Heating issues
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Dear experts: I have a question regarding heating issues. The design engineers at Changchun Design Institute proposed a design scheme for the heat source flow rate in the main heating network, specifying 10 tons of hot water per 10,000 square meters, with a heat source temperature of 65 degrees. However, the temperature supplied to the heating users is not sufficient; they attribute this to problems with the heat exchangers. Yet the amount of heat I calculated as being required is much higher than what they suggested. I’m confused about this. Currently, the return water temperature in the main network is 35 degrees, and I don’t think there’s any issue with the heat exchangers. I beg all the experts to give me some advice. My idea is to assume that the heat source flow rate is 10 tons and the temperature difference is 30 degrees (65–35); in that case, the heat output would be 10000×30×4.198/3.6 = 349800 W. In other words, the thermal index is 349800/10000 = 35 W/square meter. Is this heating index applicable in the three northeastern provinces? Is there something wrong with my calculations? Please, experts, clarify!
35 W/square meter is not sufficient in the three northeastern provinces; generally, a heat load of 65 W/square meter is adopted, with a minimum of 60 W/square meter as well.
Hero: My calculations and understanding are correct, right? The engineers at the design institute said that it’s always been this way; I really don’t know where the problem lies Moreover, the heating is provided intermittently, twice a day, for 6 hours each time
I can only say that you’ve been played by your competitors. The engineers from the design institute are estimated to be part of that group as well. Your approach is fine.
When calculating heating units, 35 W per square meter is definitely not sufficient. According to the standards for heating design in residential buildings, the heat requirement for residences is 46–70 W/㎡, while for single-story residences it is 80–105 W/㎡.
The recommended values can be estimated with reference to the \"CJJ34-2010 Code for Design of Urban Heating Pipeline Networks\":
This post was last edited by zhou742104 on 2015-8-10 at 18:07. G=0.86Q/(t1-t2); G represents the flow rate in kg/h, while Q is the heating power in watts. Q=10000X50/0.86=581 KW. The supply water temperature is 120 degrees, while the return water temperature should be 70 degrees. In the primary network, the temperature of the hot water ranges from 120 to 130 degrees, and that of the return water is 70 degrees. After heat exchange at the heat exchange station, the temperature of the water entering the secondary network is 65 degrees, with the return water temperature ranging from 40 to 50 degrees
A return water temperature of 35 degrees definitely won’t work either. Consider the temperature difference between the return water temperature and the radiator temperature, as well as the temperature of the radiator required to maintain a certain room temperature. It is more reasonable for the return water temperature to be a bit higher. Based on experience, the temperature of the radiator should be around 45 degrees.
It is recommended not to rely solely on theoretical calculations; instead, scientific simulation tests should be conducted.
First, you can take a look at the data table on the sixth floor. Secondly, based on practical experience, if the temperature of the water supplied to the primary network is too low – with the return water temperature at 35 degrees – then the return water temperature in the secondary network will be below 35 degrees as well. No matter how large the area of the heat exchangers is, a temperature difference is inevitable. The return water temperature is very low; it’s okay for users with underfloor heating, but those using radiators will have to endure the cold. The correct approach is to increase the temperature of the primary loop network, thereby raising the return water temperature of the primary network and consequently increasing the return water temperature of the secondary network. The return water temperature can also be increased by increasing the flow rate in one pass, provided that there are sufficient pipes available. Design institutes usually design systems with a primary water supply and return temperature of 120–70 degrees, but in reality the temperature of the primary water isn’t that high; it’s even lower during the initial stages of heating. In such situations, heat exchanger manufacturers end up in a difficult position – it’s not possible to design them according to the specified temperatures. The only solution is to increase the heat exchange area, so as to minimize the impact of fluctuations in the heating network.