【 Weak current College 】 terminal device selection and layout

1 Terminal device select the General principles of

VAV system is the right choice of terminal equipment and the reasonable control of the entire system of energy consumption and the comfort of the rooms, have a very significant impact, a larger capacity of the terminal device, at run time through it generated may lower pressure drop, making the energy consumption of air conditioners in blower can be reduced, however, the capacity is big terminal device minimum air control set-point are relatively high, which in turn will make the operational power consumption increased forced draft fan, also increases the heat again (if you choose to use the reheat-terminal device). On the contrary in the air volume required under the same conditions, select a size too small of a terminal device, in addition to making energy consumption will increase, you may have greater operating noise. However, due to its regulation of air flow control valve, relative to the room's load change reactions are more sensitive to its control stability than select a capacity rate of terminal device will be better.

Terminal device selection is designed according to the room need air, determine terminal device of the type and number. In General, choose a major terminal equipment should review its three performance indicators meets the design requirements: (1) standard air (or the maximum and minimum air); (2) noise value; (3) appliance inlet pressure value or unit maximum pressure drop (i.e. the total pressure drop). Finally, select the number of terminal device, it should be and architectural partition, lighting and ceiling-mounted another device on the harmonization of the layout, this building is very important.

It should be stressed that VAV choice, and control functions are inseparable. Different control on the design requirements, need to use a different type and size of the terminal device matching to obtain satisfactory operating result.

1.1 max air

In the terminal device before, we have been selected according to cooling load in summer maximum seek the maximum design of room air flow. Now concern were the selected terminal device (one or more terminal device maximum air flow) is the biggest air and the maximum air volume of design are the same. If the terminal device size is too large, will cause the entire system running a significant increase in energy consumption, when the Terminal settings using conventional "single maximum" control logic, the situation is even more so. For example, room maximum design flow for 500m3/h, now choose a terminal device for its service, the maximum air-1000m3/h, if it's the smallest air set points to its maximum air 30%, the minimum air as 300m3/h. In this case, because the terminal device maximum air is room maximum design air volume of 2 x, then the device at run time, a truly effective minimum air flow set point (i.e. 300m3/h), not the biggest design air flow (i.e. 500m3/h) 30%, but it's 60%. Thus, in most of the running hour, the terminal device is at its minimum air flow (i.e. 300m3/h), but in fact, a lot of time should be 150m3/h (i.e. 500m3/h 30%), which led to the cold wind quantity sent too much, will definitely need to heat, otherwise the room appears too cold. Of course, this is a waste of energy. This shows that the terminal device capacity selection is too large. Visible, the selected terminal device for maximum air supply and rooms need maximum design matches the amount of wind.
　　
1.2 minimum air

Similarly, at the made before the end of the device selected, for building area, we also have their summer minimum cooling load, evaluated over a minimum of room air flow design, selection of terminal device (one or more terminal device minimum air flow) of minimum air should be the minimum design air match.

As for the building area, its minimum cooling load depends on the terminal device, it's the smallest air to its maximum air 30% ~ 50%. A lower percentage of minimum air, you can provide users a wider range of temperature control set.

Again, the minimum end device air flow set point can be calculated for the value of minimum air flow, you can also calculate values lower than this number, the so-called can have high limit and lower limit of minimum air flow set points, depending on the terminal device of control logic. If you use a maximum value of the "single" control logic, the terminal device of minimum air flow setting can and calculation of the minimum ventilation rate values, is the minimum for high limits of air flow set point. Typically, only the end of the cooling unit with a maximum value of the "single" control logic. Zone reheat coil terminal device that is customary or a maximum value of the "single" control logic. If you use the "double Max" control logic, the terminal device of minimum air flow set point can be calculated the value of minimum air flow is low, low minimum air flow set point. Typically, the zone reheat coil terminal device, the recommended maximum value for the "double" control logic, that is, the smallest volume set point, not only depends on the room minimum ventilation rate requirements also depend on the terminal device is installed in velocity probe measuring accuracy and precision, and can be evaluated by calculating the lower limit of minimum air flow set point.

Visible, the terminal device, the minimum air meets the requirements to see which end device take a control logic. It should be noted that, at present only a minority of DDC control manufacturer can provide the system controller to the "double Max" control program.

1.3 noise

The noise level at the end of the device conforms to the air conditioning room noise level requirements, select the terminal device is another important factor.

Room internal noise level is determined by a variety of individual sound sources of noise in a cumulative, these sound sources include:

External noise— — outdoor generated and transmitted into the noise inside buildings;
Interference noise — — adjacent side will or corridor and passes into the room of noise;
Environmental noise — the room personnel and equipment arising from noise, its characteristics and the use of the room;
> System noise from air conditioning to decency pass into the room, or terminal device is in the room noise.
　　
Should carefully analyse the room personnel may hear voices came from a variety of sound sources. In an environment noise was very high room for a very low noise level, is unrealistic, but also economic. On the contrary, the ambient noise level is low, at the same time, many dispersed and intermittent noise and interference of external noise and can take some measures to shield off, room only from a single noise air-conditioning system, which is the ideal State.

Air tube design, on the effect of room noise level is high, while the terminal device of choice, is even more direct and room noise level. Terminal device noise in the room, spread, its sound energy propagation loss refers to the ceiling on VAV propagation attenuation of sound has, since the existence of this role makes the terminal device and the room between the sound delivery channel has been a degree of isolation. The room effect refers to the officers heard sound energy and sound source device actually issued the difference between the sound energy, it actually reflects the room itself, the sound-absorbing ability, its size and the size of rooms, the room walls of materials, indoor furniture materials relative hardness. All manufacturers of VAV product samples, in citing their standard air volume, will indicate the room effect. Samples can't find these data, a variety of different types of room room effect value can refer to table 1

Table 1 room effect on noise attenuation values

Room type
X band

250
500
1000
2000
4000

Hospital
0.8
2.5
3.5
4.0
4.8

Hotel
6.9
7.5
8.5
8.5
8.6

Office
7.2
10.3
11.0
10.5
10.5


Various sound sources on the combined effects of noise in the room, you cannot simply use digital overlays, and the application log, the added value of the sum method (table 2) plus two sound sources in higher sound energy, be integrated after the sound energy value, and then subtract the room effect, is there room in the actual noise level.
Table 2 noise superimposed when added value
The difference between the two sound sources (dB)
0
2
4
6
8
10

Added value (dB)
3
2
1
1
0
0


In General, as long as the press manufacturer product samples of air terminal device to select the callout, and run by the wind, the room will be able to reach the required level of noise. Product samples of VAV standard air flow is the recommended design flow, which is within the area and North room select terminal device, especially so because these rooms of load changes is relatively small. On the other surrounding area room, because the Sun meets the changing selection of terminal device, the volume should be slightly larger than standard air. Terminal equipment for maximum air flow value is only used on those that allow a very high level of abnormal noise, or the maximum amount of wind, the cavity of the original static value generally 500Pa following. You should understand the cavity pressure higher values in the given amount of wind noise level produced will also. Of course, the terminal device of the air flow reduction, the noise levels are reduced.

VAV manufacturers will usually provide two types of noise data diffusion noise and noise. Diffusion noise means the terminal device box downstream measured noise values, it is basically the box in the upper air pipe-air pressures; radiation noise is means underfloor in triage and in the air valve produced turbulence caused by and through the terminaldevice box body shell entered the room ceiling space of noise values. In fact, if the terminal device during Setup, the entrance to a rigid duct, and export and the straight pipe, hose and air diffusers, and the proliferation of noise will seldom occur. If the terminal device is installed in the standard acoustic ceiling above, then the terminal device-generated noise, generally not less than the noise room allows high 5NC. For example, a typical Office, its design allows the noise value is 30NC, then you can press 35NC radiation noise value to select a terminal device. In other words, the terminal units of radiated noise level is not greater than 35NC, ensures the room noise values within the 30NC.

1.4 total pressure drop

Total pressure drop is also known as the terminal device of maximum pressure drop, it should be equal to the static pressure drop and dynamic voltage drop, it was the air conditioning system design Air conveying blower will be through the terminal device may be required for the actual energy consumption indicators. However, the terminal device manufacturer generally only provide their blood pressure lowering data. It is clearly lower than the total pressure drop values because, in the terminal equipment of the entrance to the exit of the wind speed of the wind speed high number, resulting in a certain number of static regain. Therefore, when the terminal device product catalog does not provide the total pressure drop data, designers need a product sample of the air volume and size of the import, export, calculation of pressure drop of value out, resulting in the total pressure drop.

In the air the same case, you can select various sizes of different end devices. But as already stated, when you select a smaller size of the terminal device, the total running time is relatively high pressure drop, forced draft fan energy consumption will increase, the noise will be relatively high; on the contrary, when you choose a large size ofthe terminal device, the investment will increase at a time, how to determine the minimum air flow set point, will be limited, otherwise, there is likely to increase in a writ of low loadCondition of fan energy consumption and thermal energy again. Typically, you can make the calculation, energy simulation, from best energy-saving needs, considering the total pressure drop and minimum air flow setting limits, and seeking the best value for the total pressure drop. Most of the terminal device used analysis indicates that the total pressure drop of approximately 120 ~ 140Pa to choose more appropriate terminal device.

Obviously, the air supply system of the terminal device by total pressure drop is 120 ~ 140Pa to choose, and in accordance with this overall numerical calculation of pressure drop, the air supply system needs, determine the need for power blowers, close to the blower (from hydraulics sense), because there are remaining pressure can be used, where the terminal device can use larger total pressure loss value and the terminal device size selection a bit smaller?

We know that when the load on all day and all year in changes, most of the air terminal units are changing, you need a larger air and maximum air zone, the entire floor is changing, regardless of the outside area within a zone or situation. Therefore, the maximum air pressure values, in a day is changing. If you set the value of the static pressure blowers and then transferred to just meet the design State need maximum air pressure range of requirements, or if in order to offset the near end of air blower installed devices in design mode of operation, the terminal device may need more air pressure, air pressure and operating power consumption it will increase.

Therefore, because in the floor in the maximum air pressure requirements of the terminal device is over time, in a VAV system all the terminal device is installed, you should press the same total pressure drop (e.g. 120 ~140Pa) to select their size, regardless of its location is close to the blower is installed.
　　
2 several terminal device choice

Single-duct Vav (including zone reheat coil terminal device) can be according to the first section describes the General principles for selection, here no longer. Fan power terminal units, dual-channel terminal device of choice, in addition to the above general principles to be followed, there are also worthy of note in the selection, the following are described separately.
　　
2.1 series fan power terminal device

Except for some special requirements of occasions, are advised to avoid using in-line blower power terminal device, this is because the device installed recycling small fan and motor needs even broken seamlessly, while their efficiency in general are very low, typically better than centralized air conditioning blowers operating efficiency low 15%.
However, in some occasions, it is recommended that you use in-line blower power terminal device:

(1) District rooms: in design mode, the room personnel density is large, but practical in use evolution of the situation of the staff. In design mode, since ventilating and minimum requirements of the new amount of wind, meeting room requirements of the minimum ventilation rate and the maximum air volume of the often very high, even as high as 75 percent, is in the meeting room in the area, the situation is even more so. When the Conference room personnel, consistent with the decreases, such a high minimum design flow, will inevitably lead to the room is too cold. If the room uses ordinary single-duct VAV, except in the cooling will consume additional fan energy consumption, but also have reheat measures against very low room staff experienced cold. Generally, the minimum airflow over cooling with a maximum of 40% of the air flow design, you should consider choosing another type of terminal equipment, in-line blower power terminal device is one of the proposed programme. In this case use of in-line blower power terminal device may show zero set point minimum air flow, it can only send room recycling wind to meet ventilation requirements instead of air handling unit for heat, when the Conference room appears part load, and then tune and reduce primary air flow, increase the introduction of recycling of ceiling space, air and energy from the control's point of view, this is the most simple, is also one of the most effective programmes.

(2) need to have a greater minimum air flow to the maintenance room good mixed state, you can use in-line blower power terminal units, thereby preventing in part load conditions declined air flow. For example, a large entrance hall, or lobby, large space that it itself has two storeys high, usually in the first layer of height installation side sent diffuser outlet, but because of the diffuser outlet air temperature reduction, it is possible to produce the air flow, ventilation is not possible to absorb the entire space of the air volume. In this case, selection of in-line blower power terminal device, it can be used in a variety of load State remained fixed air and fixed air speed, so that space be satisfied with the air mixed state.

When the in-line blower power terminal device, the design of system control programme to be aware of when a problem is that the in-line blower power terminal units and air conditioning blowers chain control in advance, and the former to the latter starts first, preventing once the system is started, from air conditioning blowers sent a wind through the terminal device and into the ceiling space, the terminal device of circulating fan appears inverted.

In-line blower power terminal units in circulation fan, you will be 100% Design air into the room, therefore, when determining the device of the primary air inlet need minimum static value, it is not necessary to its downstream of various resistance losses (such as the air through the terminal device, appliance outlet, ventilation pipes and air diffusers, and reheat coil resistance loss) into account, these resistance loss can rely on circulating fan to overcome. In other words, in the selected in-line blower power terminal device, you should know that the device requires more than the outlet pressure value. It is clear that in determining the overall system of air conditioning blowers of wind pressure, you only need to take to overcome the primary air to a terminal device entry and adopt a secondary air valve of resistance values, so the power blowers can be reduced.

However, this concept must not be abused. Pay attention, do not put in-line blower power terminal installationCycling fans as the entire air supply system booster fan. Because, in the cooling at run time, if not enough due to wind pressure, causing the fan to the terminal device entrance, and through an air valve for primary air flow is not enough to rely on the terminal device of circulating fan from an air duct aspiration in more air flow to meet design requirements, it is impossible, if this happens, circulating fan only from the ceiling in pumping more circulating air into the room because there's resistance far less than a wind blowing tube resistance. As a result, the air temperature will increase, room cooling down, people are not comfortable. Therefore, the air conditioning blowers of wind pressure should ensure adequate primary air flow to the terminal device entry and adopt a secondary air valve.

There is no doubt that the in-line blower power terminal units of air volume in circulation fan, you should press the entire device for maximum air flow value determined.

2.2 parallel fan power terminal device

Use parallel fan power terminal device, usually can reduce, or even cancel the system and then heat requirement, and then heated terminal units compared to its initial investment and maintenance costs are very high. In addition, parallel fan power terminal units in circulation fan is run periodically, it generated noise is detected more easily. However, the loop fan and motor efficiency problems, not like in-line blower power terminal device that looked very prominent, because under normal circumstances, parallel fan power terminal units in circulation fan and motor, only the system is in heating State continuously put into operation, and in-line blower power terminal units in circulation fan and motor you need to run continuously.

If single-duct VAV reheat-terminal device can use the "double Max" control logic, and press to control logic to determine the minimum air flow set point, so that their entire life-cycle cost than parallel fan power terminal device to province. When parallel fan power device with zero set point minimum air flow, it can be completely abolished the system of heat demand, thereby significantly reducing centralized air handling unit's fan energy consumption, their life-cycle cost than single-duct reheat-terminal device to province.

Parallel fan power terminal units in circulation fan only transitional quarter and motivation gap running, it only room ceiling air sucked in and terminal units, therefore, in determining the terminal device of primary air inlet requires a minimum static values, including the air through the entire terminal device, the device outlet and air pipes and air diffuser of resistance losses are to be calculated, if with reheat coil, it should also be calculated reheat coil resistance loss. Air circulating fans are usually designed by heating the air flow required to select, it is usually the largest cooling air flow design when 50% ~ 80%.
　　　　
2.3 double-duct VAV

Select the double-duct VAV, should also follow select Single-duct VAV general principles, each terminal device for maximum air flow design to comply with the maximum air volume of requests, the total pressure drop should be not more than 120 ~ 140Pa noise level should be consistent with the room allows noise level requirements. When this type of terminal equipment with reheat coil will involve how to determine the minimum air flow control set-point, if the dynamic pressure sensitive components installed on the terminal device of Tuyere, accurately determine the calculation that you want to use basal area of the outlet, the end of this class of device is very important. You can control the minimum air flow setting point dynamic pressure sensitive element, to such end devices outlet is very important. You can control the minimum air flow setting point dynamic pressure sensitive element, controller minimum dynamic pressure set point output power, and place the dynamic pressure sensitive element of the basal area of air duct. While, generally the outlet sensitive components installed in a circular air ducts, but double-duct connection pipe end device, usually a larger rectangular duct with flange and terminal device is attached to the end of the output. Often, manufacturers are using this relatively large dimensions of the rectangular duct to measure wind tube static pressure losses. Therefore, in the calculation of the double-duct terminal device of dynamic pressure loss when using rectangular duct of basal area values.

By the end of the double-duct unit of pressure will be distinct, they depend entirely on the type of terminal equipment and the dynamic pressure-sensitive components. For cold and hot air flow can fully mixed and have a mixture of double wind vanes road terminal device, the pressure drop Max. When using hybrid control logic (not meant here in the outlet with separate sensitive element of rapid switch control, because using this control, cold and hot air valve is never in the same moment is open), cold and hot air in full hybrid is only an index, only when the terminal device for multiple supply air temperature requirements are not consistent with the room service, which may disturb the equilibrium will be experiencing this problem. When using a separate outlet dynamic pressure sensitive components, often the outlet of the section size appropriate to reduce, than using the cold, hot-air inlet dynamic pressure-sensitive components of the terminal device outlet section size to small, in this way, you make it through the dynamic pressure-sensitive components of air velocity increases, improved air flow measurement accuracy, resulting in two underfloor blend better, but at the same time, the pressure will increase. It was found that in most double-duct terminal device applications, with a separate outlet air flow sensing element, from the comfort of the room, cold, hot-air mixing control room enough. While installing mixed vanes can significantly increase the initial investment at the end of the installation and run-time total pressure drop, only in the most need this type of terminal device (e.g. hospitals) before use.