The heating or cooling equipment in the interior of the building is at the end of the energy delivery system, which is called the end device. The end device of the heating system, that is, the heat dissipation equipment, commonly used are radiators, heaters, radiant panels and fan coils (the latter two can be cooled at the same time in summer). It is the main component of the heating system. It dissipates heat into the room to supplement the heat loss in the room, maintaining the required temperature in the room.
Among them, the radiator is the most widely used and common heat dissipation equipment.
The role of the radiator: the heat carried by the heating medium (steam or hot water) of the heating system is transmitted to the room through the radiator wall to achieve the purpose of heating.
Types of radiators:
According to the heat transfer method: radiant radiator and convection radiator.
According to the material: cast iron, steel, aluminum alloy, copper aluminum composite and plastic radiator.
According to the structure: column type, airfoil type, tube type, flat type, column airfoil type.
Column radiator: material - cast iron, aluminum, copper and aluminum composite; The K value is large and beautiful
Plate radiator: material - steel; Large K value, beautiful; High pressure and long life;
Tandem radiator: material - steel, aluminum; Predominantly convection
1) Cast iron radiator:
Type: Airfoil (long wing, round wing), columnar, column airfoil.
Advantages: simple structure, corrosion resistance (material: gray cast iron), long service life, good thermal stability (large water capacity).
Disadvantages: large metal consumption (bulky, heavy labor for manufacturing, installation and transportation), lower than steel radiators.
2) Steel radiator:
Type: steel string piece, plate type, flat tube type, column type, light row tube type.
Features: low metal consumption, high metal thermal strength;
High compressive strength, suitable for high-rise buildings and high-temperature water heating;
Beautiful and neat appearance, small land occupation, easy to arrange; Less water capacity and less thermal stability;
Easy to be corroded, short service life.
Steel string radiator: composed of steel pipe, steel sheet, connected box and pipe joint, the string blade on the steel string piece adopts 0.5mm thin steel sheet string plate at both ends folded at 90 °C to form a closed shape. Many closed vertical air channels enhance convective heat release.
Available with and without cover, covered steel string radiators are typical convection radiators (convection heat dissipation accounts for almost 100%).
Steel flat tube radiator: made of rectangular water channel flat pipes superimposed and welded together, and two ends plus a joint box.
The plate type has four structural forms: single plate, double plate, single plate with convection sheet and double plate with convection sheet. Both sides of the single and double plate flat tube radiator are light plates, and the temperature of the board surface is higher and there is more radiant heat. Single and double plate flat tube radiators with convection sheets, each piece dissipates more heat than the same specification without convection sheets, and the heat is mainly transferred by convection.
Steel light strip tube radiator: welded from steel tube, easy to remove dust accumulation. Steel consumption, large footprint.
Aluminum alloy radiator: easy to process, light weight, beautiful appearance, but the cost is higher, not as durable as cast iron radiator.
Plastic radiator: light weight, metal saving, corrosion resistant, but can not withstand too high temperature and pressure.
Arrangement of radiators:
It should be installed under the exterior wall window sill.
The radiator should be installed in the open, and kindergartens and buildings for the elderly must be concealed or equipped with protective covers.
The number of pieces of cast iron radiator should not exceed: thick column type (including column airfoil) 20, thin column type 25, long airfoil type 7.
Radiators in auxiliary rooms and corridors such as storage, washing, toilets, kitchens, etc., can be connected in series with adjacent rooms.
When the hot water heating radiator is connected in series, it can be connected on the same side, but the diameter of the upper and lower series pipes should be the same as the diameter of the radiator interface.
Stairwells or other places where there is a risk of freezing should be heated by separate vertical and branch pipes. The regulating valve must not be set in front of the radiator.
The thermostat controller installed in the decorative hood must use an external sensor, which should be located in a position that correctly reflects the temperature of the room.
In the outer compartment of the two outer doors, as well as in the door bucket, radiators should not be provided to prevent freezing cracks.
The radiator of the stairwell or the hall with the back gallery should be allocated on the ground floor as much as possible, and when the number of radiators is too large to be arranged on the ground floor, it should be distributed according to the following table. Multi-storey residential stairwells generally do not have radiators.
The radiators in the stairwell or the hall with a horse gallery should be allocated on the ground floor as much as possible, and when the number of radiators is too large to be arranged on the ground floor, it should be distributed according to the specification. Multi-storey residential stairwells generally do not have radiators.
Selection and arrangement of radiators:
Selection principle: according to the actual situation, choose economical, practical, durable and beautiful radiators.
Principle: It is easy to cause indoor cooling and heating flow, outdoor intrusion of cold air heating rapidly, people's stay area warm and comfortable and less occupied indoor effective space and use area.
Calculation of radiators:
The calculation of the radiator is to determine the heat dissipation area and the number of pieces required to heat the room.
1. Calculation of radiator area:
Note: When calculating the heat dissipation area, if the heat dissipation of the exposed heating pipe exceeds 5% of the design heat load of the room heating system, it is advisable to subtract the heat release of the system pipe to the room in the Q value.
Calculation formula:
2. Calculation of the average temperature of the heat medium in the radiator:
a. Hot water heating system: tpj=(tsg+tsh)/2°C
Single pipe system: the temperature of the inlet and outlet water needs to be calculated one by one, and then find F;
Two-tube system: tpj=(t ́g+t ́h)/2 °C
b. Steam heating system:
When the steam pressure ≤ 0.03MPa, tpj takes 100°C;
When the steam pressure > 0.03MPa, tpj takes the saturation temperature tb corresponding to the steam pressure at the inlet of the radiator.
3. K value of heat transfer coefficient of radiator:
The physical meaning of a.k:
It is a sign of the heat dissipation capacity of the radiator, which refers to the heat dissipation per square meter of the radiator when (tpj-tn) is 1 °C.
b. Influencing factors: the manufacture of the radiator; the conditions of use of the radiator.
Among them, the most important factor affecting the heat transfer coefficient and heat dissipation - △t. The K value is determined experimentally.
注：Q=KA(tm-tR)W
K--- heat transfer coefficient of the heat sink.
k Amendment:
(1) Correction coefficient of the number of assembled pieces
Note: Integral radiators such as steel plate type and flat tube type are tested separately with different specifications of radiators to obtain their respective thermal performance values, and the number of pieces is not corrected.
(2) Connection form correction coefficient
In the connection form (see figure), the k-value of ipsilateral up-down output is the highest> heterolateral up-in-down-out> heterolateral down-in-down> heterolateral down-in-up-out> heterolateral up-in> ipsilateral down-in-up-out is the worst.
(3) Installation form correction coefficient
The correction of the K value is also the correction of the radiator area.
Note: The general trend of water flow during downward inlet and up-out is opposite to the gravity action of water after cooling in the radiator, which makes the heat dissipation performance worse, the heat transfer coefficient becomes smaller, and the required radiator area increases under the same heat load.
Flow: It has no great effect on the internal heat transfer coefficient, but has a great influence on the average temperature of the radiator. The average temperature is high, the external heat transfer coefficient is large, and the relationship is q=Ln(G).
Different coatings also have an effect (silver coating, i.e. aluminum powder, has a lower radiation coefficient than blended paint)
Surface parameters (radiation): paint 0.9; aluminum powder 0.4; untreated cast iron surface 0.7 - heat dissipation 15% of different coatings.
The heat medium is different, the K value is different, the inner surface of the steam radiator condenses and dissipates heat, the surface temperature is more uniform, the same TPJ, the steam is higher than that of hot water.
Note: The corrected values are shown in the appendix of "Heating Engineering" or "Heating and Ventilation Design Manual".
4. Calculation of the number and length of radiator pieces: n=A/a
Among them: a is the heat dissipation area of a radiator, ㎡/piece.
Note: n rounding, when rounding, the heat dissipation area of column type, long wing type, plate type and flat tube type can be 0.1㎡ smaller than the calculated value, and the heat dissipation area of string type, circular wing type and other radiators can be 5% smaller than the calculated value.
Note: We have calculated to determine the n value, and when calculating F, there is an item that is the chip number correction coefficient, so when calculating F, first assume β1=1.0, evaluate the n value, and then correct F and then find F.
Note: The number or length of each group of radiators should not exceed the following regulations: M-132 20 pieces; four-column, five-column type, 25 pieces; Long airfoil, 7 pieces; Round airfoil, 4m; Steel string, plate type, flat tube type, 2.4m.
Correction factor β1 for the number of assembled pieces:
The heat transfer effect of the side piece is better than that of the intermediate piece:
| Radiator form | Various cast iron and steel radiators | Steel plate type/flat tube type radiator | |||||
| Number or length of pieces per group | ≤5 | 6~10 | 11~20 | ≥21 | ≤600 | 600~1000 | ≥1000 |
| Correction factor | 0.95 | 1.00 | 1.05 | 1.1 | 0.95 | 0.98 | 1.00 |
Connection method correction factor β2:
The general trend of water flow is consistent with the direction of gravity caused by cooling.
Installation form correction factor β3:
When the same model radiator in the same heating system adopts the following four installation methods, which one has the most heat dissipation?
| Installation form | B3 |
| The upper part of the radiator installed in the groove of the wall (semi-concealed) is 100mm from the wall | 1.06 |
| Surface-mounted, but the upper part of the radiator is covered by a window sill, and the height of the radiator from the countertop is 150mm | 1.02 |
| Enclosed in the hood, the upper part is open, and the lower part is 150mm above the ground | 0.95 |
| Installed in the hood, the upper and lower openings, the opening height is 150mm | 1.04 |
Flow correction factor β4:
The temperature difference between the inlet and outlet water is 25°C, which is the standard flow rate (β4 =1)
| Heatsink type | The traffic is increased by a multiplier | ||||||
| 1.00 | 2 | 3 | 4 | 5 | 6 | 7 | |
| Column/long airfoil, multi-wing/mounted airfoil | 1.00 | 0.90 | 0.86 | 0.85 | 0.83 | 0.83 | 0.82 |
| Flat tube type | 1.00 | 0.94 | 0.93 | 0.92 | 0.91 | 0.90 | 0.90 |
Radiator example (1):
An office conference room (tn=18 °C), the calculated heating heat load is 2200W, the design uses cast iron four-column 640 radiator, installed in the cover, the upper and lower parts are open, the opening height is 150mm, the heating system is the upper supply and lower return system, the heat medium is 85/60 °C hot water, the radiator is the opposite side up and down out, what is the number of pieces of radiator selected in the conference room?
Check the sample: cast iron four-column 640 radiator single piece heat dissipation area is f=0.205m2, when △t=64.5 °C, the heat transfer coefficient of the radiator K=9.3W/(m2·°C), and the heat transfer coefficient calculation formula of 10 radiators K=2.442△t0.321.
Processing of the number of pieces to calculate the mantissa number:
Double-pipe system: the heat tail can be discarded when it does not exceed 5% of the required heat dissipation, and it should be carried out when it is greater than or equal to 5%.
Single pipe system: the number of upstream (1/3), intermediate (1/3) and downstream (1/3) radiators can be discarded when the tail number does not exceed 7.5%, 5% and 2.5% of the required heat dissipation, respectively, and vice versa.
From: 2009 National Civil Building Engineering Design Technical Measures (HVAC and Power).
Radiator example (2):
An office conference room (tn=18 °C), the calculation heating heat load is 850W, the design uses cast iron four-column 640 radiator, installed in the cover, the upper and lower opening height is 150mm, the heating system heat medium is 80/60 °C hot water, double pipe up and down, radiator is heterogeneous side up and down, what is the number of radiators selected in the conference room?
Check the sample: cast iron four-column 640 radiator single piece heat dissipation area is f=0.205m2, 10 pieces of radiator heat transfer coefficient calculation formula K=2.442△t0.321.
Radiant heat transfer - room heating load and cooling load Q:
According to the current code GB50736-2012 "Code for the design of heating, ventilation and air conditioning of civil buildings":
The design temperature of the fully radiant heating room can be reduced by 2 °C, and the design temperature of the comprehensive radiant cooling room can be increased by 0.5~1.5 °C.
The heat load of the local heating system is determined by the coefficient of the heat load of the overall heating
The heat transfer loss should not be calculated on the floor and wall of the building where the heating and cooling components are laid.
| The ratio of the local area area to the total area of the room K | K≥0.75 | K=0.55 | K=0.40 | K=0.25 | K≤0.20 |
| Calculate the coefficients | 1.00 | 0.72 | 0.54 | 0.38 | 0.30 |
The floor or wall where the heating and cooling components are laid without the heat transfer load facing outward through the floor or wall.
The radiant outward heat transfer load should be calculated in the heat supply of the heat medium in the radiant room (refrigerant cooling capacity).
Room heat load added value:
For hot water radiant heating systems that use centralized heat source heat metering or separate household independent heat source, the added value of intermittent heating and the heat transfer load between households need to be considered, and the heat load of the additional room is:
Key points of calculation:
Upward heating or cooling capacity per unit floor area:
Excerpt from: JGJ142-2012 "Technical Regulations for Radiant Heating and Cooling"
Heat transfer down Q2:
Relevant factors: surface material, thickness, water average temperature, water temperature difference (flow rate).
Radiation – average heating surface temperature:
The water supply temperature of civil buildings should be 35-45 degrees.
The temperature of the supply and return water of the hot water ground radiant heating system should be determined by calculation, the water supply temperature should not be greater than 60 degrees, and the temperature difference between the supply and return water should not be greater than 10 degrees, and should not be less than 5 degrees. The water supply temperature of civil buildings should be 35-45 degrees.
This article stipulates that the water temperature shall not exceed 60 degrees Celsius, considering the safety, longevity and comfort of radiant heating from the ground. From the consideration of comfort and energy saving, the temperature of floor heating and water supply should be used with a lower value, and domestic and foreign experience shows that 35-45 degrees is a more suitable range. Maintaining a low water supply temperature is conducive to prolonging the service life of chemical pipes and improving indoor thermal comfort; Controlling the temperature difference between the supply and return water is conducive to maintaining a large flow rate of the heat medium, facilitating the removal of air in the tube, and ensuring the uniform temperature of the ground. Severe cold and cold areas should choose the design water supply temperature on the basis of ensuring indoor temperature, and the return water temperature in cold areas is recommended to be not less than 30 degrees.
On the ground where personnel often stay, the relevant American standard research has concluded that when the ground temperature is 21-24 degrees, the dissatisfaction is less than 8%. Studies related to Japanese data show that when the upper limit of ground temperature is 31 degrees, it is acceptable from the consideration of human health and comfort. Taking into account the living habits of the mainland, it is set at 29 degrees.
Radiation - average cooling surface temperature:
The surface temperature is 1~2 °C higher than the indoor dew point. The water supply temperature is generally 14-18 °C, the greater the load, the lower the water temperature; The temperature difference between supply and return water should not be > 5 °C and should not be < 2 °C.
According to European standards, the lower limit of the floor temperature of the room where people sit for a long time is 20 degrees, and the lower limit of the temperature of the floor of the room where people are active is 18 degrees.
The radiant cooling system can only remove the sensible heat load in the room, but cannot remove the hot moisture load in the room. In order to prevent condensation on the radiant surface and increase comfort, a dehumidification ventilation system is required. The sensible heat load of the indoor part is borne by the radiant cooling system, and the air supply system bears all the latent heat load and the remaining sensible heat load in the room.
When independent control of temperature and humidity is adopted, a separate design is required.
Surface temperature check:
Heating: Ensure that the average surface temperature does not exceed the specified limits.
Cooling: not lower than the specified limit.
The ceiling is 8.48 degrees > the ground is 8.19 degrees (when the indoor temperature is 18 degrees, the unit ground upward cooling capacity is 60W), that is, the ceiling temperature can be slightly higher than the ground when cooling (the upper setting of the cold pipe is consistent with the natural convection direction).
The approximate formula for checking the average surface temperature of the heating ground is derived from the calculation method provided by the ASHRAE manual and obtained by regression. If the average surface temperature is above the specified limit, the thermal performance of the building should be improved or other auxiliary heating equipment should be installed to reduce the heat load borne by the radiant heating system on the ground and meet the limit value requirements.
Radiation-Implicit Constraints of Surface Temperature:
The flow rate of heating and cooling pipes and transmission pipes should not be less than 0.25m/s; The maximum cross-sectional flow velocity of the water divider and water collector should not be greater than 0.8m/s; Branch loops should not be more than 8 routes.
Radiant heat transfer in the ground – main design steps:
Determine room heat load Q1 (1000W)
Calculate the upward heat dissipation q1 per unit floor area (66.7W/m2, spread out in the room)
Assuming that the pipes cover the entire room (15m2), the upward heat dissipation q1* (73.2W/m2) per unit floor area is determined according to the supply and return water temperature (40/30°C), interior design temperature (18°C), surface conditions (cement), etc.
The value of the lookup table q1* is slightly greater than the calculated value q1, and the corresponding spacing pipe is taken so that the actual laying area is less than the room area (actually feasible).
Actual laying area: Q1/q1*=13.7m2
Surface temperature check tpj=tn+9.82×(q1*/100)0.969=25.3°C
Radiant heating example (1):
A residential building in a cold area adopts a hot water floor radiant heating system (intermittent heating), the heat source of each household is a gas wall-hung furnace, the living room area of a household is 32m2, and the basic heat consumption is 0.96kW, and the actual room heat load of the living room is calculated.
Radiant heating example (2):
The bathroom adopts low-temperature hot water floor radiant heating system, the design indoor temperature is 25 °C, and does not exceed the maximum upper limit of the average surface temperature, what is the maximum limit of heat dissipation per unit floor area of laying heating pipes?
Maximum upper limit: 32°C
It can be seen from TPJ=TN+9.82×(Q1/100)0.969
25+9.82×(q1/100)0.969≤32→q1≤70.5W/m2
Radiant heating example (3):
The entrance hall on the first floor of a building adopts a ground radiant heating system, the entrance hall area is 360m2, the ground area where heating pipes can be laid is 270m2, and the interior design temperature is 20, so as to ensure the maximum room heat load corresponding to the upper limit of the ground surface temperature.
Maximum upper limit: 32°C
It can be seen from TPJ=tn+9.82×(Q1/100)0.969
20+9.82×(q1/100)0.969≤32→q1≤123W/m2
Q1=270×123/1000=33.2kW
Fan heater:
The heater is the heat backup and heat supply equipment of the hot air heating system. Hot air heating is one of the more economical heating methods, convection heat dissipation accounts for almost 100%, with the characteristics of low enthusiasm and fast heating, suitable for heating of tall industrial plants and other buildings.
Regarding the design of hot air heating, it belongs to ventilation and air conditioning engineering, and this paragraph will only introduce the heater as a heating equipment. A fan heater is a combined unit composed of a fan, an electric motor and an air heater. Under the action of the fan, the air enters the unit through the suction port. After heating by the air heater, it is sent to the room from the air supply outlet to maintain the required temperature in the room.
Heaters are divided into axial flow type and centrifugal type, often called small heater fan and large heater fan.
According to its structural characteristics and applicable heating medium, it can be divided into steam heater, hot water heater, steam, hot water dual-purpose heater and cold, hot water dual-purpose heater.
Axial flow heater: small size, simple structure, easy installation. However, the hot air flow sent by it has a short range and a low outlet wind speed.
Therefore, it is mainly used to heat indoor recirculation air, generally hanging or bracketing it on walls or columns. The hot air passes through the louver adjustment plate at the air outlet and blows directly to the work area.
Centrifugal heater: heating equipment used to centrally convey a large amount of hot air.
Because it is equipped with centrifugal fan, it has a larger indenter and a higher outlet speed, and the air flow range is much longer than that of the axial flow heater, and the air supply volume and heat production are large. It is often used in central air heating systems.
Centrifugal large heaters, in addition to heating indoor recirculation air, can also be used to heat a part of outdoor fresh air. It is also used for room ventilation and heating.
This article comes from the Internet and is edited by HVAC South News Agency.