be adjusted and steps (1)-(3) should be repeated. (4) After the number of evaporator rows is determined according to step (3), at the set evaporation temperature t0, the heat transfer coefficient of the evaporation zone and the heat transfer coefficient of the superheating zone are used to calculate the heat exchange area A1JH1 of the evaporation zone and the heat exchange area A1JH2 of the superheating zone. The total heat exchange area of ​​the evaporator A1JH = A1JH1 + A1JH2; When (A-A1JH) / A<0.01%, the calibration calculation is completed; otherwise, the actual evaporation temperature t 0jh Instead of the assumed evaporation temperature t0, iterative calculation is performed. Each time the iteration is performed, the assumed t0 data is refreshed once; until (A-A1JH) / A<0.01% is satisfied; A is the total surface area per unit length of the heat exchange tube; After the calculation is completed, check whether the calculated evaporation temperature and evaporation temperature drop meet the designer's expected requirements. If not, adjust the evaporator structure according to the calculation results and recalculate.

2. A design calculation method for a variable superheat refrigeration and air conditioning evaporator according to claim 1, characterized in that: The evaporator is a finned evaporator, and the heat exchange tubes of the evaporator are fin tubes.

3. The design calculation method for a variable superheat refrigeration and air conditioning evaporator according to claim 2 is characterized in that: The calculation formulas for the heat transfer coefficient of the evaporation zone and the heat transfer coefficient of the superheating zone are as follows: Heat transfer coefficient in evaporation zone: Heat transfer coefficient in superheated zone: Where: fin and heat exchange tube contact thermal resistance rcb, heat exchanger outer surface thermal resistance Rf, heat transfer coefficient λ guan , horizontal area per unit length of fin tube Ap, vertical area per unit length of fin tube Af, rib coefficient lhxs, ratio of surface area to base tube surface area lhxso, heat transfer coefficient ab in superheat zone gas , heat transfer coefficient ab in the evaporation zone tube, convection heat transfer coefficient α outside the evaporator tube a , Inner diameter of heat exchange tube d i , air treatment moisture separation coefficient x, heat exchange tube outer diameter d0.

4. A design calculation method for a variable superheat refrigeration and air conditioning evaporator according to claim 3, characterized in that: The specific calculation of the heat exchange area A1JH1 of the evaporation zone and the heat exchange area A1JH2 of the superheating zone is: Check the calculated heat exchange area of ​​the evaporation zone A1JH1 = 1000·Q 0ZF / (K·Δt); Check the calculated heat transfer area of ​​the superheating zone A1JH2 = 1000·Q 0GR / (K gas ·Δt gr ); Where: The cooling capacity generated by the refrigerant in the superheated zone is Q 0GR , the cooling capacity Q generated by the refrigerant in the evaporation zone 0ZF , heat transfer temperature difference in superheated zone Δt gr , heat transfer temperature difference Δt in the evaporation zone.

5. A design calculation method for a variable superheat refrigeration and air conditioning evaporator according to claim 4, characterized in that: The heat transfer temperature difference Δt in the superheated zone gr The calculation method of the heat transfer temperature difference Δt in the evaporation zone is: In case of evaporator countercurrent: TZair=t1-(t1-t2)·Q 0GR / Q0 Heat transfer temperature difference in superheated area Heat transfer temperature difference in evaporation zone In downstream flow of the evaporator: TZair=(t1-t2)·Q 0GR / Q0+t2 Heat transfer temperature difference in superheated area Heat transfer temperature difference in evaporation zone Where: the air temperature Tzair at the interface between the superheat zone and the evaporation zone, the temperature drop dt1 of the refrigerant at the evaporation temperature t0, and the refrigerant outlet superheat t gr .

6. A design calculation method for a variable superheat refrigeration and air conditioning evaporator according to claim 4, characterized in that: The cooling capacity Q generated by the refrigerant in the superheated zone 0GR The cooling capacity Q generated by the refrigerant in the evaporation zone 0ZF The calculation method is: Calculate the condenser refrigerant outlet enthalpy Hi, Hi = Hpi-t gl CpLi, where the condenser refrigerant outlet saturated liquid enthalpy Hpi, condenser constant pressure specific heat CpLi, subcooling t gl ; Calculate the evaporator refrigerant outlet enthalpy He, He = Hpp·GD + (1-GD)·Hp + t gr ·Cp gas , where the gas enthalpy Hpp at the saturation temperature of the evaporator refrigerant and the refrigerant outlet superheat t gr , refrigerant outlet dryness GD, refrigerant saturated liquid enthalpy Hp, refrigerant constant pressure specific heat Cp gas ; Calculate the evaporator refrigerant inlet dryness GD1 = (Hi-Hp) / (Hpp-Hp), Refrigerant mass flow rate mr = Q0 / (He-Hi), Calculate the liquid refrigerant volume flow rate GV, GV = mr·vvL; vvL is the saturated liquid specific volume; Cooling capacity Q generated by the refrigerant in the superheated zone 0GR =mr·t gr ·Cp gas , The cooling capacity Q generated by the refrigerant in the evaporation zone 0ZF =Q0-Q 0GR .

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