Calculation of Pump Lift in Special Pipeline for Cooling Tower
Author：Trlon Views: Time:2020-11-11
The calculation of the pump head of the special pipe for cooling tower should be the system resistance (the sum of the resistance of pipes, fittings, and condenser), the height difference between the water level of the cooling tower's catchment plate and the cooling tower water distributor, the required pressure composition of the cooling tower water distributor, and Additional 5%-10% margin. The mistake that designers often make is to calculate the height difference of the system as soon as they see the open system. Although the cooling tower is an open system, because the cooling tower comes with a water collecting pan, which is equivalent to a water tank on the roof, this part of the hydrostatic pressure is offset by the static pressure required for the rise of the water supply pipe, so it only needs to be included in the cooling tower chassis and cloth The height difference of the water pipe is fine.
Take the cooling tower of an air-conditioning project as an example: 2 water pumps + 2 cooling towers are connected in parallel, with a design flow of 400t/h and a head of 40m. The following problems are encountered during debugging: When a single pump is running, if the pump outlet valve opening is greater than 30%, the pump will vibrate violently, the pressure gauges before and after the pump will be bounced, and the ammeter of the power distribution cabinet will be bounced: if the pump outlet valve is less than 25 %, the water pump can basically operate stably, the ammeter shows 90A. After calculation, when the current is 90A, the pump flow is assumed to be 400t/h, and the efficiency is 70%, and the head is about 17m. The designer probably includes the height difference between the cooling tower and the water pump into the head, so the pump head is larger. Times. Fortunately, the valve is opened too small, otherwise the pump may burn the motor.
Let's look at another situation: In actual engineering, due to many reasons, cooling towers are not allowed on the roof of the building, and the condenser is set up at a high place, forming the system shown in Figure 5.
In this system, when the water pump stops running, the cooling water in the pipeline returns to the tower to form a vacuum, resulting in a siphon and backflow, and the water collecting pan of the cooling tower will overflow. Certain measures are generally taken during design, such as installing a vacuum break valve at the top of the cooling water pipe, as shown in Figure 6. Or set a vent pipe at the top, as shown in Figure 7. The article "Hydraulic Analysis when the Cooling Tower is in the Bottom of the System" in the fourth issue of "HVAC" in 2003 proposed that when the system height is too high, an electric valve should be installed at the inlet of the cooling tower to prevent the water from flowing out when the system stops running. The author thinks that it is not convenient and simple as shown in Figures 6 and 7.
Let's analyze Figure 7 below. First, suppose that the resistance of section ab is hab, the resistance of section bc is hbc, the pump lift is H, and the required outlet water pressure of the cooling tower is hlq.
The first case: h2=hbc+hlq, the pump lift of the pipeline only needs to overcome the height difference between the resistance of the ab section and ab, that is, H=hab+h1+h2. At this time, the height of the snorkel h3 can be 0, this Is an ideal situation.
The second case: when h2 hbc+hlq-h2, water will not flow out of the snorkel.
The third case: h2>hbc+hlq, the pump head only needs to overcome the height difference between the resistance of ab section and ab, the head H=hab+h1+h2, h3=0. However, a large amount of air is mixed in the effluent water of the cooling tower, and the pump lift is wasted, which increases the power consumption, which is not an economical way. In summary, the first situation is rare, the second situation is common, and the third situation should be avoided as much as possible. In order to make the system operate normally and economically, the height of the system should not be too high. Detailed calculations should be made during the design. When the third situation occurs, it can be avoided by increasing the resistance of section bc.