An air-conditioned room is maintained at 24∘C and 50% percentage saturation. In summer, the ambient conditions are 38∘C dry bulb and 26∘C wet bulb temperatures. The space total heat gain is 50 kW with latent gain contributing 10 kW.

Question:

Q5 a) An air-conditioned room is maintained at 24∘C and 50% percentage saturation. In summer, the ambient conditions are 38∘C dry bulb and 26∘C wet bulb temperatures. The space total heat gain is 50 kW with latent gain contributing 10 kW. The return air is mixed with fresh air in the ratio 3 to 2 by mass. The temperature of the supply air to the room is 15∘C dry bulb temperature and air is taken directly to the room with no supplementary heating. b) A harvested tomato should be kept at 10∘C and 95% saturation in order to prolong its ‘best before date’. Tomatoes are kept in a warehouse in Dubai where the outside conditions are 35∘C and 10% saturation. The air conditioning for the warehouse is as follows: – Cooling of ambient air at constant specific humidity. – Humidifying the air by injecting steam (supply saturated at 1 atm.) until the temperature is 9∘C and 100% saturation. – Sensible heating of air to desired conditions, 10∘C,95%. I. Determine the temperature that ambient air should be cooled to before steam injection. [6 marks] II. Estimate the heating and cooling requirements (per kgdry air) ​ ) [2 marks] III. Explain why in the context of food storage, no recycling of air is recommended even though it is wasteful in energy. [2 marks] The following equation for humidification by steam injection can be used without proof. hw​=ω2​−ω1​h2​−h1​​

Answer:


a) Air-Conditioned Room:

Given:

  • Room temperature = 24°C, 50% RH
  • Summer conditions = 38°C dry bulb, 26°C wet bulb
  • Total heat gain = 50 kW, latent gain = 10 kW
  • Air mixing ratio: Return air – Fresh air = 3:2

To find:

  • Supply air temperature

1. Calculate the enthalpies:

  • Enthalpy of room air (h_r): Use a psychrometric chart or equation to find h_r at 24°C and 50% RH.
  • Enthalpy of outdoor air (h_o): Use a psychrometric chart or equation to find h_o at 38°C dry bulb and 26°C wet bulb.
  • Enthalpy of mixed air (h_m): Calculate the weighted average of return air and fresh air enthalpies: h_m = 3/5 * h_r + 2/5 * h_o.

2. Apply energy balance:

  • The change in enthalpy of the mixed air after being cooled equals the total heat gain: (h_m – h_s) * m_air = 50 kW.
  • Where h_s is the enthalpy of supply air, and m_air is the mass flow rate of mixed air (unknown).

3. Solve for supply air temperature:

  • Rearrange the equation to express h_s: h_s = h_m – 50 kW / m_air.
  • Now, use a psychrometric chart to find the temperature that corresponds to h_s on the constant specific humidity line for the mixed air (same humidity ratio as h_m). This is the supply air temperature.

b) Harvested Tomato Storage:

Given:

  • Desired tomato storage conditions: 10°C, 95% RH
  • Warehouse conditions: 35°C, 10% RH
  • Cooling process: Constant specific humidity, then steam injection and sensible heating.

To find:

  • I. Temperature of cooled air before steam injection.
  • II. Heating and cooling requirements per kg of dry air.
  • III. Explanation for not recycling air in food storage.

I. Cooled air temperature:

  • Follow the constant specific humidity line on the psychrometric chart from the warehouse conditions (35°C, 10% RH) until the wet bulb temperature reaches 95% RH. This point represents the cooled air before steam injection. Read the dry bulb temperature at this point, which is the answer for part I.

II. Heating and cooling requirements:

  • Calculate the enthalpy of:
    • Warehouse air (h_w)
    • Cooled air before steam injection (h_c)
    • Saturated air at 1 atm and 9°C (use steam tables or h = 2500 + 2.45T kJ/kg for estimation)
    • Desired conditions (h_d)
  • Calculate the cooling (h_w – h_c) and heating (h_d – h_s) requirements per kg of dry air, assuming constant mass flow rate.

III. No air recycling in food storage:

  • Recycling air in food storage can lead to:
    • Contamination: Accumulation of airborne contaminants like mold spores and bacteria can affect the quality and shelf life of food.
    • Excess moisture: Recycled air might already be too humid, further contributing to mold growth and food spoilage.
    • Odor build-up: Odors released from stored food can accumulate in recycled air, potentially affecting other stored items.
  • Fresh air intake ensures proper removal of contaminants and maintains optimal humidity levels for specific food storage requirements.

By following these steps and using the psychrometric chart and given equations, you can find the answers to all parts of the problem. Remember to use consistent units and consider any additional information on the chart or equations provided.

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