Research Progress on the application of the hottes

Progress in the application of gas-fired infrared radiant heating technology

gas-fired infrared radiant heating is an efficient and energy-saving heating method. It is especially suitable for places where traditional heating methods (radiators, heaters) are difficult to solve, such as open space, large space, large heat loss, rapid heating, intermittent heating, large air exchange places, etc. This paper discusses the application and research progress of gas-fired infrared radiant heating technology

1 calculation of heat load

literature [1] gives the calculation method of heat load of gas-fired infrared radiant heating system: it is recommended that the calculation method of heat load of gas-fired infrared radiant heating room is similar to that of convective heat transfer heating room, both of which adopt the same indoor temperature and regard the indoor temperature as the only control parameter. This method divides the heat load into envelope heat transfer and air infiltration heat transfer [1]

heat transfer of enclosure structure. The calculation formula of is:

where Ф B - heat transfer of enclosure structure, w

a - area of enclosure structure, m2

tin - indoor temperature, ℃

to - outdoor temperature, ℃

r - thermal resistance from inner wall surface of enclosure structure to outdoor air, (M2 · K)/w

rs - thermal resistance from indoor air to inner wall surface of enclosure structure, (M2 · K)/w

infiltration heat transfer. The calculation formula of is:

Ф A=0.018mv (tin to) (2)

where Ф A - heat transfer of air infiltration, w

m - Ventilation times, times/h

v - volume of indoor space for heating, m3

because the heat transfer properties of gas-fired infrared radiant heating and convective heat transfer heating are different, many scholars have questioned this calculation method, and established various models for discussion and verification. Ng et al. Compiled a psu-mrt program to simulate the heat load of five types of exterior enclosures when using gas-fired infrared radiant heating under the same indoor thermal comfort conditions, and compared the results with the heat load calculation method recommended by ASHRAE. The conclusion is that the thermal load obtained by ASHRAE method is 3% - 21% lower than that calculated by simulation

and well et al. Concluded through simulation calculation that: for ceiling gas-fired infrared radiant heating, the thermal load obtained by ASHRAE method is 3% - 15% higher than that calculated by simulation; For the medium and high temperature gas-fired infrared radiant heating system, the thermal load obtained by ASHRAE method is 5% - 17% higher than that calculated by simulation. Well also gives the calculated heat load correction of gas-fired infrared radiant heating system under different ventilation times. Reference [2] discusses the radiation heat transfer between human body and thermal environment and the heat dissipation of radiator

at present, the commonly used calculation methods for the heat load of gas-fired infrared radiant heating systems are:

① the conversion temperature method

still adopts the calculation method for the heat load of convective heat transfer heating systems, and the indoor calculation temperature of gas-fired infrared radiant heating systems is 2 ~ 3 ℃ lower than that of convective heat transfer heating systems

② temperature gradient reduction method

for large spaces, there is a significant temperature gradient due to the natural convection of hot air. When radiators are used for heating, the temperature gradient of large space is 0.5 ~ 1.0 ℃/m. When gas-fired infrared radiation heating is used, but its strength is 15 times higher than that of steel under the same weight, the temperature gradient can be reduced, taking 0.2 ~ 0.5 ℃/m

③ correction coefficient method

heat load of gas-fired infrared radiant heating system using correction coefficient method Ф The formula of R is:

Ф r= φ one φ two Ф C (3)

where Ф R -- heat load of gas-fired infrared radiant heating system, w

φ 1 - radiation correction coefficient

φ 2 - height correction coefficient

Ф C - heat load of convective heat transfer heating system, w

radiation correction coefficient is generally taken as 0.8. When the installation height of the radiator exceeds 6m, every 0.3m increase, Ф R increased by 1%. The installation height of the radiator applicable to this method should not exceed 18h. When it exceeds 18m, the situation will be more complex, and special design should be carried out

2 layout of radiator

ashrae recommends that the installation position of radiator should be close to the outer wall or the area with heavy load. Moving the radiator inward from the position close to the outer wall will gradually reduce the radiant heat transfer between the radiator and the outer wall, and the radiant heat lost to the outside through the outer wall will also be reduced, but it will aggravate the asymmetry of room radiant heat and reduce the uniformity of room thermal comfort [3]

literature [4] studied the best installation position of ceiling radiator under two kinds of external window positions. The research results combined with the cut in motion negotiation machine show that for rooms where the outer window is far from the ceiling, installing the radiator close to the outer wall can enhance the uniformity of thermal comfort in the room, and only increase little energy consumption. The installation position of the radiator recommended by ASHRAE is reasonable; For the room with the outer window close to the ceiling, install the radiator close to the outer window, although the room has the best thermal comfort, but the energy consumption increases significantly; The best installation position of the radiator should be 0.8m away from the outer wall

the mandatory article 4.5.4 of standard [5] points out that the installation height of gas-fired infrared radiator should be determined according to human comfort, but should not be less than 3m. Article 4.5.5 points out that when gas-fired infrared radiators are used for heating in local workplaces, their number should not be less than two, and they should be installed above the side of the human body

literature [6] points out that when the installation height of the radiator is higher than 7m, the temperature gradient increases with the increase of the installation height. However, when the installation height is 7.5m, the uniformity of air temperature in the lower space is better than that when the installation height is 5 or 6m. In the actual project, the installation height of 7.5m is relatively reasonable

3 comfort

air temperature, relative humidity, air velocity and mean radiant temperature (MRT) are the four objective elements of thermal comfort evaluation

① foreign research

in 1857, the British Public Health Bureau conducted an investigation on the thermal environment of residential heating and ventilation systems, suggesting that the wall temperature of the room should be at least equal to the indoor air temperature. In 1987, Bergland and fobelets studied the air flow and asymmetric radiation temperature. The airframe of the aircraft was designed, calculated, tested and manufactured by China! The thermal effect of the combination of difference (that is, the difference between the radiation temperatures of two opposite planes of a small plane placed in an asymmetric radiation field). The results show that when the air velocity is less than 0.25m/s and the asymmetric radiation temperature difference is less than 10 ℃, the acceptance of the thermal environment will not be affected. Fanger pointed out that when maintaining the same PMV (expected average evaluation) index between gas-fired infrared radiant heating rooms and convective heat transfer heating rooms, the indoor temperature of gas-fired infrared radiant heating rooms is about 2 ℃ lower than that of convective heat transfer heating rooms

② domestic research

domestic research started late. The thermal comfort of gas-fired infrared radiant heating is mainly studied by numerical simulation and measurement

literature [6] takes the gas-fired infrared radiant heating system in the production workshop of Tianjin Lishi Packaging Co., Ltd. in Tianjin Economic and Technological Development Zone as the research object, and uses the method of combining Airpak2.1 software with actual measurement. The temperature gradient in the main area of the heating space can be divided into two parts: in this part of the space of 0.4 ~ 3.0m, the temperature gradient is small, about 1 ℃/m; 3.0 ~ 3.5 hit this part of the space, and the temperature gradient is large. However, the space with significantly increased temperature gradient occupies a small area, which basically does not affect the indoor environment. The PMV and PPD (expected dissatisfaction rate) indexes are compared and analyzed, and it is concluded that the gas-fired infrared radiant heating system can meet the thermal comfort requirements of the working area under the design conditions

literature [7] takes a building in Wuhan as a model, and uses numerical simulation method to simulate and compare the temperature field, velocity field, PMV and PPD distribution of four heating modes: ceiling gas infrared radiation heating, hot water floor radiation heating, radiator heating and air conditioning heating in indoor space. It is concluded that the temperature gradient of gas-fired infrared radiant heating is small, which improves the indoor average radiation temperature, greatly reduces the heat dissipation of human body, and enhances the comfort of human body; Radiator heating is mainly convective heat transfer, and the indoor air velocity is uneven, which reduces the comfort; Air conditioning and heating are at a disadvantage in the comparison of indoor temperature, PMV and other factors, and should not be used

literature [8] simulated and measured the temperature field of the gas-fired infrared radiant heating system in the middle corridor of Tianjin TEDA Tropical Botanical Garden, and compared and analyzed the evaluation indexes of human comfort PMV and PPD in the vertical height of the corridor. The PMV in the working area basically meets the recommended value of ISO 7730 for PMV index (-0.5 ＜ PMV ＜ 0.5). Only when the PMV near the roof exceeds 0.5, it has little impact on the overall thermal comfort evaluation in the corridor. PPD is less than 10% when the height is less than 4m Meet the recommended value. It can be seen that better thermal comfort can be obtained by using gas-fired infrared radiant heating in large spaces

4 energy consumption and economy

theoretical research mainly includes:

① whether the gas-fired infrared radiant heating system can save energy is related to the radiation absorption rate of the inner wall of the outer enclosure and the indoor heating temperature. These parameters have a great impact on the building energy consumption index and the design heat load. If not handled properly, the energy consumption index of the gas-fired infrared radiant heating room will even be higher than that of the convective heat transfer heating room [9]

② the internal surface temperature of the outer wall of a gas-fired infrared radiant heating room is directly related to the heating energy consumption. When convective heat transfer heating is adopted, because the temperature of the inner surface of the outer wall is lower than that of gas-fired infrared radiation heating, the energy-saving advantage is limited to the case that the heater or radiator is placed in the partition wall. When the heater or radiator is located at the lower part of the outer window, this advantage does not exist. When the ventilation rate of the room is greater than the energy-saving critical value, the gas-fired infrared radiant heating is more energy-saving, otherwise the convective heat transfer heating has better energy-saving performance. The calculation formula of the energy-saving critical value MC of the number of air changes is [10]:

where MC - the energy-saving critical value of the number of air changes, times/h

k - the heat transfer coefficient of the enclosure, w/(M2 · k)

③ under the same thermal comfort conditions, the enclosure with good thermal insulation performance (such as color steel sandwich panel) is used, and the energy consumption of the gas-fired infrared radiant heating system is 93% - 97% of the energy consumption of the convective heat transfer heating system; The enclosure structure with good thermal insulation performance (such as 490mm thick brick wall) is adopted, and the energy consumption of gas-fired infrared radiant heating system is 95% - 98% of that of convective heat transfer heating system; If the enclosure structure with poor thermal insulation performance (such as 370mm thick brick wall) is adopted, sometimes the energy consumption of gas-fired infrared radiant heating system is greater than that of convective heat transfer heating system [11]

applied research mainly includes:

① hangar of Beijing western suburb airport. By comparing the economy of the gas-fired infrared soft radiation (CRV) heating system with that of the traditional radiator heating system, the cost of the gas-fired infrared soft radiation heating system is only 79.8% of the traditional radiator heating system, and the operating cost during the heating period is only 30.7% of the traditional radiator heating system [12]

② workshop of a Machinery Co., Ltd. Comparing the energy consumption of gas-fired infrared radiant heating system with that of convective heat transfer heating system, the energy consumption of gas-fired infrared radiant heating system is reduced by 30%, and the thermal efficiency is more than 95%. The cost of district hot water boiler room heating is 22.2 lower than that of gas-fired infrared radiation heating × 104 yuan, but the operation cost of the latter is low and the comprehensive benefit is considerable [13]

③ an industrial plant in Daqing Development Zone. The thermal efficiency of the gas-fired infrared radiant heating system reaches 94.7%, and the energy utilization rate is high [14]

5 conclusion

traditional heating methods (radiator, heater) can not effectively solve the heating problem of large space, resulting in a large amount of waste of energy. Gas infrared radiant heating system has high efficiency and energy saving