Heat Recovery System and Variable Flow Ventilation System
In Ankara, a heat recovery system and a variable flow ventilation system according to demand were installed on the exhaust hood of a restaurant. Run around coil type air to water heat recovery system was selected. The warm water gained from the heat recovery coil was distributed to three separate heating coils. The first caoil is heating coil of kitchen make up air AHU. The second coil is preeating coil of dining room fresh air AHU. The third coil is the preheating coil for the split air conditioner of office and staff rooms.
During the winter of 2014-2015 temperatures were monitored and recorded. Although there is no second heating coil in the kitchen fresh air air conditioning unit, there is no disturbance.
APPLICATION
The outside design temperature for winter in Ankara is -12 ° C. The outside temperature for summer is 34 ° C KT, and 18 ° C YT.
In Ankara, a restaurant kitchen is set up using a heat recovery, demand-controlled ventilation fan contol and transfer air. The evaporative cooling module is added to the fresh air air handling unit.
Two main hoods (one on a charcoal grill and one on open ranges) and the some other hoods are collected in a single exhaust system.
The exhaust ducts are insulated with 25 mm thick rockwool to protect the exhaust air temperature and to prevent the accumulation of oil and soot on the inner wall of the duct.
Total exhaust hood exhaust flow rate at full load is calculated as 15.000 m3 / h. NetZero 150 HR model is used. A self cleaning automatic washing system is established to maintain the efficiency of the electrostatic filter. The hot water gained from the heat recovery coil is distributed to fresh air AHU’s by an in-line pump. Three different AHU’s and duct lines are used to supply fresh air to the restaurant. Exhaust fan and kitchen make up air AHU fan are driven by frequency converters and they are inter locked.
One of the three AHU’s those provide fresh air to the restaurant is that make up air to the kitchen. This air handling unit with a flow rate of 10,000 m3 / h and a 100% fresh air consists of a heating coil and an evaporative cooling cell. The make up air to the kitchen is heated in the winter months by heat gained from the heat recovery coil located on the hood exhaust lines. There is no secondary heating coil in the kitchen make up air AHU. electrostatic precipitator with the self cleaning automatic washing system is very successful to protect the heat recovery coil on the exhaust side. The fan of the kitchen make up air is driven by a frequency converter. The make up air given above 30 cm from the lower level of the hood by a speed of 0.45 m / s.
The second fresh air AHU serves to dining hall. This AHU, with a flow rate of 10.000 m3 / h, gets 50% ( 5.000 m3/h ) fresh air. No exhaust is made from the dining hall, and 5,000 m3 / h of fresh air is transfered from dining hall to kitchen as the dining hall is kept at a positive pressure. In the dining hall fresh air AHU there is a preheater coil fed from heat recovery and a secondary heating coil fed from the boiler.
The third fresh air device is a ducted type split air conditioner that serves office and staff rooms. The air flow rate of the split air conditioner is 3,000 m3 / h. 2.000 m3 / h fresh air is taken from outside and 1,000 m3 / h return air is used. A portion of the hot water from the heat recovery coil located in the exhaust line was used to heat the fresh air in winter by a preheater coil placed in front of this split A/C. The air temperature at the exit of this coil is not measured, it is not followed. However, the fact that the split A/C has been able to operate with 70% fresh air even in the coldest days during the winter, shows that the split A/C’s preheating coil raises the fresh air temperature up to operating limits of the split A/C unit.
The hood exhaust fan and the kitchen make up air AHU fan are controlled by common control voltage with different frequency converters. Employees who adjust the exhaust flow according to the exhaust requirements in the hoods automatically adjust the amount of fresh air. After the common control voltage is processed in a DDC, the make up air AHU drives the frequency converter.
CONCLUTIONS
The temperature of the outside air, the air temperature before and after the heat recovery coil, the water temperatures recirculating in the heat recovery circuit, the outlet air temperatures of the air handling units and the frequency converter position are recorded.
In Table-1, values are listed while the make up air AHU and hood exhaust fan frequency converters corolated by a constant coefficient. Many different coefficients are tried to adjust the pressure balance.
In Table-2, values are listed while the make up air AHU and hood exhaust fan frequency converters corolated by variable coefficients at different fan speeds.
EVALUATIONS
When the values in Table-1 are examined, fluctuations are seen. When we look at the flow rates and temperature differences, there seems to be more heating than the heat recovery. There may be two reasons for this: 1 – There must be some latent heat recovery in the heat recovery coil. 2 – Actual fresh air flow rates are different than project design values. In fact, when the make up air fan speed is corolated at full load, then there are pressure imbalances at partial loads. And, when corolation is done acording to partial load, there are imbalances at full load. In different periods, the make up air fan and exhaust fan responses dont go parallel, therefore balancing could not be performed by constant corolation coefficients.
Table 2 gives the values obtained by using the different air fan control voltage at different speeds. The make up air fan is driven in different ratios according to the exhaust fan at different speeds and the mean factors are used in the intermediate values. At full load, the make up air frequency converter approaches the multiplication value 1 at partial loads while the exhaust fan voltage multiplied by 0.83. After this setting of the frequency converter control voltage, the rooms and the outside air and the restaurant airflows were balanced and the heat recovery values and the fresh air heating values began to overlap. Taking into account the split A/C preheater, which does not measure temperature, it is concluded that there is still some latent heat gain. This result coincided with the coil selection outputs. The coil selection outputs also show that there is some condensation on the heat recovery coil and there will be latent heat recovery. Electrostatic precipitator with self cleaning automatic washing system is very successful in protecting the exhaust side heat recovery coil.
Despite the fluctuations, an interesting result comes from Table-1. One of the assumptions that can be made before the measurements is that the make up air coil outlet temperatures will increase as the flow rates decrease. As the air flow rate decreases, the required heating capacity would decrease and the coil temperature would rise. However, at the same exhaust air temperatures at partial flow rates, make up air temperatures are the same. Actually, this result coincides with the coil selection outputs. The lower the air flow through the coil, the lower the coil capacities. Although the air temperatures are the same, the amount of heat is reduced.
The lowest kitchen fresh air suplly temperature is 11 ° C. The exhaust air temperature is assumed to be 40 ° C. The exhaust air temperature is not increased to 40 ° C and make up air supply temperature is not increased the design target 16 ° C. The actual temperature is 11 ° C and below the design target of 16 ° C. However, since the blowing air was delivered at very low speeds and in front of the hoods, a portion of the cold air was drawn by the hoods before reaching the working height. The speed of the fresh air given to the hood at very low speeds at full load decreased to very very low values at partial loads. The comfort conditions for the employees are ensured as the circulating air at the staff level is also mixed with the transfer air from the dining hall.
As we mentioned before, the main purpose of the split A/C preheater is to operate with 70% fresh air in Ankara in winter and it is satisfactory to achieve this goal.
The rate of oil and soot accumulation on the inner walls of the ducts was very low. Low velocities in partial loads did not increase accumulation. Probably the heat insulation of the duct has a positive effect.
AHSRAE Standard 92.1 is used to combine the desired 3 systems (heat recovery, demand controlled ventilation and transfer air) and make up air of the kitchen is heated without using an additional heating source. The system, which is very useful in terms of operating costs, has also decreased investment costs. The wall type condensing boiler with a total capacity of 100 kW was used for the hot water generation and dining hall AHU secondary heating coil.
Although it is not provided in the tables, it is useful to give additional information. Thanks to evaporative cooling in the summer, the fresh air blowing temperature of the kitchen was 10 ° C lower than the outside air temperature. Due to the high number of air cycles, excessive moisture is not formed in the kitchen. Evaporative cooling was not able to comfortably replace mechanical cooling. However, compared to investment and operating costs, it has been very useful for the kitchen in this study